diff mbox series

[03/11] elf: Add glibc-hwcaps support for LD_LIBRARY_PATH

Message ID 965e3bbd05f0f8227a71e73b9a1f1e4c517bdb14.1604946656.git.fweimer@redhat.com
State Committed
Headers show
Series glibc-hwcaps support | expand

Commit Message

Florian Weimer Nov. 9, 2020, 6:40 p.m. UTC
This hacks non-power-set processing into _dl_important_hwcaps.
Once the legacy hwcaps handling goes away, the subdirectory
handling needs to be reworked, but it is premature to do this
while both approaches are still supported.
---
 elf/Makefile                   |  67 ++++++++++++++--
 elf/dl-hwcaps-subdirs.c        |  29 +++++++
 elf/dl-hwcaps.c                | 138 +++++++++++++++++++++++++++-----
 elf/dl-hwcaps.h                | 105 +++++++++++++++++++++++++
 elf/dl-hwcaps_split.c          |  77 ++++++++++++++++++
 elf/dl-load.c                  |   7 +-
 elf/dl-main.h                  |  11 ++-
 elf/dl-support.c               |   5 +-
 elf/dl-usage.c                 |  68 +++++++++++++++-
 elf/markermodMARKER-VALUE.c    |  29 +++++++
 elf/rtld.c                     |  18 +++++
 elf/tst-dl-hwcaps_split.c      | 139 +++++++++++++++++++++++++++++++++
 elf/tst-glibc-hwcaps-mask.c    |  31 ++++++++
 elf/tst-glibc-hwcaps-prepend.c |  32 ++++++++
 elf/tst-glibc-hwcaps.c         |  28 +++++++
 sysdeps/generic/ldsodefs.h     |  20 +++--
 16 files changed, 771 insertions(+), 33 deletions(-)
 create mode 100644 elf/dl-hwcaps-subdirs.c
 create mode 100644 elf/dl-hwcaps_split.c
 create mode 100644 elf/markermodMARKER-VALUE.c
 create mode 100644 elf/tst-dl-hwcaps_split.c
 create mode 100644 elf/tst-glibc-hwcaps-mask.c
 create mode 100644 elf/tst-glibc-hwcaps-prepend.c
 create mode 100644 elf/tst-glibc-hwcaps.c

Comments

Adhemerval Zanella Nov. 26, 2020, 8:29 p.m. UTC | #1
On 09/11/2020 15:40, Florian Weimer via Libc-alpha wrote:
> This hacks non-power-set processing into _dl_important_hwcaps.
> Once the legacy hwcaps handling goes away, the subdirectory
> handling needs to be reworked, but it is premature to do this
> while both approaches are still supported.

Could you extend the commit message with the extra options you are adding
on ld.so and their expected semantic? I would be good to have some
documentation at least on the commit message.

Patch looks goods in general, some nits below.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

> ---
>  elf/Makefile                   |  67 ++++++++++++++--
>  elf/dl-hwcaps-subdirs.c        |  29 +++++++
>  elf/dl-hwcaps.c                | 138 +++++++++++++++++++++++++++-----
>  elf/dl-hwcaps.h                | 105 +++++++++++++++++++++++++
>  elf/dl-hwcaps_split.c          |  77 ++++++++++++++++++
>  elf/dl-load.c                  |   7 +-
>  elf/dl-main.h                  |  11 ++-
>  elf/dl-support.c               |   5 +-
>  elf/dl-usage.c                 |  68 +++++++++++++++-
>  elf/markermodMARKER-VALUE.c    |  29 +++++++
>  elf/rtld.c                     |  18 +++++
>  elf/tst-dl-hwcaps_split.c      | 139 +++++++++++++++++++++++++++++++++
>  elf/tst-glibc-hwcaps-mask.c    |  31 ++++++++
>  elf/tst-glibc-hwcaps-prepend.c |  32 ++++++++
>  elf/tst-glibc-hwcaps.c         |  28 +++++++
>  sysdeps/generic/ldsodefs.h     |  20 +++--
>  16 files changed, 771 insertions(+), 33 deletions(-)
>  create mode 100644 elf/dl-hwcaps-subdirs.c
>  create mode 100644 elf/dl-hwcaps_split.c
>  create mode 100644 elf/markermodMARKER-VALUE.c
>  create mode 100644 elf/tst-dl-hwcaps_split.c
>  create mode 100644 elf/tst-glibc-hwcaps-mask.c
>  create mode 100644 elf/tst-glibc-hwcaps-prepend.c
>  create mode 100644 elf/tst-glibc-hwcaps.c
> 
> diff --git a/elf/Makefile b/elf/Makefile
> index f10cc59e7c..d2f7e99863 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -59,7 +59,8 @@ elide-routines.os = $(all-dl-routines) dl-support enbl-secure dl-origin \
>  # ld.so uses those routines, plus some special stuff for being the program
>  # interpreter and operating independent of libc.
>  rtld-routines	= rtld $(all-dl-routines) dl-sysdep dl-environ dl-minimal \
> -  dl-error-minimal dl-conflict dl-hwcaps dl-usage
> +  dl-error-minimal dl-conflict dl-hwcaps dl-hwcaps_split dl-hwcaps-subdirs \
> +  dl-usage
>  all-rtld-routines = $(rtld-routines) $(sysdep-rtld-routines)
>  
>  CFLAGS-dl-runtime.c += -fexceptions -fasynchronous-unwind-tables

Ok.

> @@ -210,14 +211,14 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
>  	 tst-filterobj tst-filterobj-dlopen tst-auxobj tst-auxobj-dlopen \
>  	 tst-audit14 tst-audit15 tst-audit16 \
>  	 tst-single_threaded tst-single_threaded-pthread \
> -	 tst-tls-ie tst-tls-ie-dlmopen \
> -	 argv0test
> +	 tst-tls-ie tst-tls-ie-dlmopen argv0test \
> +	 tst-glibc-hwcaps tst-glibc-hwcaps-prepend tst-glibc-hwcaps-mask
>  #	 reldep9
>  tests-internal += loadtest unload unload2 circleload1 \
>  	 neededtest neededtest2 neededtest3 neededtest4 \
>  	 tst-tls3 tst-tls6 tst-tls7 tst-tls8 tst-dlmopen2 \
>  	 tst-ptrguard1 tst-stackguard1 tst-libc_dlvsym \
> -	 tst-create_format1 tst-tls-surplus
> +	 tst-create_format1 tst-tls-surplus tst-dl-hwcaps_split
>  tests-container += tst-pldd tst-dlopen-tlsmodid-container \
>    tst-dlopen-self-container
>  test-srcs = tst-pathopt

Ok.

> @@ -329,7 +330,10 @@ modules-names = testobj1 testobj2 testobj3 testobj4 testobj5 testobj6 \
>  		tst-single_threaded-mod3 tst-single_threaded-mod4 \
>  		tst-tls-ie-mod0 tst-tls-ie-mod1 tst-tls-ie-mod2 \
>  		tst-tls-ie-mod3 tst-tls-ie-mod4 tst-tls-ie-mod5 \
> -		tst-tls-ie-mod6
> +		tst-tls-ie-mod6 libmarkermod1-1 libmarkermod1-2 libmarkermod1-3 \
> +		libmarkermod2-1 libmarkermod2-2 \
> +		libmarkermod3-1 libmarkermod3-2 libmarkermod3-3 \
> +		libmarkermod4-1 libmarkermod4-2 libmarkermod4-3 libmarkermod4-4 \
>  
>  # Most modules build with _ISOMAC defined, but those filtered out
>  # depend on internal headers.

Ok.

> @@ -1812,3 +1816,56 @@ $(objpfx)argv0test.out: tst-rtld-argv0.sh $(objpfx)ld.so \
>              '$(test-wrapper-env)' '$(run_program_env)' \
>              '$(rpath-link)' 'test-argv0' > $@; \
>      $(evaluate-test)
> +
> +# Most likely search subdirectories, for each supported architecture.
> +# Used to obtain test coverage wide test coverage.

This comments sounds strange, specially the second line.

> +glibc-hwcaps-first-subdirs-for-tests =
> +
> +# The test modules are parameterized by preprocessor macros.
> +LDFLAGS-libmarkermod1-1.so += -Wl,-soname,libmarkermod1.so
> +LDFLAGS-libmarkermod2-1.so += -Wl,-soname,libmarkermod2.so
> +LDFLAGS-libmarkermod3-1.so += -Wl,-soname,libmarkermod3.so
> +LDFLAGS-libmarkermod4-1.so += -Wl,-soname,libmarkermod4.so
> +$(objpfx)libmarkermod%.os : markermodMARKER-VALUE.c
> +	$(compile-command.c) \
> +	  -DMARKER=marker$(firstword $(subst -, ,$*)) \
> +	  -DVALUE=$(lastword $(subst -, ,$*))
> +$(objpfx)libmarkermod1.so: $(objpfx)libmarkermod1-1.so
> +	cp $< $@
> +$(objpfx)libmarkermod2.so: $(objpfx)libmarkermod2-1.so
> +	cp $< $@
> +$(objpfx)libmarkermod3.so: $(objpfx)libmarkermod3-1.so
> +	cp $< $@
> +$(objpfx)libmarkermod4.so: $(objpfx)libmarkermod4-1.so
> +	cp $< $@
> +

Ok.

> +# tst-glibc-hwcaps-prepend checks that --glibc-hwcaps-prepend is
> +# preferred over auto-detected subdirectories.
> +$(objpfx)tst-glibc-hwcaps-prepend: $(objpfx)libmarkermod1-1.so
> +$(objpfx)glibc-hwcaps/prepend-markermod1/libmarkermod1.so: \
> +  $(objpfx)libmarkermod1-2.so
> +	$(make-target-directory)
> +	cp $< $@
> +$(objpfx)glibc-hwcaps/%/libmarkermod1.so: $(objpfx)libmarkermod1-3.so
> +	$(make-target-directory)
> +	cp $< $@
> +$(objpfx)tst-glibc-hwcaps-prepend.out: \
> +  $(objpfx)tst-glibc-hwcaps-prepend $(objpfx)libmarkermod1.so \
> +  $(patsubst %,$(objpfx)glibc-hwcaps/%/libmarkermod1.so,prepend-markermod1 \
> +    $(glibc-hwcaps-first-subdirs-for-tests))
> +	$(test-wrapper) $(rtld-prefix) \
> +	  --glibc-hwcaps-prepend prepend-markermod1 \
> +	  $< > $@; \
> +	$(evaluate-test)
> +

Ok.

> +# tst-glibc-hwcaps-mask checks that --glibc-hwcaps-mask can be used to
> +# suppress all auto-detected subdirectories.
> +$(objpfx)tst-glibc-hwcaps-mask: $(objpfx)libmarkermod1-1.so
> +$(objpfx)tst-glibc-hwcaps-mask.out: \
> +  $(objpfx)tst-glibc-hwcaps-mask $(objpfx)libmarkermod1.so \
> +  $(patsubst %,$(objpfx)glibc-hwcaps/%/libmarkermod1.so,\
> +    $(glibc-hwcaps-first-subdirs-for-tests))
> +	$(test-wrapper) $(rtld-prefix) \
> +	  --glibc-hwcaps-mask does-not-exist \
> +	  $< > $@; \
> +	$(evaluate-test)

Ok.

> diff --git a/elf/dl-hwcaps-subdirs.c b/elf/dl-hwcaps-subdirs.c
> new file mode 100644
> index 0000000000..60c6d59731
> --- /dev/null
> +++ b/elf/dl-hwcaps-subdirs.c
> @@ -0,0 +1,29 @@
> +/* Architecture-specific glibc-hwcaps subdirectories.  Generic version.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <dl-hwcaps.h>
> +
> +/* In the generic version, there are no subdirectories defined.  */
> +
> +const char _dl_hwcaps_subdirs[] = "";
> +
> +uint32_t
> +_dl_hwcaps_subdirs_active (void)
> +{
> +  return 0;
> +}

Ok.

> diff --git a/elf/dl-hwcaps.c b/elf/dl-hwcaps.c
> index 44dbac099f..f611f3a1a6 100644
> --- a/elf/dl-hwcaps.c
> +++ b/elf/dl-hwcaps.c
> @@ -26,20 +26,97 @@
>  #include <dl-procinfo.h>
>  #include <dl-hwcaps.h>
>  
> +/* This is the result of counting the substrings in a colon-separated
> +   hwcaps string.  */
> +struct hwcaps_counts
> +{
> +  /* Number of substrings.  */
> +  size_t count;
> +
> +  /* Sum of the individual substring lengths (without separators or
> +     null terminators).  */
> +  size_t total_length;
> +
> +  /* Maximum length of an individual substring.  */
> +  size_t maximum_length;
> +};
> +

Ok.

> +/* Update *COUNTS according to the contents of HWCAPS.  Skip over
> +   entries whose bit is not set in MASK.  */
> +static void
> +update_hwcaps_counts (struct hwcaps_counts *counts, const char *hwcaps,
> +		      uint32_t bitmask, const char *mask)
> +{
> +  struct dl_hwcaps_split_masked sp;
> +  _dl_hwcaps_split_masked_init (&sp, hwcaps, bitmask, mask);
> +  while (_dl_hwcaps_split_masked (&sp))
> +    {
> +      ++counts->count;
> +      counts->total_length += sp.split.length;
> +      if (sp.split.length > counts->maximum_length)
> +	counts->maximum_length = sp.split.length;
> +    }
> +}
> +

Ok.

> +/* State for copy_hwcaps.  Must be initialized to point to
> +   the storage areas for the array and the strings themselves.  */
> +struct copy_hwcaps
> +{
> +  struct r_strlenpair *next_pair;
> +  char *next_string;
> +};
> +
> +/* Copy HWCAPS into the string pairs and strings, advancing *TARGET.
> +   Skip over entries whose bit is not set in MASK.  */
> +static void
> +copy_hwcaps (struct copy_hwcaps *target, const char *hwcaps,
> +	     uint32_t bitmask, const char *mask)
> +{
> +  struct dl_hwcaps_split_masked sp;
> +  _dl_hwcaps_split_masked_init (&sp, hwcaps, bitmask, mask);
> +  while (_dl_hwcaps_split_masked (&sp))
> +    {
> +      target->next_pair->str = target->next_string;
> +      char *slash = __mempcpy (__mempcpy (target->next_string,
> +					  GLIBC_HWCAPS_PREFIX,
> +					  strlen (GLIBC_HWCAPS_PREFIX)),
> +			       sp.split.segment, sp.split.length);
> +      *slash = '/';
> +      target->next_pair->len
> +	= strlen (GLIBC_HWCAPS_PREFIX) + sp.split.length + 1;
> +      ++target->next_pair;
> +      target->next_string = slash + 1;
> +    }
> +}
> +

Ok.

>  /* Return an array of useful/necessary hardware capability names.  */
>  const struct r_strlenpair *
> -_dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
> +_dl_important_hwcaps (const char *glibc_hwcaps_prepend,
> +		      const char *glibc_hwcaps_mask,
> +		      size_t *sz, size_t *max_capstrlen)
>  {
>    uint64_t hwcap_mask = GET_HWCAP_MASK();
>    /* Determine how many important bits are set.  */
>    uint64_t masked = GLRO(dl_hwcap) & hwcap_mask;
>    size_t cnt = GLRO (dl_platform) != NULL;
>    size_t n, m;
> -  size_t total;
>    struct r_strlenpair *result;
>    struct r_strlenpair *rp;
>    char *cp;
>  
> +  /* glibc-hwcaps subdirectories.  These are exempted from the power
> +     set construction below.  */
> +  uint32_t hwcaps_subdirs_active = _dl_hwcaps_subdirs_active ();
> +  struct hwcaps_counts hwcaps_counts =  { 0, };
> +  update_hwcaps_counts (&hwcaps_counts, glibc_hwcaps_prepend, -1, NULL);
> +  update_hwcaps_counts (&hwcaps_counts, _dl_hwcaps_subdirs,
> +			hwcaps_subdirs_active, glibc_hwcaps_mask);
> +
> +  /* Each hwcaps subdirectory has a GLIBC_HWCAPS_PREFIX string prefix
> +     and a "/" suffix once stored in the result.  */
> +  size_t total = (hwcaps_counts.count * (strlen (GLIBC_HWCAPS_PREFIX) + 1)
> +		  + hwcaps_counts.total_length);
> +
>    /* Count the number of bits set in the masked value.  */
>    for (n = 0; (~((1ULL << n) - 1) & masked) != 0; ++n)
>      if ((masked & (1ULL << n)) != 0)

Ok.

> @@ -74,10 +151,10 @@ _dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
>  
>    /* Determine the total size of all strings together.  */
>    if (cnt == 1)
> -    total = temp[0].len + 1;
> +    total += temp[0].len + 1;
>    else
>      {
> -      total = temp[0].len + temp[cnt - 1].len + 2;
> +      total += temp[0].len + temp[cnt - 1].len + 2;
>        if (cnt > 2)
>  	{
>  	  total <<= 1;

Ok.

> @@ -94,26 +171,48 @@ _dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
>  	}
>      }
>  
> -  /* The result structure: we use a very compressed way to store the
> -     various combinations of capability names.  */
> -  *sz = 1 << cnt;
> -  result = (struct r_strlenpair *) malloc (*sz * sizeof (*result) + total);
> -  if (result == NULL)
> +  *sz = hwcaps_counts.count + (1 << cnt);
> +
> +  /* This is the overall result, including both glibc-hwcaps
> +     subdirectories and the legacy hwcaps subdirectories using the
> +     power set construction.  */
> +  struct r_strlenpair *overall_result
> +    = malloc (*sz * sizeof (*result) + total);
> +  if (overall_result == NULL)
>      _dl_signal_error (ENOMEM, NULL, NULL,
>  		      N_("cannot create capability list"));
>  
> +  /* Fill in the glibc-hwcaps subdirectories.  */
> +  {
> +    struct copy_hwcaps target;
> +    target.next_pair = overall_result;
> +    target.next_string = (char *) (overall_result + *sz);
> +    copy_hwcaps (&target, glibc_hwcaps_prepend, -1, NULL);
> +    copy_hwcaps (&target, _dl_hwcaps_subdirs,
> +		 hwcaps_subdirs_active, glibc_hwcaps_mask);
> +    /* Set up the write target for the power set construction.  */
> +    result = target.next_pair;
> +    cp = target.next_string;
> +  }
> +
> +
> +  /* Power set construction begins here.  We use a very compressed way
> +     to store the various combinations of capability names.  */
> +
>    if (cnt == 1)
>      {
> -      result[0].str = (char *) (result + *sz);
> +      result[0].str = cp;
>        result[0].len = temp[0].len + 1;
> -      result[1].str = (char *) (result + *sz);
> +      result[1].str = cp;
>        result[1].len = 0;
> -      cp = __mempcpy ((char *) (result + *sz), temp[0].str, temp[0].len);
> +      cp = __mempcpy (cp, temp[0].str, temp[0].len);
>        *cp = '/';
> -      *sz = 2;
> -      *max_capstrlen = result[0].len;
> +      if (result[0].len > hwcaps_counts.maximum_length)
> +	*max_capstrlen = result[0].len;
> +      else
> +	*max_capstrlen = hwcaps_counts.maximum_length;
>  
> -      return result;
> +      return overall_result;
>      }
>  
>    /* Fill in the information.  This follows the following scheme

Ok.

> @@ -124,7 +223,7 @@ _dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
>  	      #3: 0, 3			1001
>       This allows the representation of all possible combinations of
>       capability names in the string.  First generate the strings.  */
> -  result[1].str = result[0].str = cp = (char *) (result + *sz);
> +  result[1].str = result[0].str = cp;
>  #define add(idx) \
>        cp = __mempcpy (__mempcpy (cp, temp[idx].str, temp[idx].len), "/", 1);
>    if (cnt == 2)
> @@ -191,7 +290,10 @@ _dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
>    while (--n != 0);
>  
>    /* The maximum string length.  */
> -  *max_capstrlen = result[0].len;
> +  if (result[0].len > hwcaps_counts.maximum_length)
> +    *max_capstrlen = result[0].len;
> +  else
> +    *max_capstrlen = hwcaps_counts.maximum_length;
>  
> -  return result;
> +  return overall_result;
>  }

Ok.

> diff --git a/elf/dl-hwcaps.h b/elf/dl-hwcaps.h
> index b66da59b89..ab39d8a46d 100644
> --- a/elf/dl-hwcaps.h
> +++ b/elf/dl-hwcaps.h
> @@ -16,6 +16,11 @@
>     License along with the GNU C Library; if not, see
>     <https://www.gnu.org/licenses/>.  */
>  
> +#ifndef _DL_HWCAPS_H
> +#define _DL_HWCAPS_H
> +
> +#include <stdint.h>
> +
>  #include <elf/dl-tunables.h>
>  
>  #if HAVE_TUNABLES
> @@ -28,3 +33,103 @@
>  #  define GET_HWCAP_MASK() (0)
>  # endif
>  #endif
> +
> +#define GLIBC_HWCAPS_SUBDIRECTORY "glibc-hwcaps"
> +#define GLIBC_HWCAPS_PREFIX GLIBC_HWCAPS_SUBDIRECTORY "/"
> +
> +/* Used by _dl_hwcaps_split below, to split strings at ':'
> +   separators.  */
> +struct dl_hwcaps_split
> +{
> +  const char *segment;          /* Start of the current segment.  */
> +  size_t length;                /* Number of bytes until ':' or NUL.  */
> +};
> +
> +/* Prepare *S to parse SUBJECT, for future _dl_hwcaps_split calls.  If
> +   SUBJECT is NULL, it is treated as the empty string.  */
> +static inline void
> +_dl_hwcaps_split_init (struct dl_hwcaps_split *s, const char *subject)
> +{
> +  s->segment = subject;
> +  /* The initial call to _dl_hwcaps_split will not skip anything.  */
> +  s->length = 0;
> +}

Ok.

> +
> +/* Extract the next non-empty string segment, up to ':' or the null
> +   terminator.  Return true if one more segment was found, or false if
> +   the end of the string was reached.  On success, S->segment is the
> +   start of the segment found, and S->length is its length.
> +   (Typically, S->segment[S->length] is not null.)  */
> +_Bool _dl_hwcaps_split (struct dl_hwcaps_split *s) attribute_hidden;
> +
> +/* Similar to dl_hwcaps_split, but with bit-based and name-based
> +   masking.  */
> +struct dl_hwcaps_split_masked
> +{
> +  struct dl_hwcaps_split split;
> +
> +  /* For used by the iterator implementation.  */
> +  const char *mask;
> +  uint32_t bitmask;
> +};
> +
> +/* Prepare *S for iteration with _dl_hwcaps_split_masked.  Only HWCAP
> +   names in SUBJECT whose bit is set in BITMASK and whose name is in
> +   MASK will be returned.  SUBJECT must not contain empty HWCAP names.
> +   If MASK is NULL, no name-based masking is applied.  Likewise for
> +   BITMASK if BITMASK is -1 (infinite number of bits).  */
> +static inline void
> +_dl_hwcaps_split_masked_init (struct dl_hwcaps_split_masked *s,
> +                              const char *subject,
> +                              uint32_t bitmask, const char *mask)
> +{
> +  _dl_hwcaps_split_init (&s->split, subject);
> +  s->bitmask = bitmask;
> +  s->mask = mask;
> +}
> +

Ok.

> +/* Like _dl_hwcaps_split, but apply masking.  */
> +_Bool _dl_hwcaps_split_masked (struct dl_hwcaps_split_masked *s)
> +  attribute_hidden;
> +
> +/* Returns true if the colon-separated HWCAP list HWCAPS contains the
> +   capability NAME (with length NAME_LENGTH).  If HWCAPS is NULL, the
> +   function returns true.  */
> +_Bool _dl_hwcaps_contains (const char *hwcaps, const char *name,
> +                           size_t name_length) attribute_hidden;

I find it confusing that an null hwcap result true while an empty
relies on name.  Do we need that NULL to be handled special here?

> +
> +/* Colon-separated string of glibc-hwcaps subdirectories, without the
> +   "glibc-hwcaps/" prefix.  The most preferred subdirectory needs to
> +   be listed first.  Up to 32 subdirectories are supported, limited by
> +   the width of the uint32_t mask.  */
> +extern const char _dl_hwcaps_subdirs[] attribute_hidden;
> +
> +/* Returns a bitmap of active subdirectories in _dl_hwcaps_subdirs.
> +   Bit 0 (the LSB) corresponds to the first substring in
> +   _dl_hwcaps_subdirs, bit 1 to the second substring, and so on.
> +   There is no direct correspondence between HWCAP bitmasks and this
> +   bitmask.  */
> +uint32_t _dl_hwcaps_subdirs_active (void) attribute_hidden;
> +
> +/* Returns a bitmask that marks the last ACTIVE subdirectories in a
> +   _dl_hwcaps_subdirs_active string (containing SUBDIRS directories in
> +   total) as active.  Intended for use in _dl_hwcaps_subdirs_active
> +   implementations (if a contiguous tail of the list in
> +   _dl_hwcaps_subdirs is selected).  */
> +static inline uint32_t
> +_dl_hwcaps_subdirs_build_bitmask (int subdirs, int active)
> +{

Should it any assert to check if subdirs and/or active is within the
expected range?

> +  /* Leading subdirectories that are not active.  */
> +  int inactive = subdirs - active;
> +  if (inactive == 32)
> +    return 0;
> +
> +  uint32_t mask;
> +  if (subdirs != 32)
> +    mask = (1U << subdirs) - 1;
> +  else
> +    mask = -1;
> +  return mask ^ ((1U << inactive) - 1);
> +}
> +
> +#endif /* _DL_HWCAPS_H */
> diff --git a/elf/dl-hwcaps_split.c b/elf/dl-hwcaps_split.c
> new file mode 100644
> index 0000000000..95225e9f40
> --- /dev/null
> +++ b/elf/dl-hwcaps_split.c
> @@ -0,0 +1,77 @@
> +/* Hardware capability support for run-time dynamic loader.  String splitting.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <dl-hwcaps.h>
> +#include <stdbool.h>
> +#include <string.h>
> +
> +_Bool
> +_dl_hwcaps_split (struct dl_hwcaps_split *s)
> +{
> +  if (s->segment == NULL)
> +    return false;
> +
> +  /* Skip over the previous segment.   */
> +  s->segment += s->length;
> +
> +  /* Consume delimiters.  This also avoids returning an empty
> +     segment.  */
> +  while (*s->segment == ':')
> +    ++s->segment;
> +  if (*s->segment == '\0')
> +    return false;
> +
> +  /* This could use strchrnul, but we would have to link the function
> +     into ld.so for that.  */
> +  const char *colon = strchr (s->segment, ':');
> +  if (colon == NULL)
> +    s->length = strlen (s->segment);
> +  else
> +    s->length = colon - s->segment;
> +  return true;
> +}
> +

Ok.

> +_Bool
> +_dl_hwcaps_split_masked (struct dl_hwcaps_split_masked *s)
> +{
> +  while (true)
> +    {
> +      if (!_dl_hwcaps_split (&s->split))
> +        return false;
> +      bool active = s->bitmask & 1;
> +      s->bitmask >>= 1;
> +      if (active && _dl_hwcaps_contains (s->mask,
> +                                         s->split.segment, s->split.length))
> +        return true;
> +    }
> +}
> +

Ok.

> +_Bool
> +_dl_hwcaps_contains (const char *hwcaps, const char *name, size_t name_length)
> +{
> +  if (hwcaps == NULL)
> +    return true;
> +
> +  struct dl_hwcaps_split split;
> +  _dl_hwcaps_split_init (&split, hwcaps);
> +  while (_dl_hwcaps_split (&split))
> +    if (split.length == name_length
> +        && memcmp (split.segment, name, name_length) == 0)
> +      return true;
> +  return false;
> +}

Ok.

> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index f3201e7c14..9020f1646f 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -682,7 +682,9 @@ cache_rpath (struct link_map *l,
>  
>  
>  void
> -_dl_init_paths (const char *llp, const char *source)
> +_dl_init_paths (const char *llp, const char *source,
> +		const char *glibc_hwcaps_prepend,
> +		const char *glibc_hwcaps_mask)
>  {
>    size_t idx;
>    const char *strp;
> @@ -697,7 +699,8 @@ _dl_init_paths (const char *llp, const char *source)
>  
>  #ifdef SHARED
>    /* Get the capabilities.  */
> -  capstr = _dl_important_hwcaps (&ncapstr, &max_capstrlen);
> +  capstr = _dl_important_hwcaps (glibc_hwcaps_prepend, glibc_hwcaps_mask,
> +				 &ncapstr, &max_capstrlen);
>  #endif
>  
>    /* First set up the rest of the default search directory entries.  */

Ok.

> diff --git a/elf/dl-main.h b/elf/dl-main.h
> index b51256d3b4..566713a0d1 100644
> --- a/elf/dl-main.h
> +++ b/elf/dl-main.h
> @@ -84,6 +84,14 @@ struct dl_main_state
>    /* The preload list passed as a command argument.  */
>    const char *preloadarg;
>  
> +  /* Additional glibc-hwcaps subdirectories to search first.
> +     Colon-separated list.  */
> +  const char *glibc_hwcaps_prepend;
> +
> +  /* Mask for the internal glibc-hwcaps subdirectories.
> +     Colon-separated list.  */
> +  const char *glibc_hwcaps_mask;
> +
>    enum rtld_mode mode;
>  
>    /* True if any of the debugging options is enabled.  */

Ok.

> @@ -98,7 +106,8 @@ struct dl_main_state
>  static inline void
>  call_init_paths (const struct dl_main_state *state)
>  {
> -  _dl_init_paths (state->library_path, state->library_path_source);
> +  _dl_init_paths (state->library_path, state->library_path_source,
> +                  state->glibc_hwcaps_prepend, state->glibc_hwcaps_mask);
>  }
>  
>  /* Print ld.so usage information and exit.  */

Ok.

> diff --git a/elf/dl-support.c b/elf/dl-support.c
> index afbc94df54..3264262f4e 100644
> --- a/elf/dl-support.c
> +++ b/elf/dl-support.c
> @@ -323,7 +323,10 @@ _dl_non_dynamic_init (void)
>  
>    /* Initialize the data structures for the search paths for shared
>       objects.  */
> -  _dl_init_paths (getenv ("LD_LIBRARY_PATH"), "LD_LIBRARY_PATH");
> +  _dl_init_paths (getenv ("LD_LIBRARY_PATH"), "LD_LIBRARY_PATH",
> +		  /* No glibc-hwcaps selection support in statically
> +		     linked binaries.  */
> +		  NULL, NULL);
>  
>    /* Remember the last search directory added at startup.  */
>    _dl_init_all_dirs = GL(dl_all_dirs);

Ok.

> diff --git a/elf/dl-usage.c b/elf/dl-usage.c
> index 796ad38b43..e22a9c3942 100644
> --- a/elf/dl-usage.c
> +++ b/elf/dl-usage.c
> @@ -83,7 +83,7 @@ print_search_path_for_help (struct dl_main_state *state)
>  {
>    if (__rtld_search_dirs.dirs == NULL)
>      /* The run-time search paths have not yet been initialized.  */
> -    _dl_init_paths (state->library_path, state->library_path_source);
> +    call_init_paths (state);
>  
>    _dl_printf ("\nShared library search path:\n");
>  
> @@ -132,6 +132,67 @@ print_hwcap_1_finish (bool *first)
>      _dl_printf (")\n");
>  }
>  
> +/* Print the header for print_hwcaps_subdirectories.  */
> +static void
> +print_hwcaps_subdirectories_header (bool *nothing_printed)
> +{
> +  if (*nothing_printed)
> +    {
> +      _dl_printf ("\n\
> +Subdirectories of glibc-hwcaps directories, in priority order:\n");
> +      *nothing_printed = false;
> +    }
> +}
> +
> +/* Print the HWCAP name itself, indented.  */
> +static void
> +print_hwcaps_subdirectories_name (const struct dl_hwcaps_split *split)
> +{
> +  _dl_write (STDOUT_FILENO, "  ", 2);
> +  _dl_write (STDOUT_FILENO, split->segment, split->length);
> +}
> +
> +/* Print the list of recognized glibc-hwcaps subdirectories.  */
> +static void
> +print_hwcaps_subdirectories (const struct dl_main_state *state)
> +{
> +  bool nothing_printed = true;
> +  struct dl_hwcaps_split split;
> +
> +  /* The prepended glibc-hwcaps subdirectories.  */
> +  _dl_hwcaps_split_init (&split, state->glibc_hwcaps_prepend);
> +  while (_dl_hwcaps_split (&split))
> +    {
> +      print_hwcaps_subdirectories_header (&nothing_printed);
> +      print_hwcaps_subdirectories_name (&split);
> +      bool first = true;
> +      print_hwcap_1 (&first, true, "searched");
> +      print_hwcap_1_finish (&first);
> +    }
> +
> +  /* The built-in glibc-hwcaps subdirectories.  Do the filtering
> +     manually, so that more precise diagnostics are possible.  */
> +  uint32_t mask = _dl_hwcaps_subdirs_active ();
> +  _dl_hwcaps_split_init (&split, _dl_hwcaps_subdirs);
> +  while (_dl_hwcaps_split (&split))
> +    {
> +      print_hwcaps_subdirectories_header (&nothing_printed);
> +      print_hwcaps_subdirectories_name (&split);
> +      bool first = true;
> +      print_hwcap_1 (&first, mask & 1, "supported");
> +      bool listed = _dl_hwcaps_contains (state->glibc_hwcaps_mask,
> +                                         split.segment, split.length);
> +      print_hwcap_1 (&first, !listed, "masked");
> +      print_hwcap_1 (&first, (mask & 1) && listed, "searched");
> +      print_hwcap_1_finish (&first);
> +      mask >>= 1;
> +    }
> +
> +  if (nothing_printed)
> +    _dl_printf ("\n\
> +No subdirectories of glibc-hwcaps directories are searched.\n");
> +}
> +
>  /* Write a list of hwcap subdirectories to standard output.  See
>   _dl_important_hwcaps in dl-hwcaps.c.  */
>  static void
> @@ -186,6 +247,10 @@ setting environment variables (which would be inherited by subprocesses).\n\
>    --inhibit-cache       Do not use " LD_SO_CACHE "\n\
>    --library-path PATH   use given PATH instead of content of the environment\n\
>                          variable LD_LIBRARY_PATH\n\
> +  --glibc-hwcaps-prepend LIST\n\
> +                        search glibc-hwcaps subdirectories in LIST\n\
> +  --glibc-hwcaps-mask LIST\n\
> +                        only search built-in subdirectories if in LIST\n\
>    --inhibit-rpath LIST  ignore RUNPATH and RPATH information in object names\n\
>                          in LIST\n\
>    --audit LIST          use objects named in LIST as auditors\n\
> @@ -198,6 +263,7 @@ This program interpreter self-identifies as: " RTLD "\n\
>  ",
>                argv0);
>    print_search_path_for_help (state);
> +  print_hwcaps_subdirectories (state);
>    print_legacy_hwcap_directories ();
>    _exit (EXIT_SUCCESS);
>  }

Do we really need to export the new options with the '--glibc' prefix?
The loader options are an implementation detail (same for the internal
variable names).

> diff --git a/elf/markermodMARKER-VALUE.c b/elf/markermodMARKER-VALUE.c
> new file mode 100644
> index 0000000000..99bdcf71a4
> --- /dev/null
> +++ b/elf/markermodMARKER-VALUE.c
> @@ -0,0 +1,29 @@
> +/* Source file template for building shared objects with marker functions.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +/* MARKER and VALUE must be set on the compiler command line.  */
> +
> +#ifndef MARKER
> +# error MARKER not defined
> +#endif

Maybe also add a check for the VALUE?

> +
> +int
> +MARKER (void)
> +{
> +  return VALUE;
> +}
> diff --git a/elf/rtld.c b/elf/rtld.c
> index 5d117d0d2c..e3ec66c030 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -289,6 +289,8 @@ dl_main_state_init (struct dl_main_state *state)
>    state->library_path_source = NULL;
>    state->preloadlist = NULL;
>    state->preloadarg = NULL;
> +  state->glibc_hwcaps_prepend = NULL;
> +  state->glibc_hwcaps_mask = NULL;
>    state->mode = rtld_mode_normal;
>    state->any_debug = false;
>    state->version_info = false;
> @@ -1244,6 +1246,22 @@ dl_main (const ElfW(Phdr) *phdr,
>  	  {
>  	    argv0 = _dl_argv[2];
>  
> +	    _dl_skip_args += 2;
> +	    _dl_argc -= 2;
> +	    _dl_argv += 2;
> +	  }
> +	else if (strcmp (_dl_argv[1], "--glibc-hwcaps-prepend") == 0
> +		 && _dl_argc > 2)
> +	  {
> +	    state.glibc_hwcaps_prepend = _dl_argv[2];
> +	    _dl_skip_args += 2;
> +	    _dl_argc -= 2;
> +	    _dl_argv += 2;
> +	  }
> +	else if (strcmp (_dl_argv[1], "--glibc-hwcaps-mask") == 0
> +		 && _dl_argc > 2)
> +	  {
> +	    state.glibc_hwcaps_mask = _dl_argv[2];
>  	    _dl_skip_args += 2;
>  	    _dl_argc -= 2;
>  	    _dl_argv += 2;

Ok.

> diff --git a/elf/tst-dl-hwcaps_split.c b/elf/tst-dl-hwcaps_split.c
> new file mode 100644
> index 0000000000..929c99a23b
> --- /dev/null
> +++ b/elf/tst-dl-hwcaps_split.c
> @@ -0,0 +1,139 @@
> +/* Unit tests for dl-hwcaps.c.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <array_length.h>
> +#include <dl-hwcaps.h>
> +#include <string.h>
> +#include <support/check.h>
> +
> +static void
> +check_split_masked (const char *input, int32_t bitmask, const char *mask,
> +                    const char *expected[], size_t expected_length)
> +{
> +  struct dl_hwcaps_split_masked split;
> +  _dl_hwcaps_split_masked_init (&split, input, bitmask, mask);
> +  size_t index = 0;
> +  while (_dl_hwcaps_split_masked (&split))
> +    {
> +      TEST_VERIFY_EXIT (index < expected_length);
> +      TEST_COMPARE_BLOB (expected[index], strlen (expected[index]),
> +                         split.split.segment, split.split.length);
> +      ++index;
> +    }
> +  TEST_COMPARE (index, expected_length);
> +}
> +
> +static void
> +check_split (const char *input,
> +             const char *expected[], size_t expected_length)
> +{
> +  struct dl_hwcaps_split split;
> +  _dl_hwcaps_split_init (&split, input);
> +  size_t index = 0;
> +  while (_dl_hwcaps_split (&split))
> +    {
> +      TEST_VERIFY_EXIT (index < expected_length);
> +      TEST_COMPARE_BLOB (expected[index], strlen (expected[index]),
> +                         split.segment, split.length);
> +      ++index;
> +    }
> +  TEST_COMPARE (index, expected_length);
> +
> +  /* Reuse the test cases with masking that does not actually remove
> +     anything.  */
> +  check_split_masked (input, -1, NULL, expected, expected_length);
> +  check_split_masked (input, -1, input, expected, expected_length);
> +}
> +
> +static int
> +do_test (void)
> +{
> +  /* Splitting tests, without masking.  */
> +  check_split (NULL, NULL, 0);
> +  check_split ("", NULL, 0);
> +  check_split (":", NULL, 0);
> +  check_split ("::", NULL, 0);

Ok.

> +
> +  {
> +    const char *expected[] = { "first" };
> +    check_split ("first", expected, array_length (expected));
> +    check_split (":first", expected, array_length (expected));
> +    check_split ("first:", expected, array_length (expected));
> +    check_split (":first:", expected, array_length (expected));
> +  }
> +

Ok.

> +  {
> +    const char *expected[] = { "first", "second" };
> +    check_split ("first:second", expected, array_length (expected));
> +    check_split ("first::second", expected, array_length (expected));
> +    check_split (":first:second", expected, array_length (expected));
> +    check_split ("first:second:", expected, array_length (expected));
> +    check_split (":first:second:", expected, array_length (expected));
> +  }
> +


Ok.
> +  /* Splitting tests with masking.  */
> +  {
> +    const char *expected[] = { "first" };
> +    check_split_masked ("first", 3, "first:second",
> +                        expected, array_length (expected));
> +    check_split_masked ("first:second", 3, "first:",
> +                        expected, array_length (expected));
> +    check_split_masked ("first:second", 1, NULL,
> +                        expected, array_length (expected));
> +  }

Ok.

> +  {
> +    const char *expected[] = { "second" };
> +    check_split_masked ("first:second", 3, "second",
> +                        expected, array_length (expected));
> +    check_split_masked ("first:second:third", -1, "second:",
> +                        expected, array_length (expected));
> +    check_split_masked ("first:second", 2, NULL,
> +                        expected, array_length (expected));
> +    check_split_masked ("first:second:third", 2, "first:second",
> +                        expected, array_length (expected));
> +  }
> +

Ok.

> +  /* Tests for _dl_hwcaps_contains.  */
> +  TEST_VERIFY (_dl_hwcaps_contains (NULL, "first", strlen ("first")));
> +  TEST_VERIFY (_dl_hwcaps_contains (NULL, "", 0));
> +  TEST_VERIFY (! _dl_hwcaps_contains ("", "first", strlen ("first")));
> +  TEST_VERIFY (! _dl_hwcaps_contains ("firs", "first", strlen ("first")));
> +  TEST_VERIFY (_dl_hwcaps_contains ("firs", "first", strlen ("first") - 1));
> +  for (int i = 0; i < strlen ("first"); ++i)
> +    TEST_VERIFY (! _dl_hwcaps_contains ("first", "first", i));
> +  TEST_VERIFY (_dl_hwcaps_contains ("first", "first", strlen ("first")));
> +  TEST_VERIFY (_dl_hwcaps_contains ("first:", "first", strlen ("first")));
> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second",
> +                                    "first", strlen ("first")));
> +  TEST_VERIFY (_dl_hwcaps_contains (":first:second", "first",
> +                                    strlen ("first")));
> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second", "second",
> +                                    strlen ("second")));
> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second:", "second",
> +                                    strlen ("second")));
> +  for (int i = 0; i < strlen ("second"); ++i)
> +    TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "sec", i));

Maybe add some tests with multiple ':'.

> +
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>
> +
> +/* Rebuild the sources here because the object file is built for
> +   inclusion into the dynamic loader.  */
> +#include "dl-hwcaps_split.c"

Ok.

> diff --git a/elf/tst-glibc-hwcaps-mask.c b/elf/tst-glibc-hwcaps-mask.c
> new file mode 100644
> index 0000000000..27b09b358c
> --- /dev/null
> +++ b/elf/tst-glibc-hwcaps-mask.c
> @@ -0,0 +1,31 @@
> +/* Test that --glibc-hwcaps-mask works.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <support/check.h>
> +
> +extern int marker1 (void);
> +
> +static int
> +do_test (void)
> +{
> +  /* The marker1 function in elf/markermod1.so returns 1.  */
> +  TEST_COMPARE (marker1 (), 1);
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>

Ok.

> diff --git a/elf/tst-glibc-hwcaps-prepend.c b/elf/tst-glibc-hwcaps-prepend.c
> new file mode 100644
> index 0000000000..57d7319f14
> --- /dev/null
> +++ b/elf/tst-glibc-hwcaps-prepend.c
> @@ -0,0 +1,32 @@
> +/* Test that --glibc-hwcaps-prepend works.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <support/check.h>
> +
> +extern int marker1 (void);
> +
> +static int
> +do_test (void)
> +{
> +  /* The marker1 function in
> +     glibc-hwcaps/prepend-markermod1/markermod1.so returns 2.  */
> +  TEST_COMPARE (marker1 (), 2);
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>

Ok.

> diff --git a/elf/tst-glibc-hwcaps.c b/elf/tst-glibc-hwcaps.c
> new file mode 100644
> index 0000000000..28f47cf891
> --- /dev/null
> +++ b/elf/tst-glibc-hwcaps.c
> @@ -0,0 +1,28 @@
> +/* Stub test for glibc-hwcaps.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <stdio.h>
> +
> +static int
> +do_test (void)
> +{
> +  puts ("info: generic tst-glibc-hwcaps (tests nothing)");
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>

Ok.

> diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
> index 382eeb9be0..0b2babc70c 100644
> --- a/sysdeps/generic/ldsodefs.h
> +++ b/sysdeps/generic/ldsodefs.h
> @@ -1047,8 +1047,13 @@ extern struct r_debug *_dl_debug_initialize (ElfW(Addr) ldbase, Lmid_t ns)
>       attribute_hidden;
>  
>  /* Initialize the basic data structure for the search paths.  SOURCE
> -   is either "LD_LIBRARY_PATH" or "--library-path".  */
> -extern void _dl_init_paths (const char *library_path, const char *source)
> +   is either "LD_LIBRARY_PATH" or "--library-path".
> +   GLIBC_HWCAPS_PREPEND adds additional glibc-hwcaps subdirectories to
> +   search.  GLIBC_HWCAPS_MASK is used to filter the built-in
> +   subdirectories if not NULL.  */
> +extern void _dl_init_paths (const char *library_path, const char *source,
> +			    const char *glibc_hwcaps_prepend,
> +			    const char *glibc_hwcaps_mask)
>    attribute_hidden;
>  
>  /* Gather the information needed to install the profiling tables and start

Ok.

> @@ -1072,9 +1077,14 @@ extern void _dl_show_auxv (void) attribute_hidden;
>  extern char *_dl_next_ld_env_entry (char ***position) attribute_hidden;
>  
>  /* Return an array with the names of the important hardware
> -   capabilities.  The length of the array is written to *SZ, and the
> -   maximum of all strings length is written to *MAX_CAPSTRLEN.  */
> -const struct r_strlenpair *_dl_important_hwcaps (size_t *sz,
> +   capabilities.  PREPEND is a colon-separated list of glibc-hwcaps
> +   directories to search first.  MASK is a colon-separated list used
> +   to filter the built-in glibc-hwcaps subdirectories.  The length of
> +   the array is written to *SZ, and the maximum of all strings length
> +   is written to *MAX_CAPSTRLEN.  */
> +const struct r_strlenpair *_dl_important_hwcaps (const char *prepend,
> +						 const char *mask,
> +						 size_t *sz,
>  						 size_t *max_capstrlen)
>    attribute_hidden;
>  
> 

Ok.
Florian Weimer Nov. 27, 2020, 7:49 p.m. UTC | #2
* Adhemerval Zanella via Libc-alpha:

> On 09/11/2020 15:40, Florian Weimer via Libc-alpha wrote:
>> This hacks non-power-set processing into _dl_important_hwcaps.
>> Once the legacy hwcaps handling goes away, the subdirectory
>> handling needs to be reworked, but it is premature to do this
>> while both approaches are still supported.
>
> Could you extend the commit message with the extra options you are adding
> on ld.so and their expected semantic? I would be good to have some
> documentation at least on the commit message.

I came up with this:

ld.so supports two new arguments, --glibc-hwcaps-prepend and
--glibc-hwcaps-mask.  Each accepts a colon-separated list of
glibc-hwcaps subdirectory names.  The prepend option adds additional 
subdirectories that are searched first, in the specified order.  The 
mask option restricts the automatically selected subdirectories to 
those listed in the option argument.

I'll also try to come up with a way to document ld.so invocations in the
manual, but this will take some time (and probably some command line
interface improvements).

>> @@ -1812,3 +1816,56 @@ $(objpfx)argv0test.out: tst-rtld-argv0.sh $(objpfx)ld.so \
>>              '$(test-wrapper-env)' '$(run_program_env)' \
>>              '$(rpath-link)' 'test-argv0' > $@; \
>>      $(evaluate-test)
>> +
>> +# Most likely search subdirectories, for each supported architecture.
>> +# Used to obtain test coverage wide test coverage.
>
> This comments sounds strange, specially the second line.

Eh, that was quite garbled.  I 

# A list containing the name of the most likely searched subdirectory
# of the glibc-hwcaps directory, for each supported architecture (in
# other words, the oldest hardware level recognized by the
# glibc-hwcaps mechanism for this architecture).  Used to obtain test
# coverage for some glibc-hwcaps tests for the widest possible range
# of systems.
glibc-hwcaps-first-subdirs-for-tests =

>> +/* Like _dl_hwcaps_split, but apply masking.  */
>> +_Bool _dl_hwcaps_split_masked (struct dl_hwcaps_split_masked *s)
>> +  attribute_hidden;
>> +
>> +/* Returns true if the colon-separated HWCAP list HWCAPS contains the
>> +   capability NAME (with length NAME_LENGTH).  If HWCAPS is NULL, the
>> +   function returns true.  */
>> +_Bool _dl_hwcaps_contains (const char *hwcaps, const char *name,
>> +                           size_t name_length) attribute_hidden;
>
> I find it confusing that an null hwcap result true while an empty
> relies on name.  Do we need that NULL to be handled special here?

NULL means there is no mask.  It denotes the universe in set terms.
The caller assumes this.  The empty set can be written as "".

>> +/* Colon-separated string of glibc-hwcaps subdirectories, without the
>> +/* Returns a bitmask that marks the last ACTIVE subdirectories in a
>> +   _dl_hwcaps_subdirs_active string (containing SUBDIRS directories in
>> +   total) as active.  Intended for use in _dl_hwcaps_subdirs_active
>> +   implementations (if a contiguous tail of the list in
>> +   _dl_hwcaps_subdirs is selected).  */
>> +static inline uint32_t
>> +_dl_hwcaps_subdirs_build_bitmask (int subdirs, int active)
>> +{
>
> Should it any assert to check if subdirs and/or active is within the
> expected range?

We discussed this before, I think.  I think GCC should warn if the
numbers are out of range because the result is undefined.  I filed GCC
PR 97424 for this.  GCC knows about the undefined nature and exploits it
(but not as agressively as Clang), but does not warn about it because
this issue occurs very often in code that is eventually optimized away
completely.

>>  /* Write a list of hwcap subdirectories to standard output.  See
>>   _dl_important_hwcaps in dl-hwcaps.c.  */
>>  static void
>> @@ -186,6 +247,10 @@ setting environment variables (which would be inherited by subprocesses).\n\
>>    --inhibit-cache       Do not use " LD_SO_CACHE "\n\
>>    --library-path PATH   use given PATH instead of content of the environment\n\
>>                          variable LD_LIBRARY_PATH\n\
>> +  --glibc-hwcaps-prepend LIST\n\
>> +                        search glibc-hwcaps subdirectories in LIST\n\
>> +  --glibc-hwcaps-mask LIST\n\
>> +                        only search built-in subdirectories if in LIST\n\
>>    --inhibit-rpath LIST  ignore RUNPATH and RPATH information in object names\n\
>>                          in LIST\n\
>>    --audit LIST          use objects named in LIST as auditors\n\
>> @@ -198,6 +263,7 @@ This program interpreter self-identifies as: " RTLD "\n\
>>  ",
>>                argv0);
>>    print_search_path_for_help (state);
>> +  print_hwcaps_subdirectories (state);
>>    print_legacy_hwcap_directories ();
>>    _exit (EXIT_SUCCESS);
>>  }
>
> Do we really need to export the new options with the '--glibc' prefix?
> The loader options are an implementation detail (same for the internal
> variable names).

I did that to match the glibc-hwcaps directory name.  We already have
LD_HWCAP_MASK, and --hwcaps-mask looks like it would be related to that.

>> +#ifndef MARKER
>> +# error MARKER not defined
>> +#endif
>
> Maybe also add a check for the VALUE?

Compilation will fail anyway if VALUE is not defined:

>> +
>> +int
>> +MARKER (void)
>> +{
>> +  return VALUE;
>> +}

>> +  /* Tests for _dl_hwcaps_contains.  */
>> +  TEST_VERIFY (_dl_hwcaps_contains (NULL, "first", strlen ("first")));
>> +  TEST_VERIFY (_dl_hwcaps_contains (NULL, "", 0));
>> +  TEST_VERIFY (! _dl_hwcaps_contains ("", "first", strlen ("first")));
>> +  TEST_VERIFY (! _dl_hwcaps_contains ("firs", "first", strlen ("first")));
>> +  TEST_VERIFY (_dl_hwcaps_contains ("firs", "first", strlen ("first") - 1));
>> +  for (int i = 0; i < strlen ("first"); ++i)
>> +    TEST_VERIFY (! _dl_hwcaps_contains ("first", "first", i));
>> +  TEST_VERIFY (_dl_hwcaps_contains ("first", "first", strlen ("first")));
>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:", "first", strlen ("first")));
>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second",
>> +                                    "first", strlen ("first")));
>> +  TEST_VERIFY (_dl_hwcaps_contains (":first:second", "first",
>> +                                    strlen ("first")));
>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second", "second",
>> +                                    strlen ("second")));
>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second:", "second",
>> +                                    strlen ("second")));
>> +  for (int i = 0; i < strlen ("second"); ++i)
>> +    TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "sec", i));
>
> Maybe add some tests with multiple ':'.

Good idea, I came up with this:

@@ -126,8 +126,17 @@ do_test (void)
                                     strlen ("second")));
   TEST_VERIFY (_dl_hwcaps_contains ("first:second:", "second",
                                     strlen ("second")));
+  TEST_VERIFY (_dl_hwcaps_contains ("first::second:", "second",
+                                    strlen ("second")));
+  TEST_VERIFY (_dl_hwcaps_contains ("first:second::", "second",
+                                    strlen ("second")));
   for (int i = 0; i < strlen ("second"); ++i)
-    TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "sec", i));
+    {
+      TEST_VERIFY (!_dl_hwcaps_contains ("first:second", "second", i));
+      TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "second", i));
+      TEST_VERIFY (!_dl_hwcaps_contains ("first:second::", "second", i));
+      TEST_VERIFY (!_dl_hwcaps_contains ("first::second", "second", i));
+    }
 
   return 0;
 }

i == 0 in the loop handles empty strings.

Thanks,
Florian
Adhemerval Zanella Nov. 30, 2020, 6:59 p.m. UTC | #3
On 27/11/2020 16:49, Florian Weimer wrote:
> * Adhemerval Zanella via Libc-alpha:
> 
>> On 09/11/2020 15:40, Florian Weimer via Libc-alpha wrote:
>>> This hacks non-power-set processing into _dl_important_hwcaps.
>>> Once the legacy hwcaps handling goes away, the subdirectory
>>> handling needs to be reworked, but it is premature to do this
>>> while both approaches are still supported.
>>
>> Could you extend the commit message with the extra options you are adding
>> on ld.so and their expected semantic? I would be good to have some
>> documentation at least on the commit message.
> 
> I came up with this:
> 
> ld.so supports two new arguments, --glibc-hwcaps-prepend and
> --glibc-hwcaps-mask.  Each accepts a colon-separated list of
> glibc-hwcaps subdirectory names.  The prepend option adds additional 
> subdirectories that are searched first, in the specified order.  The 
> mask option restricts the automatically selected subdirectories to 
> those listed in the option argument.

Looks good, I would also add an example usage (maybe based on how
tst-glibc-hwcaps-prepend-cache is organized).

> 
> I'll also try to come up with a way to document ld.so invocations in the
> manual, but this will take some time (and probably some command line
> interface improvements).

Ack.

> 
>>> @@ -1812,3 +1816,56 @@ $(objpfx)argv0test.out: tst-rtld-argv0.sh $(objpfx)ld.so \
>>>              '$(test-wrapper-env)' '$(run_program_env)' \
>>>              '$(rpath-link)' 'test-argv0' > $@; \
>>>      $(evaluate-test)
>>> +
>>> +# Most likely search subdirectories, for each supported architecture.
>>> +# Used to obtain test coverage wide test coverage.
>>
>> This comments sounds strange, specially the second line.
> 
> Eh, that was quite garbled.  I 
> 
> # A list containing the name of the most likely searched subdirectory
> # of the glibc-hwcaps directory, for each supported architecture (in
> # other words, the oldest hardware level recognized by the
> # glibc-hwcaps mechanism for this architecture).  Used to obtain test
> # coverage for some glibc-hwcaps tests for the widest possible range
> # of systems.
> glibc-hwcaps-first-subdirs-for-tests =
> 

It sounds better.

>>> +/* Like _dl_hwcaps_split, but apply masking.  */
>>> +_Bool _dl_hwcaps_split_masked (struct dl_hwcaps_split_masked *s)
>>> +  attribute_hidden;
>>> +
>>> +/* Returns true if the colon-separated HWCAP list HWCAPS contains the
>>> +   capability NAME (with length NAME_LENGTH).  If HWCAPS is NULL, the
>>> +   function returns true.  */
>>> +_Bool _dl_hwcaps_contains (const char *hwcaps, const char *name,
>>> +                           size_t name_length) attribute_hidden;
>>
>> I find it confusing that an null hwcap result true while an empty
>> relies on name.  Do we need that NULL to be handled special here?
> 
> NULL means there is no mask.  It denotes the universe in set terms.
> The caller assumes this.  The empty set can be written as "".

Alright.

> 
>>> +/* Colon-separated string of glibc-hwcaps subdirectories, without the
>>> +/* Returns a bitmask that marks the last ACTIVE subdirectories in a
>>> +   _dl_hwcaps_subdirs_active string (containing SUBDIRS directories in
>>> +   total) as active.  Intended for use in _dl_hwcaps_subdirs_active
>>> +   implementations (if a contiguous tail of the list in
>>> +   _dl_hwcaps_subdirs is selected).  */
>>> +static inline uint32_t
>>> +_dl_hwcaps_subdirs_build_bitmask (int subdirs, int active)
>>> +{
>>
>> Should it any assert to check if subdirs and/or active is within the
>> expected range?
> 
> We discussed this before, I think.  

I can't recall, but sorry if I am bring it again.

> I think GCC should warn if the
> numbers are out of range because the result is undefined.  I filed GCC
> PR 97424 for this.  GCC knows about the undefined nature and exploits it
> (but not as agressively as Clang), but does not warn about it because
> this issue occurs very often in code that is eventually optimized away
> completely.

It seems that gcc will warn with the newer options -Wanalyzer-shift-count-negative
and -Wanalyzer-shift-count-overflow, but it seems it won't be enabled as default
(it requires -fanalyzer).  It also seems that __builtin_warning is still WIP,
so it might take even longer to enable it.

I still think an assert here would improve the code, although at least
since we can get them properly integrated on glibc.

> 
>>>  /* Write a list of hwcap subdirectories to standard output.  See
>>>   _dl_important_hwcaps in dl-hwcaps.c.  */
>>>  static void
>>> @@ -186,6 +247,10 @@ setting environment variables (which would be inherited by subprocesses).\n\
>>>    --inhibit-cache       Do not use " LD_SO_CACHE "\n\
>>>    --library-path PATH   use given PATH instead of content of the environment\n\
>>>                          variable LD_LIBRARY_PATH\n\
>>> +  --glibc-hwcaps-prepend LIST\n\
>>> +                        search glibc-hwcaps subdirectories in LIST\n\
>>> +  --glibc-hwcaps-mask LIST\n\
>>> +                        only search built-in subdirectories if in LIST\n\
>>>    --inhibit-rpath LIST  ignore RUNPATH and RPATH information in object names\n\
>>>                          in LIST\n\
>>>    --audit LIST          use objects named in LIST as auditors\n\
>>> @@ -198,6 +263,7 @@ This program interpreter self-identifies as: " RTLD "\n\
>>>  ",
>>>                argv0);
>>>    print_search_path_for_help (state);
>>> +  print_hwcaps_subdirectories (state);
>>>    print_legacy_hwcap_directories ();
>>>    _exit (EXIT_SUCCESS);
>>>  }
>>
>> Do we really need to export the new options with the '--glibc' prefix?
>> The loader options are an implementation detail (same for the internal
>> variable names).
> 
> I did that to match the glibc-hwcaps directory name.  We already have
> LD_HWCAP_MASK, and --hwcaps-mask looks like it would be related to that.

Ack.

> 
>>> +#ifndef MARKER
>>> +# error MARKER not defined
>>> +#endif
>>
>> Maybe also add a check for the VALUE?
> 
> Compilation will fail anyway if VALUE is not defined:

Ack.

> 
>>> +
>>> +int
>>> +MARKER (void)
>>> +{
>>> +  return VALUE;
>>> +}
> 
>>> +  /* Tests for _dl_hwcaps_contains.  */
>>> +  TEST_VERIFY (_dl_hwcaps_contains (NULL, "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains (NULL, "", 0));
>>> +  TEST_VERIFY (! _dl_hwcaps_contains ("", "first", strlen ("first")));
>>> +  TEST_VERIFY (! _dl_hwcaps_contains ("firs", "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("firs", "first", strlen ("first") - 1));
>>> +  for (int i = 0; i < strlen ("first"); ++i)
>>> +    TEST_VERIFY (! _dl_hwcaps_contains ("first", "first", i));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first", "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:", "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second",
>>> +                                    "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains (":first:second", "first",
>>> +                                    strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second", "second",
>>> +                                    strlen ("second")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second:", "second",
>>> +                                    strlen ("second")));
>>> +  for (int i = 0; i < strlen ("second"); ++i)
>>> +    TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "sec", i));
>>
>> Maybe add some tests with multiple ':'.
> 
> Good idea, I came up with this:
> 
> @@ -126,8 +126,17 @@ do_test (void)
>                                      strlen ("second")));
>    TEST_VERIFY (_dl_hwcaps_contains ("first:second:", "second",
>                                      strlen ("second")));
> +  TEST_VERIFY (_dl_hwcaps_contains ("first::second:", "second",
> +                                    strlen ("second")));
> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second::", "second",
> +                                    strlen ("second")));
>    for (int i = 0; i < strlen ("second"); ++i)
> -    TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "sec", i));
> +    {
> +      TEST_VERIFY (!_dl_hwcaps_contains ("first:second", "second", i));
> +      TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "second", i));
> +      TEST_VERIFY (!_dl_hwcaps_contains ("first:second::", "second", i));
> +      TEST_VERIFY (!_dl_hwcaps_contains ("first::second", "second", i));
> +    }
>  
>    return 0;
>  }
> 
> i == 0 in the loop handles empty strings.

LGTM, thanks.

> 
> Thanks,
> Florian
>
Matheus Castanho Dec. 4, 2020, 12:25 p.m. UTC | #4
Hi Florian, after merging this patch builds with --disable-tunables started failing on ppc*:

In file included from dl-hwcaps_split.c:19:
./dl-hwcaps.h:45:3: error: unknown type name ‘size_t’
   size_t length;                /* Number of bytes until ':' or NUL.  */
   ^~~~~~
./dl-hwcaps.h:99:28: error: unknown type name ‘size_t’
                            size_t name_length) attribute_hidden;
                            ^~~~~~
./dl-hwcaps.h:99:28: note: ‘size_t’ is defined in header ‘<stddef.h>’; did you forget to ‘#include <stddef.h>’?
./dl-hwcaps.h:25:1:
+#include <stddef.h>
 
./dl-hwcaps.h:99:28:
                            size_t name_length) attribute_hidden;
                            ^~~~~~
In file included from dl-hwcaps-subdirs.c:19:
./dl-hwcaps.h:45:3: error: unknown type name ‘size_t’
   size_t length;                /* Number of bytes until ':' or NUL.  */
   ^~~~~~
./dl-hwcaps.h:99:28: error: unknown type name ‘size_t’
                            size_t name_length) attribute_hidden;
                            ^~~~~~
./dl-hwcaps.h:99:28: note: ‘size_t’ is defined in header ‘<stddef.h>’; did you forget to ‘#include <stddef.h>’?
./dl-hwcaps.h:25:1:
+#include <stddef.h>
 
./dl-hwcaps.h:99:28:
                            size_t name_length) attribute_hidden;
                            ^~~~~~

Including stddef.h on dl-hwcaps.h fixes the issue:

diff --git a/elf/dl-hwcaps.h b/elf/dl-hwcaps.h
index e6fcac6edc..147dc3d2a8 100644
--- a/elf/dl-hwcaps.h
+++ b/elf/dl-hwcaps.h
@@ -20,6 +20,7 @@
 #define _DL_HWCAPS_H
 
 #include <stdint.h>
+#include <stddef.h>
 
 #include <elf/dl-tunables.h>

Thanks,
Matheus Castanho

On 11/27/20 4:49 PM, Florian Weimer via Libc-alpha wrote:
> * Adhemerval Zanella via Libc-alpha:
> 
>> On 09/11/2020 15:40, Florian Weimer via Libc-alpha wrote:
>>> This hacks non-power-set processing into _dl_important_hwcaps.
>>> Once the legacy hwcaps handling goes away, the subdirectory
>>> handling needs to be reworked, but it is premature to do this
>>> while both approaches are still supported.
>>
>> Could you extend the commit message with the extra options you are adding
>> on ld.so and their expected semantic? I would be good to have some
>> documentation at least on the commit message.
> 
> I came up with this:
> 
> ld.so supports two new arguments, --glibc-hwcaps-prepend and
> --glibc-hwcaps-mask.  Each accepts a colon-separated list of
> glibc-hwcaps subdirectory names.  The prepend option adds additional 
> subdirectories that are searched first, in the specified order.  The 
> mask option restricts the automatically selected subdirectories to 
> those listed in the option argument.
> 
> I'll also try to come up with a way to document ld.so invocations in the
> manual, but this will take some time (and probably some command line
> interface improvements).
> 
>>> @@ -1812,3 +1816,56 @@ $(objpfx)argv0test.out: tst-rtld-argv0.sh $(objpfx)ld.so \
>>>              '$(test-wrapper-env)' '$(run_program_env)' \
>>>              '$(rpath-link)' 'test-argv0' > $@; \
>>>      $(evaluate-test)
>>> +
>>> +# Most likely search subdirectories, for each supported architecture.
>>> +# Used to obtain test coverage wide test coverage.
>>
>> This comments sounds strange, specially the second line.
> 
> Eh, that was quite garbled.  I 
> 
> # A list containing the name of the most likely searched subdirectory
> # of the glibc-hwcaps directory, for each supported architecture (in
> # other words, the oldest hardware level recognized by the
> # glibc-hwcaps mechanism for this architecture).  Used to obtain test
> # coverage for some glibc-hwcaps tests for the widest possible range
> # of systems.
> glibc-hwcaps-first-subdirs-for-tests =
> 
>>> +/* Like _dl_hwcaps_split, but apply masking.  */
>>> +_Bool _dl_hwcaps_split_masked (struct dl_hwcaps_split_masked *s)
>>> +  attribute_hidden;
>>> +
>>> +/* Returns true if the colon-separated HWCAP list HWCAPS contains the
>>> +   capability NAME (with length NAME_LENGTH).  If HWCAPS is NULL, the
>>> +   function returns true.  */
>>> +_Bool _dl_hwcaps_contains (const char *hwcaps, const char *name,
>>> +                           size_t name_length) attribute_hidden;
>>
>> I find it confusing that an null hwcap result true while an empty
>> relies on name.  Do we need that NULL to be handled special here?
> 
> NULL means there is no mask.  It denotes the universe in set terms.
> The caller assumes this.  The empty set can be written as "".
> 
>>> +/* Colon-separated string of glibc-hwcaps subdirectories, without the
>>> +/* Returns a bitmask that marks the last ACTIVE subdirectories in a
>>> +   _dl_hwcaps_subdirs_active string (containing SUBDIRS directories in
>>> +   total) as active.  Intended for use in _dl_hwcaps_subdirs_active
>>> +   implementations (if a contiguous tail of the list in
>>> +   _dl_hwcaps_subdirs is selected).  */
>>> +static inline uint32_t
>>> +_dl_hwcaps_subdirs_build_bitmask (int subdirs, int active)
>>> +{
>>
>> Should it any assert to check if subdirs and/or active is within the
>> expected range?
> 
> We discussed this before, I think.  I think GCC should warn if the
> numbers are out of range because the result is undefined.  I filed GCC
> PR 97424 for this.  GCC knows about the undefined nature and exploits it
> (but not as agressively as Clang), but does not warn about it because
> this issue occurs very often in code that is eventually optimized away
> completely.
> 
>>>  /* Write a list of hwcap subdirectories to standard output.  See
>>>   _dl_important_hwcaps in dl-hwcaps.c.  */
>>>  static void
>>> @@ -186,6 +247,10 @@ setting environment variables (which would be inherited by subprocesses).\n\
>>>    --inhibit-cache       Do not use " LD_SO_CACHE "\n\
>>>    --library-path PATH   use given PATH instead of content of the environment\n\
>>>                          variable LD_LIBRARY_PATH\n\
>>> +  --glibc-hwcaps-prepend LIST\n\
>>> +                        search glibc-hwcaps subdirectories in LIST\n\
>>> +  --glibc-hwcaps-mask LIST\n\
>>> +                        only search built-in subdirectories if in LIST\n\
>>>    --inhibit-rpath LIST  ignore RUNPATH and RPATH information in object names\n\
>>>                          in LIST\n\
>>>    --audit LIST          use objects named in LIST as auditors\n\
>>> @@ -198,6 +263,7 @@ This program interpreter self-identifies as: " RTLD "\n\
>>>  ",
>>>                argv0);
>>>    print_search_path_for_help (state);
>>> +  print_hwcaps_subdirectories (state);
>>>    print_legacy_hwcap_directories ();
>>>    _exit (EXIT_SUCCESS);
>>>  }
>>
>> Do we really need to export the new options with the '--glibc' prefix?
>> The loader options are an implementation detail (same for the internal
>> variable names).
> 
> I did that to match the glibc-hwcaps directory name.  We already have
> LD_HWCAP_MASK, and --hwcaps-mask looks like it would be related to that.
> 
>>> +#ifndef MARKER
>>> +# error MARKER not defined
>>> +#endif
>>
>> Maybe also add a check for the VALUE?
> 
> Compilation will fail anyway if VALUE is not defined:
> 
>>> +
>>> +int
>>> +MARKER (void)
>>> +{
>>> +  return VALUE;
>>> +}
> 
>>> +  /* Tests for _dl_hwcaps_contains.  */
>>> +  TEST_VERIFY (_dl_hwcaps_contains (NULL, "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains (NULL, "", 0));
>>> +  TEST_VERIFY (! _dl_hwcaps_contains ("", "first", strlen ("first")));
>>> +  TEST_VERIFY (! _dl_hwcaps_contains ("firs", "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("firs", "first", strlen ("first") - 1));
>>> +  for (int i = 0; i < strlen ("first"); ++i)
>>> +    TEST_VERIFY (! _dl_hwcaps_contains ("first", "first", i));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first", "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:", "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second",
>>> +                                    "first", strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains (":first:second", "first",
>>> +                                    strlen ("first")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second", "second",
>>> +                                    strlen ("second")));
>>> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second:", "second",
>>> +                                    strlen ("second")));
>>> +  for (int i = 0; i < strlen ("second"); ++i)
>>> +    TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "sec", i));
>>
>> Maybe add some tests with multiple ':'.
> 
> Good idea, I came up with this:
> 
> @@ -126,8 +126,17 @@ do_test (void)
>                                      strlen ("second")));
>    TEST_VERIFY (_dl_hwcaps_contains ("first:second:", "second",
>                                      strlen ("second")));
> +  TEST_VERIFY (_dl_hwcaps_contains ("first::second:", "second",
> +                                    strlen ("second")));
> +  TEST_VERIFY (_dl_hwcaps_contains ("first:second::", "second",
> +                                    strlen ("second")));
>    for (int i = 0; i < strlen ("second"); ++i)
> -    TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "sec", i));
> +    {
> +      TEST_VERIFY (!_dl_hwcaps_contains ("first:second", "second", i));
> +      TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "second", i));
> +      TEST_VERIFY (!_dl_hwcaps_contains ("first:second::", "second", i));
> +      TEST_VERIFY (!_dl_hwcaps_contains ("first::second", "second", i));
> +    }
>  
>    return 0;
>  }
> 
> i == 0 in the loop handles empty strings.
> 
> Thanks,
> Florian
>
Florian Weimer Dec. 4, 2020, 12:43 p.m. UTC | #5
* Matheus Castanho:

> Including stddef.h on dl-hwcaps.h fixes the issue:
>
> diff --git a/elf/dl-hwcaps.h b/elf/dl-hwcaps.h
> index e6fcac6edc..147dc3d2a8 100644
> --- a/elf/dl-hwcaps.h
> +++ b/elf/dl-hwcaps.h
> @@ -20,6 +20,7 @@
>  #define _DL_HWCAPS_H
>  
>  #include <stdint.h>
> +#include <stddef.h>
>  
>  #include <elf/dl-tunables.h>

Sorry for that.  Fix looks good to me.  Do you want me to push the fix,
or do you want to do it yourself?

Thanks,
Florian
Matheus Castanho Dec. 4, 2020, 12:59 p.m. UTC | #6
On 12/4/20 9:43 AM, Florian Weimer wrote:
> * Matheus Castanho:
> 
>> Including stddef.h on dl-hwcaps.h fixes the issue:
>>
>> diff --git a/elf/dl-hwcaps.h b/elf/dl-hwcaps.h
>> index e6fcac6edc..147dc3d2a8 100644
>> --- a/elf/dl-hwcaps.h
>> +++ b/elf/dl-hwcaps.h
>> @@ -20,6 +20,7 @@
>>  #define _DL_HWCAPS_H
>>  
>>  #include <stdint.h>
>> +#include <stddef.h>
>>  
>>  #include <elf/dl-tunables.h>
> 
> Sorry for that.  Fix looks good to me.  Do you want me to push the fix,
> or do you want to do it yourself?
> 
> Thanks,
> Florian
> 

No problem. I've pushed it already: 4f4bd9e47ba98ccfeeaa8c600c0b0c8bbabcebb3

Thanks,
Matheus Castanho
diff mbox series

Patch

diff --git a/elf/Makefile b/elf/Makefile
index f10cc59e7c..d2f7e99863 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -59,7 +59,8 @@  elide-routines.os = $(all-dl-routines) dl-support enbl-secure dl-origin \
 # ld.so uses those routines, plus some special stuff for being the program
 # interpreter and operating independent of libc.
 rtld-routines	= rtld $(all-dl-routines) dl-sysdep dl-environ dl-minimal \
-  dl-error-minimal dl-conflict dl-hwcaps dl-usage
+  dl-error-minimal dl-conflict dl-hwcaps dl-hwcaps_split dl-hwcaps-subdirs \
+  dl-usage
 all-rtld-routines = $(rtld-routines) $(sysdep-rtld-routines)
 
 CFLAGS-dl-runtime.c += -fexceptions -fasynchronous-unwind-tables
@@ -210,14 +211,14 @@  tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 tst-filterobj tst-filterobj-dlopen tst-auxobj tst-auxobj-dlopen \
 	 tst-audit14 tst-audit15 tst-audit16 \
 	 tst-single_threaded tst-single_threaded-pthread \
-	 tst-tls-ie tst-tls-ie-dlmopen \
-	 argv0test
+	 tst-tls-ie tst-tls-ie-dlmopen argv0test \
+	 tst-glibc-hwcaps tst-glibc-hwcaps-prepend tst-glibc-hwcaps-mask
 #	 reldep9
 tests-internal += loadtest unload unload2 circleload1 \
 	 neededtest neededtest2 neededtest3 neededtest4 \
 	 tst-tls3 tst-tls6 tst-tls7 tst-tls8 tst-dlmopen2 \
 	 tst-ptrguard1 tst-stackguard1 tst-libc_dlvsym \
-	 tst-create_format1 tst-tls-surplus
+	 tst-create_format1 tst-tls-surplus tst-dl-hwcaps_split
 tests-container += tst-pldd tst-dlopen-tlsmodid-container \
   tst-dlopen-self-container
 test-srcs = tst-pathopt
@@ -329,7 +330,10 @@  modules-names = testobj1 testobj2 testobj3 testobj4 testobj5 testobj6 \
 		tst-single_threaded-mod3 tst-single_threaded-mod4 \
 		tst-tls-ie-mod0 tst-tls-ie-mod1 tst-tls-ie-mod2 \
 		tst-tls-ie-mod3 tst-tls-ie-mod4 tst-tls-ie-mod5 \
-		tst-tls-ie-mod6
+		tst-tls-ie-mod6 libmarkermod1-1 libmarkermod1-2 libmarkermod1-3 \
+		libmarkermod2-1 libmarkermod2-2 \
+		libmarkermod3-1 libmarkermod3-2 libmarkermod3-3 \
+		libmarkermod4-1 libmarkermod4-2 libmarkermod4-3 libmarkermod4-4 \
 
 # Most modules build with _ISOMAC defined, but those filtered out
 # depend on internal headers.
@@ -1812,3 +1816,56 @@  $(objpfx)argv0test.out: tst-rtld-argv0.sh $(objpfx)ld.so \
             '$(test-wrapper-env)' '$(run_program_env)' \
             '$(rpath-link)' 'test-argv0' > $@; \
     $(evaluate-test)
+
+# Most likely search subdirectories, for each supported architecture.
+# Used to obtain test coverage wide test coverage.
+glibc-hwcaps-first-subdirs-for-tests =
+
+# The test modules are parameterized by preprocessor macros.
+LDFLAGS-libmarkermod1-1.so += -Wl,-soname,libmarkermod1.so
+LDFLAGS-libmarkermod2-1.so += -Wl,-soname,libmarkermod2.so
+LDFLAGS-libmarkermod3-1.so += -Wl,-soname,libmarkermod3.so
+LDFLAGS-libmarkermod4-1.so += -Wl,-soname,libmarkermod4.so
+$(objpfx)libmarkermod%.os : markermodMARKER-VALUE.c
+	$(compile-command.c) \
+	  -DMARKER=marker$(firstword $(subst -, ,$*)) \
+	  -DVALUE=$(lastword $(subst -, ,$*))
+$(objpfx)libmarkermod1.so: $(objpfx)libmarkermod1-1.so
+	cp $< $@
+$(objpfx)libmarkermod2.so: $(objpfx)libmarkermod2-1.so
+	cp $< $@
+$(objpfx)libmarkermod3.so: $(objpfx)libmarkermod3-1.so
+	cp $< $@
+$(objpfx)libmarkermod4.so: $(objpfx)libmarkermod4-1.so
+	cp $< $@
+
+# tst-glibc-hwcaps-prepend checks that --glibc-hwcaps-prepend is
+# preferred over auto-detected subdirectories.
+$(objpfx)tst-glibc-hwcaps-prepend: $(objpfx)libmarkermod1-1.so
+$(objpfx)glibc-hwcaps/prepend-markermod1/libmarkermod1.so: \
+  $(objpfx)libmarkermod1-2.so
+	$(make-target-directory)
+	cp $< $@
+$(objpfx)glibc-hwcaps/%/libmarkermod1.so: $(objpfx)libmarkermod1-3.so
+	$(make-target-directory)
+	cp $< $@
+$(objpfx)tst-glibc-hwcaps-prepend.out: \
+  $(objpfx)tst-glibc-hwcaps-prepend $(objpfx)libmarkermod1.so \
+  $(patsubst %,$(objpfx)glibc-hwcaps/%/libmarkermod1.so,prepend-markermod1 \
+    $(glibc-hwcaps-first-subdirs-for-tests))
+	$(test-wrapper) $(rtld-prefix) \
+	  --glibc-hwcaps-prepend prepend-markermod1 \
+	  $< > $@; \
+	$(evaluate-test)
+
+# tst-glibc-hwcaps-mask checks that --glibc-hwcaps-mask can be used to
+# suppress all auto-detected subdirectories.
+$(objpfx)tst-glibc-hwcaps-mask: $(objpfx)libmarkermod1-1.so
+$(objpfx)tst-glibc-hwcaps-mask.out: \
+  $(objpfx)tst-glibc-hwcaps-mask $(objpfx)libmarkermod1.so \
+  $(patsubst %,$(objpfx)glibc-hwcaps/%/libmarkermod1.so,\
+    $(glibc-hwcaps-first-subdirs-for-tests))
+	$(test-wrapper) $(rtld-prefix) \
+	  --glibc-hwcaps-mask does-not-exist \
+	  $< > $@; \
+	$(evaluate-test)
diff --git a/elf/dl-hwcaps-subdirs.c b/elf/dl-hwcaps-subdirs.c
new file mode 100644
index 0000000000..60c6d59731
--- /dev/null
+++ b/elf/dl-hwcaps-subdirs.c
@@ -0,0 +1,29 @@ 
+/* Architecture-specific glibc-hwcaps subdirectories.  Generic version.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <dl-hwcaps.h>
+
+/* In the generic version, there are no subdirectories defined.  */
+
+const char _dl_hwcaps_subdirs[] = "";
+
+uint32_t
+_dl_hwcaps_subdirs_active (void)
+{
+  return 0;
+}
diff --git a/elf/dl-hwcaps.c b/elf/dl-hwcaps.c
index 44dbac099f..f611f3a1a6 100644
--- a/elf/dl-hwcaps.c
+++ b/elf/dl-hwcaps.c
@@ -26,20 +26,97 @@ 
 #include <dl-procinfo.h>
 #include <dl-hwcaps.h>
 
+/* This is the result of counting the substrings in a colon-separated
+   hwcaps string.  */
+struct hwcaps_counts
+{
+  /* Number of substrings.  */
+  size_t count;
+
+  /* Sum of the individual substring lengths (without separators or
+     null terminators).  */
+  size_t total_length;
+
+  /* Maximum length of an individual substring.  */
+  size_t maximum_length;
+};
+
+/* Update *COUNTS according to the contents of HWCAPS.  Skip over
+   entries whose bit is not set in MASK.  */
+static void
+update_hwcaps_counts (struct hwcaps_counts *counts, const char *hwcaps,
+		      uint32_t bitmask, const char *mask)
+{
+  struct dl_hwcaps_split_masked sp;
+  _dl_hwcaps_split_masked_init (&sp, hwcaps, bitmask, mask);
+  while (_dl_hwcaps_split_masked (&sp))
+    {
+      ++counts->count;
+      counts->total_length += sp.split.length;
+      if (sp.split.length > counts->maximum_length)
+	counts->maximum_length = sp.split.length;
+    }
+}
+
+/* State for copy_hwcaps.  Must be initialized to point to
+   the storage areas for the array and the strings themselves.  */
+struct copy_hwcaps
+{
+  struct r_strlenpair *next_pair;
+  char *next_string;
+};
+
+/* Copy HWCAPS into the string pairs and strings, advancing *TARGET.
+   Skip over entries whose bit is not set in MASK.  */
+static void
+copy_hwcaps (struct copy_hwcaps *target, const char *hwcaps,
+	     uint32_t bitmask, const char *mask)
+{
+  struct dl_hwcaps_split_masked sp;
+  _dl_hwcaps_split_masked_init (&sp, hwcaps, bitmask, mask);
+  while (_dl_hwcaps_split_masked (&sp))
+    {
+      target->next_pair->str = target->next_string;
+      char *slash = __mempcpy (__mempcpy (target->next_string,
+					  GLIBC_HWCAPS_PREFIX,
+					  strlen (GLIBC_HWCAPS_PREFIX)),
+			       sp.split.segment, sp.split.length);
+      *slash = '/';
+      target->next_pair->len
+	= strlen (GLIBC_HWCAPS_PREFIX) + sp.split.length + 1;
+      ++target->next_pair;
+      target->next_string = slash + 1;
+    }
+}
+
 /* Return an array of useful/necessary hardware capability names.  */
 const struct r_strlenpair *
-_dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
+_dl_important_hwcaps (const char *glibc_hwcaps_prepend,
+		      const char *glibc_hwcaps_mask,
+		      size_t *sz, size_t *max_capstrlen)
 {
   uint64_t hwcap_mask = GET_HWCAP_MASK();
   /* Determine how many important bits are set.  */
   uint64_t masked = GLRO(dl_hwcap) & hwcap_mask;
   size_t cnt = GLRO (dl_platform) != NULL;
   size_t n, m;
-  size_t total;
   struct r_strlenpair *result;
   struct r_strlenpair *rp;
   char *cp;
 
+  /* glibc-hwcaps subdirectories.  These are exempted from the power
+     set construction below.  */
+  uint32_t hwcaps_subdirs_active = _dl_hwcaps_subdirs_active ();
+  struct hwcaps_counts hwcaps_counts =  { 0, };
+  update_hwcaps_counts (&hwcaps_counts, glibc_hwcaps_prepend, -1, NULL);
+  update_hwcaps_counts (&hwcaps_counts, _dl_hwcaps_subdirs,
+			hwcaps_subdirs_active, glibc_hwcaps_mask);
+
+  /* Each hwcaps subdirectory has a GLIBC_HWCAPS_PREFIX string prefix
+     and a "/" suffix once stored in the result.  */
+  size_t total = (hwcaps_counts.count * (strlen (GLIBC_HWCAPS_PREFIX) + 1)
+		  + hwcaps_counts.total_length);
+
   /* Count the number of bits set in the masked value.  */
   for (n = 0; (~((1ULL << n) - 1) & masked) != 0; ++n)
     if ((masked & (1ULL << n)) != 0)
@@ -74,10 +151,10 @@  _dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
 
   /* Determine the total size of all strings together.  */
   if (cnt == 1)
-    total = temp[0].len + 1;
+    total += temp[0].len + 1;
   else
     {
-      total = temp[0].len + temp[cnt - 1].len + 2;
+      total += temp[0].len + temp[cnt - 1].len + 2;
       if (cnt > 2)
 	{
 	  total <<= 1;
@@ -94,26 +171,48 @@  _dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
 	}
     }
 
-  /* The result structure: we use a very compressed way to store the
-     various combinations of capability names.  */
-  *sz = 1 << cnt;
-  result = (struct r_strlenpair *) malloc (*sz * sizeof (*result) + total);
-  if (result == NULL)
+  *sz = hwcaps_counts.count + (1 << cnt);
+
+  /* This is the overall result, including both glibc-hwcaps
+     subdirectories and the legacy hwcaps subdirectories using the
+     power set construction.  */
+  struct r_strlenpair *overall_result
+    = malloc (*sz * sizeof (*result) + total);
+  if (overall_result == NULL)
     _dl_signal_error (ENOMEM, NULL, NULL,
 		      N_("cannot create capability list"));
 
+  /* Fill in the glibc-hwcaps subdirectories.  */
+  {
+    struct copy_hwcaps target;
+    target.next_pair = overall_result;
+    target.next_string = (char *) (overall_result + *sz);
+    copy_hwcaps (&target, glibc_hwcaps_prepend, -1, NULL);
+    copy_hwcaps (&target, _dl_hwcaps_subdirs,
+		 hwcaps_subdirs_active, glibc_hwcaps_mask);
+    /* Set up the write target for the power set construction.  */
+    result = target.next_pair;
+    cp = target.next_string;
+  }
+
+
+  /* Power set construction begins here.  We use a very compressed way
+     to store the various combinations of capability names.  */
+
   if (cnt == 1)
     {
-      result[0].str = (char *) (result + *sz);
+      result[0].str = cp;
       result[0].len = temp[0].len + 1;
-      result[1].str = (char *) (result + *sz);
+      result[1].str = cp;
       result[1].len = 0;
-      cp = __mempcpy ((char *) (result + *sz), temp[0].str, temp[0].len);
+      cp = __mempcpy (cp, temp[0].str, temp[0].len);
       *cp = '/';
-      *sz = 2;
-      *max_capstrlen = result[0].len;
+      if (result[0].len > hwcaps_counts.maximum_length)
+	*max_capstrlen = result[0].len;
+      else
+	*max_capstrlen = hwcaps_counts.maximum_length;
 
-      return result;
+      return overall_result;
     }
 
   /* Fill in the information.  This follows the following scheme
@@ -124,7 +223,7 @@  _dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
 	      #3: 0, 3			1001
      This allows the representation of all possible combinations of
      capability names in the string.  First generate the strings.  */
-  result[1].str = result[0].str = cp = (char *) (result + *sz);
+  result[1].str = result[0].str = cp;
 #define add(idx) \
       cp = __mempcpy (__mempcpy (cp, temp[idx].str, temp[idx].len), "/", 1);
   if (cnt == 2)
@@ -191,7 +290,10 @@  _dl_important_hwcaps (size_t *sz, size_t *max_capstrlen)
   while (--n != 0);
 
   /* The maximum string length.  */
-  *max_capstrlen = result[0].len;
+  if (result[0].len > hwcaps_counts.maximum_length)
+    *max_capstrlen = result[0].len;
+  else
+    *max_capstrlen = hwcaps_counts.maximum_length;
 
-  return result;
+  return overall_result;
 }
diff --git a/elf/dl-hwcaps.h b/elf/dl-hwcaps.h
index b66da59b89..ab39d8a46d 100644
--- a/elf/dl-hwcaps.h
+++ b/elf/dl-hwcaps.h
@@ -16,6 +16,11 @@ 
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#ifndef _DL_HWCAPS_H
+#define _DL_HWCAPS_H
+
+#include <stdint.h>
+
 #include <elf/dl-tunables.h>
 
 #if HAVE_TUNABLES
@@ -28,3 +33,103 @@ 
 #  define GET_HWCAP_MASK() (0)
 # endif
 #endif
+
+#define GLIBC_HWCAPS_SUBDIRECTORY "glibc-hwcaps"
+#define GLIBC_HWCAPS_PREFIX GLIBC_HWCAPS_SUBDIRECTORY "/"
+
+/* Used by _dl_hwcaps_split below, to split strings at ':'
+   separators.  */
+struct dl_hwcaps_split
+{
+  const char *segment;          /* Start of the current segment.  */
+  size_t length;                /* Number of bytes until ':' or NUL.  */
+};
+
+/* Prepare *S to parse SUBJECT, for future _dl_hwcaps_split calls.  If
+   SUBJECT is NULL, it is treated as the empty string.  */
+static inline void
+_dl_hwcaps_split_init (struct dl_hwcaps_split *s, const char *subject)
+{
+  s->segment = subject;
+  /* The initial call to _dl_hwcaps_split will not skip anything.  */
+  s->length = 0;
+}
+
+/* Extract the next non-empty string segment, up to ':' or the null
+   terminator.  Return true if one more segment was found, or false if
+   the end of the string was reached.  On success, S->segment is the
+   start of the segment found, and S->length is its length.
+   (Typically, S->segment[S->length] is not null.)  */
+_Bool _dl_hwcaps_split (struct dl_hwcaps_split *s) attribute_hidden;
+
+/* Similar to dl_hwcaps_split, but with bit-based and name-based
+   masking.  */
+struct dl_hwcaps_split_masked
+{
+  struct dl_hwcaps_split split;
+
+  /* For used by the iterator implementation.  */
+  const char *mask;
+  uint32_t bitmask;
+};
+
+/* Prepare *S for iteration with _dl_hwcaps_split_masked.  Only HWCAP
+   names in SUBJECT whose bit is set in BITMASK and whose name is in
+   MASK will be returned.  SUBJECT must not contain empty HWCAP names.
+   If MASK is NULL, no name-based masking is applied.  Likewise for
+   BITMASK if BITMASK is -1 (infinite number of bits).  */
+static inline void
+_dl_hwcaps_split_masked_init (struct dl_hwcaps_split_masked *s,
+                              const char *subject,
+                              uint32_t bitmask, const char *mask)
+{
+  _dl_hwcaps_split_init (&s->split, subject);
+  s->bitmask = bitmask;
+  s->mask = mask;
+}
+
+/* Like _dl_hwcaps_split, but apply masking.  */
+_Bool _dl_hwcaps_split_masked (struct dl_hwcaps_split_masked *s)
+  attribute_hidden;
+
+/* Returns true if the colon-separated HWCAP list HWCAPS contains the
+   capability NAME (with length NAME_LENGTH).  If HWCAPS is NULL, the
+   function returns true.  */
+_Bool _dl_hwcaps_contains (const char *hwcaps, const char *name,
+                           size_t name_length) attribute_hidden;
+
+/* Colon-separated string of glibc-hwcaps subdirectories, without the
+   "glibc-hwcaps/" prefix.  The most preferred subdirectory needs to
+   be listed first.  Up to 32 subdirectories are supported, limited by
+   the width of the uint32_t mask.  */
+extern const char _dl_hwcaps_subdirs[] attribute_hidden;
+
+/* Returns a bitmap of active subdirectories in _dl_hwcaps_subdirs.
+   Bit 0 (the LSB) corresponds to the first substring in
+   _dl_hwcaps_subdirs, bit 1 to the second substring, and so on.
+   There is no direct correspondence between HWCAP bitmasks and this
+   bitmask.  */
+uint32_t _dl_hwcaps_subdirs_active (void) attribute_hidden;
+
+/* Returns a bitmask that marks the last ACTIVE subdirectories in a
+   _dl_hwcaps_subdirs_active string (containing SUBDIRS directories in
+   total) as active.  Intended for use in _dl_hwcaps_subdirs_active
+   implementations (if a contiguous tail of the list in
+   _dl_hwcaps_subdirs is selected).  */
+static inline uint32_t
+_dl_hwcaps_subdirs_build_bitmask (int subdirs, int active)
+{
+  /* Leading subdirectories that are not active.  */
+  int inactive = subdirs - active;
+  if (inactive == 32)
+    return 0;
+
+  uint32_t mask;
+  if (subdirs != 32)
+    mask = (1U << subdirs) - 1;
+  else
+    mask = -1;
+  return mask ^ ((1U << inactive) - 1);
+}
+
+#endif /* _DL_HWCAPS_H */
diff --git a/elf/dl-hwcaps_split.c b/elf/dl-hwcaps_split.c
new file mode 100644
index 0000000000..95225e9f40
--- /dev/null
+++ b/elf/dl-hwcaps_split.c
@@ -0,0 +1,77 @@ 
+/* Hardware capability support for run-time dynamic loader.  String splitting.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <dl-hwcaps.h>
+#include <stdbool.h>
+#include <string.h>
+
+_Bool
+_dl_hwcaps_split (struct dl_hwcaps_split *s)
+{
+  if (s->segment == NULL)
+    return false;
+
+  /* Skip over the previous segment.   */
+  s->segment += s->length;
+
+  /* Consume delimiters.  This also avoids returning an empty
+     segment.  */
+  while (*s->segment == ':')
+    ++s->segment;
+  if (*s->segment == '\0')
+    return false;
+
+  /* This could use strchrnul, but we would have to link the function
+     into ld.so for that.  */
+  const char *colon = strchr (s->segment, ':');
+  if (colon == NULL)
+    s->length = strlen (s->segment);
+  else
+    s->length = colon - s->segment;
+  return true;
+}
+
+_Bool
+_dl_hwcaps_split_masked (struct dl_hwcaps_split_masked *s)
+{
+  while (true)
+    {
+      if (!_dl_hwcaps_split (&s->split))
+        return false;
+      bool active = s->bitmask & 1;
+      s->bitmask >>= 1;
+      if (active && _dl_hwcaps_contains (s->mask,
+                                         s->split.segment, s->split.length))
+        return true;
+    }
+}
+
+_Bool
+_dl_hwcaps_contains (const char *hwcaps, const char *name, size_t name_length)
+{
+  if (hwcaps == NULL)
+    return true;
+
+  struct dl_hwcaps_split split;
+  _dl_hwcaps_split_init (&split, hwcaps);
+  while (_dl_hwcaps_split (&split))
+    if (split.length == name_length
+        && memcmp (split.segment, name, name_length) == 0)
+      return true;
+  return false;
+}
diff --git a/elf/dl-load.c b/elf/dl-load.c
index f3201e7c14..9020f1646f 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -682,7 +682,9 @@  cache_rpath (struct link_map *l,
 
 
 void
-_dl_init_paths (const char *llp, const char *source)
+_dl_init_paths (const char *llp, const char *source,
+		const char *glibc_hwcaps_prepend,
+		const char *glibc_hwcaps_mask)
 {
   size_t idx;
   const char *strp;
@@ -697,7 +699,8 @@  _dl_init_paths (const char *llp, const char *source)
 
 #ifdef SHARED
   /* Get the capabilities.  */
-  capstr = _dl_important_hwcaps (&ncapstr, &max_capstrlen);
+  capstr = _dl_important_hwcaps (glibc_hwcaps_prepend, glibc_hwcaps_mask,
+				 &ncapstr, &max_capstrlen);
 #endif
 
   /* First set up the rest of the default search directory entries.  */
diff --git a/elf/dl-main.h b/elf/dl-main.h
index b51256d3b4..566713a0d1 100644
--- a/elf/dl-main.h
+++ b/elf/dl-main.h
@@ -84,6 +84,14 @@  struct dl_main_state
   /* The preload list passed as a command argument.  */
   const char *preloadarg;
 
+  /* Additional glibc-hwcaps subdirectories to search first.
+     Colon-separated list.  */
+  const char *glibc_hwcaps_prepend;
+
+  /* Mask for the internal glibc-hwcaps subdirectories.
+     Colon-separated list.  */
+  const char *glibc_hwcaps_mask;
+
   enum rtld_mode mode;
 
   /* True if any of the debugging options is enabled.  */
@@ -98,7 +106,8 @@  struct dl_main_state
 static inline void
 call_init_paths (const struct dl_main_state *state)
 {
-  _dl_init_paths (state->library_path, state->library_path_source);
+  _dl_init_paths (state->library_path, state->library_path_source,
+                  state->glibc_hwcaps_prepend, state->glibc_hwcaps_mask);
 }
 
 /* Print ld.so usage information and exit.  */
diff --git a/elf/dl-support.c b/elf/dl-support.c
index afbc94df54..3264262f4e 100644
--- a/elf/dl-support.c
+++ b/elf/dl-support.c
@@ -323,7 +323,10 @@  _dl_non_dynamic_init (void)
 
   /* Initialize the data structures for the search paths for shared
      objects.  */
-  _dl_init_paths (getenv ("LD_LIBRARY_PATH"), "LD_LIBRARY_PATH");
+  _dl_init_paths (getenv ("LD_LIBRARY_PATH"), "LD_LIBRARY_PATH",
+		  /* No glibc-hwcaps selection support in statically
+		     linked binaries.  */
+		  NULL, NULL);
 
   /* Remember the last search directory added at startup.  */
   _dl_init_all_dirs = GL(dl_all_dirs);
diff --git a/elf/dl-usage.c b/elf/dl-usage.c
index 796ad38b43..e22a9c3942 100644
--- a/elf/dl-usage.c
+++ b/elf/dl-usage.c
@@ -83,7 +83,7 @@  print_search_path_for_help (struct dl_main_state *state)
 {
   if (__rtld_search_dirs.dirs == NULL)
     /* The run-time search paths have not yet been initialized.  */
-    _dl_init_paths (state->library_path, state->library_path_source);
+    call_init_paths (state);
 
   _dl_printf ("\nShared library search path:\n");
 
@@ -132,6 +132,67 @@  print_hwcap_1_finish (bool *first)
     _dl_printf (")\n");
 }
 
+/* Print the header for print_hwcaps_subdirectories.  */
+static void
+print_hwcaps_subdirectories_header (bool *nothing_printed)
+{
+  if (*nothing_printed)
+    {
+      _dl_printf ("\n\
+Subdirectories of glibc-hwcaps directories, in priority order:\n");
+      *nothing_printed = false;
+    }
+}
+
+/* Print the HWCAP name itself, indented.  */
+static void
+print_hwcaps_subdirectories_name (const struct dl_hwcaps_split *split)
+{
+  _dl_write (STDOUT_FILENO, "  ", 2);
+  _dl_write (STDOUT_FILENO, split->segment, split->length);
+}
+
+/* Print the list of recognized glibc-hwcaps subdirectories.  */
+static void
+print_hwcaps_subdirectories (const struct dl_main_state *state)
+{
+  bool nothing_printed = true;
+  struct dl_hwcaps_split split;
+
+  /* The prepended glibc-hwcaps subdirectories.  */
+  _dl_hwcaps_split_init (&split, state->glibc_hwcaps_prepend);
+  while (_dl_hwcaps_split (&split))
+    {
+      print_hwcaps_subdirectories_header (&nothing_printed);
+      print_hwcaps_subdirectories_name (&split);
+      bool first = true;
+      print_hwcap_1 (&first, true, "searched");
+      print_hwcap_1_finish (&first);
+    }
+
+  /* The built-in glibc-hwcaps subdirectories.  Do the filtering
+     manually, so that more precise diagnostics are possible.  */
+  uint32_t mask = _dl_hwcaps_subdirs_active ();
+  _dl_hwcaps_split_init (&split, _dl_hwcaps_subdirs);
+  while (_dl_hwcaps_split (&split))
+    {
+      print_hwcaps_subdirectories_header (&nothing_printed);
+      print_hwcaps_subdirectories_name (&split);
+      bool first = true;
+      print_hwcap_1 (&first, mask & 1, "supported");
+      bool listed = _dl_hwcaps_contains (state->glibc_hwcaps_mask,
+                                         split.segment, split.length);
+      print_hwcap_1 (&first, !listed, "masked");
+      print_hwcap_1 (&first, (mask & 1) && listed, "searched");
+      print_hwcap_1_finish (&first);
+      mask >>= 1;
+    }
+
+  if (nothing_printed)
+    _dl_printf ("\n\
+No subdirectories of glibc-hwcaps directories are searched.\n");
+}
+
 /* Write a list of hwcap subdirectories to standard output.  See
  _dl_important_hwcaps in dl-hwcaps.c.  */
 static void
@@ -186,6 +247,10 @@  setting environment variables (which would be inherited by subprocesses).\n\
   --inhibit-cache       Do not use " LD_SO_CACHE "\n\
   --library-path PATH   use given PATH instead of content of the environment\n\
                         variable LD_LIBRARY_PATH\n\
+  --glibc-hwcaps-prepend LIST\n\
+                        search glibc-hwcaps subdirectories in LIST\n\
+  --glibc-hwcaps-mask LIST\n\
+                        only search built-in subdirectories if in LIST\n\
   --inhibit-rpath LIST  ignore RUNPATH and RPATH information in object names\n\
                         in LIST\n\
   --audit LIST          use objects named in LIST as auditors\n\
@@ -198,6 +263,7 @@  This program interpreter self-identifies as: " RTLD "\n\
 ",
               argv0);
   print_search_path_for_help (state);
+  print_hwcaps_subdirectories (state);
   print_legacy_hwcap_directories ();
   _exit (EXIT_SUCCESS);
 }
diff --git a/elf/markermodMARKER-VALUE.c b/elf/markermodMARKER-VALUE.c
new file mode 100644
index 0000000000..99bdcf71a4
--- /dev/null
+++ b/elf/markermodMARKER-VALUE.c
@@ -0,0 +1,29 @@ 
+/* Source file template for building shared objects with marker functions.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* MARKER and VALUE must be set on the compiler command line.  */
+
+#ifndef MARKER
+# error MARKER not defined
+#endif
+
+int
+MARKER (void)
+{
+  return VALUE;
+}
diff --git a/elf/rtld.c b/elf/rtld.c
index 5d117d0d2c..e3ec66c030 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -289,6 +289,8 @@  dl_main_state_init (struct dl_main_state *state)
   state->library_path_source = NULL;
   state->preloadlist = NULL;
   state->preloadarg = NULL;
+  state->glibc_hwcaps_prepend = NULL;
+  state->glibc_hwcaps_mask = NULL;
   state->mode = rtld_mode_normal;
   state->any_debug = false;
   state->version_info = false;
@@ -1244,6 +1246,22 @@  dl_main (const ElfW(Phdr) *phdr,
 	  {
 	    argv0 = _dl_argv[2];
 
+	    _dl_skip_args += 2;
+	    _dl_argc -= 2;
+	    _dl_argv += 2;
+	  }
+	else if (strcmp (_dl_argv[1], "--glibc-hwcaps-prepend") == 0
+		 && _dl_argc > 2)
+	  {
+	    state.glibc_hwcaps_prepend = _dl_argv[2];
+	    _dl_skip_args += 2;
+	    _dl_argc -= 2;
+	    _dl_argv += 2;
+	  }
+	else if (strcmp (_dl_argv[1], "--glibc-hwcaps-mask") == 0
+		 && _dl_argc > 2)
+	  {
+	    state.glibc_hwcaps_mask = _dl_argv[2];
 	    _dl_skip_args += 2;
 	    _dl_argc -= 2;
 	    _dl_argv += 2;
diff --git a/elf/tst-dl-hwcaps_split.c b/elf/tst-dl-hwcaps_split.c
new file mode 100644
index 0000000000..929c99a23b
--- /dev/null
+++ b/elf/tst-dl-hwcaps_split.c
@@ -0,0 +1,139 @@ 
+/* Unit tests for dl-hwcaps.c.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <array_length.h>
+#include <dl-hwcaps.h>
+#include <string.h>
+#include <support/check.h>
+
+static void
+check_split_masked (const char *input, int32_t bitmask, const char *mask,
+                    const char *expected[], size_t expected_length)
+{
+  struct dl_hwcaps_split_masked split;
+  _dl_hwcaps_split_masked_init (&split, input, bitmask, mask);
+  size_t index = 0;
+  while (_dl_hwcaps_split_masked (&split))
+    {
+      TEST_VERIFY_EXIT (index < expected_length);
+      TEST_COMPARE_BLOB (expected[index], strlen (expected[index]),
+                         split.split.segment, split.split.length);
+      ++index;
+    }
+  TEST_COMPARE (index, expected_length);
+}
+
+static void
+check_split (const char *input,
+             const char *expected[], size_t expected_length)
+{
+  struct dl_hwcaps_split split;
+  _dl_hwcaps_split_init (&split, input);
+  size_t index = 0;
+  while (_dl_hwcaps_split (&split))
+    {
+      TEST_VERIFY_EXIT (index < expected_length);
+      TEST_COMPARE_BLOB (expected[index], strlen (expected[index]),
+                         split.segment, split.length);
+      ++index;
+    }
+  TEST_COMPARE (index, expected_length);
+
+  /* Reuse the test cases with masking that does not actually remove
+     anything.  */
+  check_split_masked (input, -1, NULL, expected, expected_length);
+  check_split_masked (input, -1, input, expected, expected_length);
+}
+
+static int
+do_test (void)
+{
+  /* Splitting tests, without masking.  */
+  check_split (NULL, NULL, 0);
+  check_split ("", NULL, 0);
+  check_split (":", NULL, 0);
+  check_split ("::", NULL, 0);
+
+  {
+    const char *expected[] = { "first" };
+    check_split ("first", expected, array_length (expected));
+    check_split (":first", expected, array_length (expected));
+    check_split ("first:", expected, array_length (expected));
+    check_split (":first:", expected, array_length (expected));
+  }
+
+  {
+    const char *expected[] = { "first", "second" };
+    check_split ("first:second", expected, array_length (expected));
+    check_split ("first::second", expected, array_length (expected));
+    check_split (":first:second", expected, array_length (expected));
+    check_split ("first:second:", expected, array_length (expected));
+    check_split (":first:second:", expected, array_length (expected));
+  }
+
+  /* Splitting tests with masking.  */
+  {
+    const char *expected[] = { "first" };
+    check_split_masked ("first", 3, "first:second",
+                        expected, array_length (expected));
+    check_split_masked ("first:second", 3, "first:",
+                        expected, array_length (expected));
+    check_split_masked ("first:second", 1, NULL,
+                        expected, array_length (expected));
+  }
+  {
+    const char *expected[] = { "second" };
+    check_split_masked ("first:second", 3, "second",
+                        expected, array_length (expected));
+    check_split_masked ("first:second:third", -1, "second:",
+                        expected, array_length (expected));
+    check_split_masked ("first:second", 2, NULL,
+                        expected, array_length (expected));
+    check_split_masked ("first:second:third", 2, "first:second",
+                        expected, array_length (expected));
+  }
+
+  /* Tests for _dl_hwcaps_contains.  */
+  TEST_VERIFY (_dl_hwcaps_contains (NULL, "first", strlen ("first")));
+  TEST_VERIFY (_dl_hwcaps_contains (NULL, "", 0));
+  TEST_VERIFY (! _dl_hwcaps_contains ("", "first", strlen ("first")));
+  TEST_VERIFY (! _dl_hwcaps_contains ("firs", "first", strlen ("first")));
+  TEST_VERIFY (_dl_hwcaps_contains ("firs", "first", strlen ("first") - 1));
+  for (int i = 0; i < strlen ("first"); ++i)
+    TEST_VERIFY (! _dl_hwcaps_contains ("first", "first", i));
+  TEST_VERIFY (_dl_hwcaps_contains ("first", "first", strlen ("first")));
+  TEST_VERIFY (_dl_hwcaps_contains ("first:", "first", strlen ("first")));
+  TEST_VERIFY (_dl_hwcaps_contains ("first:second",
+                                    "first", strlen ("first")));
+  TEST_VERIFY (_dl_hwcaps_contains (":first:second", "first",
+                                    strlen ("first")));
+  TEST_VERIFY (_dl_hwcaps_contains ("first:second", "second",
+                                    strlen ("second")));
+  TEST_VERIFY (_dl_hwcaps_contains ("first:second:", "second",
+                                    strlen ("second")));
+  for (int i = 0; i < strlen ("second"); ++i)
+    TEST_VERIFY (!_dl_hwcaps_contains ("first:second:", "sec", i));
+
+  return 0;
+}
+
+#include <support/test-driver.c>
+
+/* Rebuild the sources here because the object file is built for
+   inclusion into the dynamic loader.  */
+#include "dl-hwcaps_split.c"
diff --git a/elf/tst-glibc-hwcaps-mask.c b/elf/tst-glibc-hwcaps-mask.c
new file mode 100644
index 0000000000..27b09b358c
--- /dev/null
+++ b/elf/tst-glibc-hwcaps-mask.c
@@ -0,0 +1,31 @@ 
+/* Test that --glibc-hwcaps-mask works.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+
+extern int marker1 (void);
+
+static int
+do_test (void)
+{
+  /* The marker1 function in elf/markermod1.so returns 1.  */
+  TEST_COMPARE (marker1 (), 1);
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/elf/tst-glibc-hwcaps-prepend.c b/elf/tst-glibc-hwcaps-prepend.c
new file mode 100644
index 0000000000..57d7319f14
--- /dev/null
+++ b/elf/tst-glibc-hwcaps-prepend.c
@@ -0,0 +1,32 @@ 
+/* Test that --glibc-hwcaps-prepend works.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+
+extern int marker1 (void);
+
+static int
+do_test (void)
+{
+  /* The marker1 function in
+     glibc-hwcaps/prepend-markermod1/markermod1.so returns 2.  */
+  TEST_COMPARE (marker1 (), 2);
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/elf/tst-glibc-hwcaps.c b/elf/tst-glibc-hwcaps.c
new file mode 100644
index 0000000000..28f47cf891
--- /dev/null
+++ b/elf/tst-glibc-hwcaps.c
@@ -0,0 +1,28 @@ 
+/* Stub test for glibc-hwcaps.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+
+static int
+do_test (void)
+{
+  puts ("info: generic tst-glibc-hwcaps (tests nothing)");
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 382eeb9be0..0b2babc70c 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -1047,8 +1047,13 @@  extern struct r_debug *_dl_debug_initialize (ElfW(Addr) ldbase, Lmid_t ns)
      attribute_hidden;
 
 /* Initialize the basic data structure for the search paths.  SOURCE
-   is either "LD_LIBRARY_PATH" or "--library-path".  */
-extern void _dl_init_paths (const char *library_path, const char *source)
+   is either "LD_LIBRARY_PATH" or "--library-path".
+   GLIBC_HWCAPS_PREPEND adds additional glibc-hwcaps subdirectories to
+   search.  GLIBC_HWCAPS_MASK is used to filter the built-in
+   subdirectories if not NULL.  */
+extern void _dl_init_paths (const char *library_path, const char *source,
+			    const char *glibc_hwcaps_prepend,
+			    const char *glibc_hwcaps_mask)
   attribute_hidden;
 
 /* Gather the information needed to install the profiling tables and start
@@ -1072,9 +1077,14 @@  extern void _dl_show_auxv (void) attribute_hidden;
 extern char *_dl_next_ld_env_entry (char ***position) attribute_hidden;
 
 /* Return an array with the names of the important hardware
-   capabilities.  The length of the array is written to *SZ, and the
-   maximum of all strings length is written to *MAX_CAPSTRLEN.  */
-const struct r_strlenpair *_dl_important_hwcaps (size_t *sz,
+   capabilities.  PREPEND is a colon-separated list of glibc-hwcaps
+   directories to search first.  MASK is a colon-separated list used
+   to filter the built-in glibc-hwcaps subdirectories.  The length of
+   the array is written to *SZ, and the maximum of all strings length
+   is written to *MAX_CAPSTRLEN.  */
+const struct r_strlenpair *_dl_important_hwcaps (const char *prepend,
+						 const char *mask,
+						 size_t *sz,
 						 size_t *max_capstrlen)
   attribute_hidden;