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V5 [PATCH] sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]

Message ID CAMe9rOqtkRBtoQH7v3kESoUX5LF6xun+d2CCOiXEOVCdy7vy0g@mail.gmail.com
State Superseded
Headers show
Series V5 [PATCH] sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305] | expand

Commit Message

H.J. Lu Oct. 13, 2020, 8:32 p.m. UTC
On Mon, Oct 12, 2020 at 3:04 PM Joseph Myers <joseph@codesourcery.com> wrote:
>
> On Sat, 10 Oct 2020, H.J. Lu via Libc-alpha wrote:
>
> > +* Add _SC_MINSIGSTKSZ and _SC_SIGSTKSZ.  When _SC_SIGSTKSZ_SOURCE is
> > +  defined, MINSIGSTKSZ and SIGSTKSZ are no longer constant on Linux.
> > +  MINSIGSTKSZ is redefined to sysconf(_SC_MINSIGSTKSZ) and SIGSTKSZ
> > +  is redefined to sysconf (_SC_SIGSTKSZ).
>
> I don't think the _SC_SIGSTKSZ_SOURCE option is a good idea.  But in any
> case, all feature test macros need to be documented in creature.texi, and
> the new _SC_* constants would need documenting in conf.texi, with
> appropriate documentation that they are GNU extensions.

Fixed.

> > +#ifdef __USE_SC_SIGSTKSZ
>
> And the internal macros for new feature test macros ought to be tested via
> __GLIBC_USE (thus, always defined to either 0 or 1) rather than #ifdef.
>

Fixed.

Here is the updated patch with the conform/data/unistd.h-data change
removed.

Comments

Dave Martin Oct. 14, 2020, 5:47 p.m. UTC | #1
On Tue, Oct 13, 2020 at 01:32:23PM -0700, H.J. Lu via Libc-alpha wrote:
> On Mon, Oct 12, 2020 at 3:04 PM Joseph Myers <joseph@codesourcery.com> wrote:
> >
> > On Sat, 10 Oct 2020, H.J. Lu via Libc-alpha wrote:
> >
> > > +* Add _SC_MINSIGSTKSZ and _SC_SIGSTKSZ.  When _SC_SIGSTKSZ_SOURCE is
> > > +  defined, MINSIGSTKSZ and SIGSTKSZ are no longer constant on Linux.
> > > +  MINSIGSTKSZ is redefined to sysconf(_SC_MINSIGSTKSZ) and SIGSTKSZ
> > > +  is redefined to sysconf (_SC_SIGSTKSZ).
> >
> > I don't think the _SC_SIGSTKSZ_SOURCE option is a good idea.  But in any
> > case, all feature test macros need to be documented in creature.texi, and
> > the new _SC_* constants would need documenting in conf.texi, with
> > appropriate documentation that they are GNU extensions.
> 
> Fixed.
> 
> > > +#ifdef __USE_SC_SIGSTKSZ
> >
> > And the internal macros for new feature test macros ought to be tested via
> > __GLIBC_USE (thus, always defined to either 0 or 1) rather than #ifdef.
> >
> 
> Fixed.
> 
> Here is the updated patch with the conform/data/unistd.h-data change
> removed.
> 
> -- 
> H.J.

> From 8f6a13de0049eec9c68e72d98fef12507c7f5af3 Mon Sep 17 00:00:00 2001
> From: "H.J. Lu" <hjl.tools@gmail.com>
> Date: Fri, 25 Sep 2020 13:10:08 -0700
> Subject: [PATCH] sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]
> 
> Add _SC_MINSIGSTKSZ for the minimum signal stack size derived from
> AT_MINSIGSTKSZ, which is the minimum number of bytes of free stack
> space required in order to gurantee successful, non-nested handling
> of a single signal whose handler is an empty function, and _SC_SIGSTKSZ
> which is the suggested minimum number of bytes of stack space required
> for a signal stack.
> 
> If AT_MINSIGSTKSZ isn't available, sysconf (_SC_MINSIGSTKSZ) returns
> MINSIGSTKSZ.  On Linux/x86 with XSAVE, the signal frame used by kernel
> is composed of the following areas and laid out as:
> 
>  ------------------------------
>  | alignment padding          |
>  ------------------------------
>  | xsave buffer               |
>  ------------------------------
>  | fsave header (32-bit only) |
>  ------------------------------
>  | siginfo + ucontext         |
>  ------------------------------
> 
> Compute AT_MINSIGSTKSZ value as size of xsave buffer + size of fsave
> header (32-bit only) + size of siginfo and ucontext + alignment padding.
> 
> If _SC_SIGSTKSZ_SOURCE is defined, MINSIGSTKSZ and SIGSTKSZ are redefined
> as
> 
> /* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ).  */
>  # undef SIGSTKSZ
>  # define SIGSTKSZ sysconf (_SC_SIGSTKSZ)
> 
> /* Minimum stack size for a signal handler: SIGSTKSZ.  */
>  # undef MINSIGSTKSZ
>  # define MINSIGSTKSZ SIGSTKSZ
> 
> Compilation will fail if the source assumes constant MINSIGSTKSZ or
> SIGSTKSZ.
> 
> The reason for not simply increasing the kernel's MINSIGSTKSZ #define
> (apart from the fact that it is rarely used, due to glibc's shadowing
> definitions) was that userspace binaries will have baked in the old
> value of the constant and may be making assumptions about it.
> 
> For example, the type (char [MINSIGSTKSZ]) changes if this #define
> changes.  This could be a problem if an newly built library tries to
> memcpy() or dump such an object defined by and old binary.
> Bounds-checking and the stack sizes passed to things like sigaltstack()
> and makecontext() could similarly go wrong.
> ---
>  NEWS                                          |  5 ++
>  bits/confname.h                               |  8 +-
>  bits/sigstksz.h                               | 21 +++++
>  elf/dl-support.c                              |  5 ++
>  elf/dl-sysdep.c                               |  9 ++
>  include/features.h                            | 10 +++
>  manual/conf.texi                              | 21 +++++
>  manual/creature.texi                          |  6 ++
>  posix/sysconf.c                               |  3 +
>  signal/Makefile                               |  5 +-
>  signal/signal.h                               |  1 +
>  signal/tst-minsigstksz-5.c                    | 84 +++++++++++++++++++
>  sysdeps/generic/ldsodefs.h                    |  3 +
>  sysdeps/unix/sysv/linux/Makefile              |  8 ++
>  sysdeps/unix/sysv/linux/bits/sigstksz.h       | 33 ++++++++
>  .../unix/sysv/linux/ia64/sysconf-sigstksz.h   | 27 ++++++
>  sysdeps/unix/sysv/linux/sysconf-sigstksz.h    | 38 +++++++++
>  sysdeps/unix/sysv/linux/sysconf.c             |  9 ++
>  .../unix/sysv/linux/x86/dl-minsigstacksize.h  | 77 +++++++++++++++++
>  sysdeps/x86/cpu-features.c                    | 16 ++--
>  sysdeps/x86/dl-minsigstacksize.h              | 23 +++++
>  21 files changed, 404 insertions(+), 8 deletions(-)
>  create mode 100644 bits/sigstksz.h
>  create mode 100644 signal/tst-minsigstksz-5.c
>  create mode 100644 sysdeps/unix/sysv/linux/bits/sigstksz.h
>  create mode 100644 sysdeps/unix/sysv/linux/ia64/sysconf-sigstksz.h
>  create mode 100644 sysdeps/unix/sysv/linux/sysconf-sigstksz.h
>  create mode 100644 sysdeps/unix/sysv/linux/x86/dl-minsigstacksize.h
>  create mode 100644 sysdeps/x86/dl-minsigstacksize.h
> 
> diff --git a/NEWS b/NEWS
> index e84c39aeb1..a7995f8c6a 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -9,6 +9,11 @@ Version 2.33
>  
>  Major new features:
>  
> +* Add _SC_MINSIGSTKSZ and _SC_SIGSTKSZ.  When _SC_SIGSTKSZ_SOURCE is
> +  defined, MINSIGSTKSZ and SIGSTKSZ are no longer constant on Linux.
> +  MINSIGSTKSZ is redefined to sysconf(_SC_MINSIGSTKSZ) and SIGSTKSZ
> +  is redefined to sysconf (_SC_SIGSTKSZ).
> +
>  * The dynamic linker accepts the --argv0 argument and provides opportunity
>    to change argv[0] string.
>  
> diff --git a/bits/confname.h b/bits/confname.h
> index 5dc8215093..451d3eb636 100644
> --- a/bits/confname.h
> +++ b/bits/confname.h
> @@ -525,8 +525,14 @@ enum
>  
>      _SC_THREAD_ROBUST_PRIO_INHERIT,
>  #define _SC_THREAD_ROBUST_PRIO_INHERIT	_SC_THREAD_ROBUST_PRIO_INHERIT
> -    _SC_THREAD_ROBUST_PRIO_PROTECT
> +    _SC_THREAD_ROBUST_PRIO_PROTECT,
>  #define _SC_THREAD_ROBUST_PRIO_PROTECT	_SC_THREAD_ROBUST_PRIO_PROTECT
> +
> +    _SC_MINSIGSTKSZ,
> +#define	_SC_MINSIGSTKSZ			_SC_MINSIGSTKSZ
> +
> +    _SC_SIGSTKSZ
> +#define	_SC_SIGSTKSZ			_SC_SIGSTKSZ
>    };
>  
>  /* Values for the NAME argument to `confstr'.  */
> diff --git a/bits/sigstksz.h b/bits/sigstksz.h
> new file mode 100644
> index 0000000000..5535d873b5
> --- /dev/null
> +++ b/bits/sigstksz.h
> @@ -0,0 +1,21 @@
> +/* Definition of MINSIGSTKSZ.  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/>.  */
> +
> +#ifndef _SIGNAL_H
> +# error "Never include <bits/sigstksz.h> directly; use <signal.h> instead."
> +#endif
> diff --git a/elf/dl-support.c b/elf/dl-support.c
> index afbc94df54..18ac0bdd44 100644
> --- a/elf/dl-support.c
> +++ b/elf/dl-support.c
> @@ -136,6 +136,8 @@ void (*_dl_init_static_tls) (struct link_map *) = &_dl_nothread_init_static_tls;
>  
>  size_t _dl_pagesize = EXEC_PAGESIZE;
>  
> +size_t _dl_minsigstacksize = MINSIGSTKSZ;
> +
>  int _dl_inhibit_cache;
>  
>  unsigned int _dl_osversion;
> @@ -294,6 +296,9 @@ _dl_aux_init (ElfW(auxv_t) *av)
>        case AT_RANDOM:
>  	_dl_random = (void *) av->a_un.a_val;
>  	break;
> +      case AT_MINSIGSTKSZ:
> +	GLRO(dl_minsigstacksize) = av->a_un.a_val;
> +	break;
>        DL_PLATFORM_AUXV
>        }
>    if (seen == 0xf)
> diff --git a/elf/dl-sysdep.c b/elf/dl-sysdep.c
> index 854570821c..2b1bce8cf5 100644
> --- a/elf/dl-sysdep.c
> +++ b/elf/dl-sysdep.c
> @@ -117,6 +117,11 @@ _dl_sysdep_start (void **start_argptr,
>    user_entry = (ElfW(Addr)) ENTRY_POINT;
>    GLRO(dl_platform) = NULL; /* Default to nothing known about the platform.  */
>  
> +  /* NB: Default to a constant MINSIGSTKSZ.  */
> +  _Static_assert (__builtin_constant_p (MINSIGSTKSZ),
> +		  "MINSIGSTKSZ is constant");
> +  GLRO(dl_minsigstacksize) = MINSIGSTKSZ;
> +
>    for (av = GLRO(dl_auxv); av->a_type != AT_NULL; set_seen (av++))
>      switch (av->a_type)
>        {
> @@ -181,6 +186,9 @@ _dl_sysdep_start (void **start_argptr,
>        case AT_RANDOM:
>  	_dl_random = (void *) av->a_un.a_val;
>  	break;
> +      case AT_MINSIGSTKSZ:
> +	GLRO(dl_minsigstacksize) = av->a_un.a_val;
> +	break;
>        DL_PLATFORM_AUXV
>        }
>  
> @@ -308,6 +316,7 @@ _dl_show_auxv (void)
>  	  [AT_SYSINFO_EHDR - 2] =	{ "SYSINFO_EHDR:      0x", hex },
>  	  [AT_RANDOM - 2] =		{ "RANDOM:            0x", hex },
>  	  [AT_HWCAP2 - 2] =		{ "HWCAP2:            0x", hex },
> +	  [AT_MINSIGSTKSZ - 2] =	{ "MINSIGSTKSZ        ", dec },

(Random, unrelated question ^^ is the missing "0x" on AT_HWCAP
and AT_NOTELF intentional?  Are we just stuck with those now?)


Also, I discovered while trying to test my version of this that invoking
ld.so explicitly messes up the auxv passed to the target program,
probably as a side-effect of mungeing the args and environment.  This
leads to getauxval () returning garbage in the target program.

Should I throw that in bugzilla, or is it likely to be trivial to sort
out?


>  	  [AT_L1I_CACHESIZE - 2] =	{ "L1I_CACHESIZE:     ", dec },
>  	  [AT_L1I_CACHEGEOMETRY - 2] =	{ "L1I_CACHEGEOMETRY: 0x", hex },
>  	  [AT_L1D_CACHESIZE - 2] =	{ "L1D_CACHESIZE:     ", dec },
> diff --git a/include/features.h b/include/features.h
> index f3e62d3362..38b528e027 100644
> --- a/include/features.h
> +++ b/include/features.h
> @@ -55,6 +55,8 @@
>     _FORTIFY_SOURCE	Add security hardening to many library functions.
>  			Set to 1 or 2; 2 performs stricter checks than 1.
>  
> +   _SC_SIGSTKSZ_SOURCE	Select non-constant MINSIGSTKSZ and SIGSTKSZ.
> +

Maybe "correct (but non compiletime constant)"

>     _REENTRANT, _THREAD_SAFE
>  			Obsolete; equivalent to _POSIX_C_SOURCE=199506L.
>  
> @@ -96,6 +98,7 @@
>     __USE_ATFILE		Define *at interfaces and AT_* constants for them.
>     __USE_GNU		Define GNU extensions.
>     __USE_FORTIFY_LEVEL	Additional security measures used, according to level.
> +   __USE_SC_SIGSTKSZ	Define non-constant MINSIGSTKSZ and SIGSTKSZ.
>  
>     The macros `__GNU_LIBRARY__', `__GLIBC__', and `__GLIBC_MINOR__' are
>     defined by this file unconditionally.  `__GNU_LIBRARY__' is provided
> @@ -139,6 +142,7 @@
>  #undef	__USE_ATFILE
>  #undef	__USE_GNU
>  #undef	__USE_FORTIFY_LEVEL
> +#undef	__USE_SC_SIGSTKSZ
>  #undef	__KERNEL_STRICT_NAMES
>  #undef	__GLIBC_USE_ISOC2X
>  #undef	__GLIBC_USE_DEPRECATED_GETS
> @@ -407,6 +411,12 @@
>  # define __USE_FORTIFY_LEVEL 0
>  #endif
>  
> +#ifdef	_SC_SIGSTKSZ_SOURCE
> +# define __USE_SC_SIGSTKSZ	1
> +#else
> +# define __USE_SC_SIGSTKSZ	0
> +#endif
> +
>  /* The function 'gets' existed in C89, but is impossible to use
>     safely.  It has been removed from ISO C11 and ISO C++14.  Note: for
>     compatibility with various implementations of <cstdio>, this test
> diff --git a/manual/conf.texi b/manual/conf.texi
> index f959b00bb6..ba9847aaa4 100644
> --- a/manual/conf.texi
> +++ b/manual/conf.texi
> @@ -913,6 +913,27 @@ Inquire about the parameter corresponding to @code{NL_SETMAX}.
>  @item _SC_NL_TEXTMAX
>  @standards{X/Open, unistd.h}
>  Inquire about the parameter corresponding to @code{NL_TEXTMAX}.
> +
> +@item _SC_MINSIGSTKSZ
> +@standards{GNU, unistd.h}
> +Inquire about the minimum number of bytes of free stack space required
> +in order to guarantee successful, non-nested handling of a single signal
> +whose handler is an empty function.
> +
> +@item _SC_SIGSTKSZ
> +@standards{GNU, unistd.h}
> +Inquire about the suggested minimum number of bytes of stack space
> +required for a signal stack.
> +
> +This is not guaranteed to be enough for any specific purpose other than
> +the invocation of a single, non-nested, empty handler, but nonetheless
> +should be enough for basic scenarios involving simple signal handlers
> +and very low levels of signal nesting (say, 2 or 3 levels at the very
> +most).
> +
> +This value is provided for developer convenience and to ease migration
> +from the legacy @code{SIGSTKSZ} constant.  Programs requiring stronger
> +guarantees should avoid using it if at all possible.
>  @end vtable
>  
>  @node Examples of Sysconf
> diff --git a/manual/creature.texi b/manual/creature.texi
> index be5050468b..21c6d3d5d0 100644
> --- a/manual/creature.texi
> +++ b/manual/creature.texi
> @@ -257,6 +257,12 @@ various library functions.  If defined to @math{2}, even stricter
>  checks are applied.
>  @end defvr
>  
> +@defvr Macro _SC_SIGSTKSZ_SOURCE
> +@standards{GNU, (none)}
> +If this macro is defined, non-constant MINSIGSTKSZ and SIGSTKSZ are
> +defined.
> +@end defvr
> +
>  @defvr Macro _REENTRANT
>  @defvrx Macro _THREAD_SAFE
>  @standards{Obsolete, (none)}
> diff --git a/posix/sysconf.c b/posix/sysconf.c
> index ba303384c1..ca7833e6c4 100644
> --- a/posix/sysconf.c
> +++ b/posix/sysconf.c
> @@ -266,6 +266,9 @@ __sysconf (int name)
>      case _SC_XOPEN_REALTIME:
>      case _SC_XOPEN_REALTIME_THREADS:
>  
> +    case _SC_MINSIGSTKSZ:
> +    case _SC_SIGSTKSZ:
> +
>        break;
>      }
>  
> diff --git a/signal/Makefile b/signal/Makefile
> index 2ec3ddd74f..641d30582d 100644
> --- a/signal/Makefile
> +++ b/signal/Makefile
> @@ -31,7 +31,8 @@ headers := signal.h sys/signal.h \
>  	   bits/types/sigevent_t.h bits/types/siginfo_t.h \
>  	   bits/types/sigset_t.h bits/types/sigval_t.h \
>  	   bits/types/stack_t.h bits/types/struct_sigstack.h \
> -	   bits/types/__sigval_t.h bits/signal_ext.h
> +	   bits/types/__sigval_t.h bits/signal_ext.h \
> +	   bits/sigstksz.h
>  
>  routines	:= signal raise killpg \
>  		   sigaction sigprocmask kill \
> @@ -48,7 +49,7 @@ routines	:= signal raise killpg \
>  tests		:= tst-signal tst-sigset tst-sigsimple tst-raise tst-sigset2 \
>  		   tst-sigwait-eintr tst-sigaction \
>  		   tst-minsigstksz-1 tst-minsigstksz-2 tst-minsigstksz-3 \
> -		   tst-minsigstksz-3a tst-minsigstksz-4 \
> +		   tst-minsigstksz-3a tst-minsigstksz-4 tst-minsigstksz-5 \
>  		   tst-sigisemptyset
>  
>  include ../Rules
> diff --git a/signal/signal.h b/signal/signal.h
> index effe3d698f..0311eb2a66 100644
> --- a/signal/signal.h
> +++ b/signal/signal.h
> @@ -312,6 +312,7 @@ extern int siginterrupt (int __sig, int __interrupt) __THROW
>    __attribute_deprecated_msg__ ("Use sigaction with SA_RESTART instead");
>  
>  # include <bits/sigstack.h>
> +# include <bits/sigstksz.h>
>  # include <bits/ss_flags.h>
>  
>  /* Alternate signal handler stack interface.
> diff --git a/signal/tst-minsigstksz-5.c b/signal/tst-minsigstksz-5.c
> new file mode 100644
> index 0000000000..d657d2f4e6
> --- /dev/null
> +++ b/signal/tst-minsigstksz-5.c
> @@ -0,0 +1,84 @@
> +/* Test of signal delivery on an alternate stack with MINSIGSTKSZ size.
> +   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 <signal.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <unistd.h>
> +#include <support/check.h>
> +#include <support/support.h>
> +
> +static volatile sig_atomic_t handler_run;
> +
> +static void
> +handler (int signo)
> +{
> +  /* Clear a bit of on-stack memory.  */
> +  volatile char buffer[256];
> +  for (size_t i = 0; i < sizeof (buffer); ++i)
> +    buffer[i] = 0;
> +  handler_run = 1;
> +}
> +
> +int
> +do_test (void)
> +{
> +  size_t stack_buffer_size = 64 * 1024 * 1024;
> +  void *stack_buffer = xmalloc (stack_buffer_size);
> +  void *stack_end = stack_buffer + stack_buffer_size;
> +  memset (stack_buffer, 0xCC, stack_buffer_size);
> +
> +  void *stack_bottom = stack_buffer + (stack_buffer_size + MINSIGSTKSZ) / 2;
> +  void *stack_top = stack_bottom + MINSIGSTKSZ;
> +  stack_t stack =
> +    {
> +      .ss_sp = stack_bottom,
> +      .ss_size = MINSIGSTKSZ,
> +    };
> +  if (sigaltstack (&stack, NULL) < 0)
> +    FAIL_RET ("sigaltstack: %m\n");
> +
> +  struct sigaction act =
> +    {
> +      .sa_handler = handler,
> +      .sa_flags = SA_ONSTACK,
> +    };
> +  if (sigaction (SIGUSR1, &act, NULL) < 0)
> +    FAIL_RET ("sigaction: %m\n");
> +
> +  if (kill (getpid (), SIGUSR1) < 0)
> +    FAIL_RET ("kill: %m\n");
> +
> +  if (handler_run != 1)
> +    FAIL_RET ("handler did not run\n");
> +
> +  for (void *p = stack_buffer; p < stack_bottom; ++p)
> +    if (*(unsigned char *) p != 0xCC)
> +      FAIL_RET ("changed byte %ld bytes below configured stack\n",
> +		(long) (stack_bottom - p));
> +  for (void *p = stack_top; p < stack_end; ++p)
> +    if (*(unsigned char *) p != 0xCC)
> +      FAIL_RET ("changed byte %ld bytes above configured stack\n",
> +		(long) (p - stack_top));
> +
> +  free (stack_buffer);
> +
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>
> diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
> index 382eeb9be0..5ce847aeac 100644
> --- a/sysdeps/generic/ldsodefs.h
> +++ b/sysdeps/generic/ldsodefs.h
> @@ -528,6 +528,9 @@ struct rtld_global_ro
>    /* Cached value of `getpagesize ()'.  */
>    EXTERN size_t _dl_pagesize;
>  
> +  /* Cached value of `sysconf (_SC_MINSIGSTKSZ)'.  */
> +  EXTERN size_t _dl_minsigstacksize;
> +
>    /* Do we read from ld.so.cache?  */
>    EXTERN int _dl_inhibit_cache;
>  
> diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
> index d760debf40..a17ca28de8 100644
> --- a/sysdeps/unix/sysv/linux/Makefile
> +++ b/sysdeps/unix/sysv/linux/Makefile
> @@ -190,6 +190,9 @@ sysdep_routines += ntp_gettime ntp_gettimex
>  endif
>  
>  ifeq ($(subdir),signal)
> +# Compile tst-minsigstksz-5.c with _SC_SIGSTKSZ_SOURCE.
> +CFLAGS-tst-minsigstksz-5.c += -D_SC_SIGSTKSZ_SOURCE
> +
>  tests-special += $(objpfx)tst-signal-numbers.out
>  # Depending on signal.o* is a hack.  What we actually want is a dependency
>  # on signal.h and everything it includes.  That's impractical to write
> @@ -228,6 +231,11 @@ ifeq ($(subdir),sunrpc)
>  sysdep_headers += nfs/nfs.h
>  endif
>  
> +ifeq ($(subdir),support)
> +# Compile xsigstack.c with _SC_SIGSTKSZ_SOURCE.
> +CFLAGS-xsigstack.c += -D_SC_SIGSTKSZ_SOURCE
> +endif
> +
>  ifeq ($(subdir),termios)
>  sysdep_headers += termio.h
>  endif
> diff --git a/sysdeps/unix/sysv/linux/bits/sigstksz.h b/sysdeps/unix/sysv/linux/bits/sigstksz.h
> new file mode 100644
> index 0000000000..cd5b3cc895
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/bits/sigstksz.h
> @@ -0,0 +1,33 @@
> +/* Definition of MINSIGSTKSZ and SIGSTKSZ.  Linux 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/>.  */
> +
> +#ifndef _SIGNAL_H
> +# error "Never include <bits/sigstksz.h> directly; use <signal.h> instead."
> +#endif
> +
> +#if __USE_SC_SIGSTKSZ
> +# include <unistd.h>
> +
> +/* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ).  */
> +# undef SIGSTKSZ
> +# define SIGSTKSZ sysconf (_SC_SIGSTKSZ)
> +
> +/* Minimum stack size for a signal handler: SIGSTKSZ.  */
> +# undef MINSIGSTKSZ
> +# define MINSIGSTKSZ SIGSTKSZ
> +#endif

To help raise awareness, is it worth adding deprecation warnings on
these?

Could we still consider them deprecated even with _SC_SIGSTKSZ_SOURCE?
Ideally they should be (or even removed), since even if these values are
"correct", using them is still a potential portability problem when
building for other library stacks.

I think the rule ought to be to use these only if _SC_SIGSTKSZ /
_SC_MINSIGSTKSZ aren't available, and with the caveat that the values
may be wrong --  similar to the situation with PAGESIZE.


It could be worth making this feature test macro more general and
harvesting any other broken legacy macros we're aware of (such as
PAGESIZE, but there are probably others).  Probably out of scope for
this patch, though.


> diff --git a/sysdeps/unix/sysv/linux/ia64/sysconf-sigstksz.h b/sysdeps/unix/sysv/linux/ia64/sysconf-sigstksz.h
> new file mode 100644
> index 0000000000..7e5ceba151
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/ia64/sysconf-sigstksz.h
> @@ -0,0 +1,27 @@
> +/* sysconf_sigstksz ().  Linux/ia64 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/>.  */
> +
> +/* Return sysconf (_SC_SIGSTKSZ).  */
> +
> +static long int
> +sysconf_sigstksz (void)
> +{
> +  _Static_assert (__builtin_constant_p (SIGSTKSZ),
> +		  "SIGSTKSZ is constant");
> +  return SIGSTKSZ;
> +}
> diff --git a/sysdeps/unix/sysv/linux/sysconf-sigstksz.h b/sysdeps/unix/sysv/linux/sysconf-sigstksz.h
> new file mode 100644
> index 0000000000..64d450b22c
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/sysconf-sigstksz.h
> @@ -0,0 +1,38 @@
> +/* sysconf_sigstksz ().  Linux 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/>.  */
> +
> +/* Return sysconf (_SC_SIGSTKSZ).  */
> +
> +static long int
> +sysconf_sigstksz (void)
> +{
> +  long int minsigstacksize = GLRO(dl_minsigstacksize);
> +  assert (minsigstacksize != 0);
> +  _Static_assert (__builtin_constant_p (MINSIGSTKSZ),
> +		  "MINSIGSTKSZ is constant");
> +  if (minsigstacksize < MINSIGSTKSZ)
> +    minsigstacksize = MINSIGSTKSZ;
> +  /* MAX (MINSIGSTKSZ, sysconf (_SC_MINSIGSTKSZ)) * 4.  */
> +  long int sigstacksize = minsigstacksize * 4;
> +  /* Return MAX (SIGSTKSZ, sigstacksize).  */
> +  _Static_assert (__builtin_constant_p (SIGSTKSZ),
> +		  "SIGSTKSZ is constant");
> +  if (sigstacksize < SIGSTKSZ)
> +    sigstacksize = SIGSTKSZ;
> +  return sigstacksize;
> +}
> diff --git a/sysdeps/unix/sysv/linux/sysconf.c b/sysdeps/unix/sysv/linux/sysconf.c
> index 7958a74164..d1d86df3fb 100644
> --- a/sysdeps/unix/sysv/linux/sysconf.c
> +++ b/sysdeps/unix/sysv/linux/sysconf.c
> @@ -16,6 +16,7 @@
>     License along with the GNU C Library; if not, see
>     <https://www.gnu.org/licenses/>.  */
>  
> +#include <assert.h>
>  #include <errno.h>
>  #include <fcntl.h>
>  #include <stdlib.h>
> @@ -26,6 +27,7 @@
>  #include <sys/param.h>
>  #include <not-cancel.h>
>  #include <ldsodefs.h>
> +#include <sysconf-sigstksz.h>
>  
>  /* Legacy value of ARG_MAX.  The macro is now not defined since the
>     actual value varies based on the stack size.  */
> @@ -75,6 +77,13 @@ __sysconf (int name)
>        }
>        break;
>  
> +    case _SC_MINSIGSTKSZ:
> +      assert (GLRO(dl_minsigstacksize) != 0);
> +      return GLRO(dl_minsigstacksize);
> +
> +    case _SC_SIGSTKSZ:
> +      return sysconf_sigstksz ();
> +
>      default:
>        break;
>      }
> diff --git a/sysdeps/unix/sysv/linux/x86/dl-minsigstacksize.h b/sysdeps/unix/sysv/linux/x86/dl-minsigstacksize.h
> new file mode 100644
> index 0000000000..d2dc436572
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/x86/dl-minsigstacksize.h
> @@ -0,0 +1,77 @@
> +/* Emulate AT_MINSIGSTKSZ.  Linux/x86 version.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +
> +   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 <unistd.h>
> +
> +/* Emulate AT_MINSIGSTKSZ with XSAVE. */
> +
> +static inline void
> +dl_check_minsigstacksize (void)
> +{
> +  /* NB: Default to a constant MINSIGSTKSZ.  */
> +  _Static_assert (__builtin_constant_p (MINSIGSTKSZ),
> +		  "MINSIGSTKSZ is constant");
> +  /* Return if AT_MINSIGSTKSZ is provide by kernel.  */
> +  if (GLRO(dl_minsigstacksize) != MINSIGSTKSZ)
> +    return;

Couldn't the kernel actually yield MINSIGSTKSZ or a smaller value, say,
if running on hardware that doesn't have AVX-512?

We might want a separate flag to indicate whether we obtained a value
from the auxv, rather relying on MINSIGSTKSZ having this magic meaning.

> +
> +  /* Emulate AT_MINSIGSTKSZ.  In Linux kernel, the signal frame data
> +     with XSAVE is composed of the following areas and laid out as:
> +     ------------------------------
> +     | alignment padding          |
> +     ------------------------------
> +     | xsave buffer               |
> +     ------------------------------
> +     | fsave header (32-bit only) |
> +     ------------------------------
> +     | siginfo + ucontext         |
> +     ------------------------------
> + */
> +
> +  unsigned int sigframe_size;
> +
> +#ifdef __x86_64__
> +  /* NB: sizeof(struct rt_sigframe) + 8-byte return address in Linux
> +     kernel.  */
> +  sigframe_size = 440 + 8;
> +#else
> +  /* NB: sizeof(struct sigframe_ia32) + sizeof(struct fregs_state)) +
> +     4-byte return address + 3 * 4-byte arguments in Linux kernel.  */
> +  sigframe_size = 736 + 112 + 4 + 3 * 4;
> +#endif
> +
> +  /* Add 15 bytes to align the stack to 16 bytes.  */
> +  sigframe_size += 15;
> +
> +  /* Make the space before xsave buffer multiple of 16 bytes.  */
> +  sigframe_size = ALIGN_UP (sigframe_size, 16);
> +
> +  /* Add (64 - 16)-byte padding to align xsave buffer at 64 bytes.  */
> +  sigframe_size += 64 - 16;
> +
> +  unsigned int eax, ebx, ecx, edx;
> +  __cpuid_count (0xd, 0, eax, ebx, ecx, edx);
> +
> +  /* Add the size of xsave buffer.  */
> +  sigframe_size += ebx;
> +
> +  /* Add the size of FP_XSTATE_MAGIC2.  */
> +#define FP_XSTATE_MAGIC2 0x46505845U
> +  sigframe_size += sizeof (FP_XSTATE_MAGIC2);
> +
> +  GLRO(dl_minsigstacksize) = sigframe_size;
> +}

Ack that we don't need anything similar for AArch64.
(Since AT_MINSIGSTKSZ was merged in the kernel along with the first
problematic arch extension, SVE).

[...]

Cheers
---Dave
Florian Weimer Oct. 14, 2020, 6:07 p.m. UTC | #2
* Dave Martin via Libc-alpha:

> (Random, unrelated question ^^ is the missing "0x" on AT_HWCAP
> and AT_NOTELF intentional?  Are we just stuck with those now?)

I think it would make sense to file a bug in Bugzilla for that.

> Also, I discovered while trying to test my version of this that invoking
> ld.so explicitly messes up the auxv passed to the target program,
> probably as a side-effect of mungeing the args and environment.  This
> leads to getauxval () returning garbage in the target program.
>
> Should I throw that in bugzilla, or is it likely to be trivial to sort
> out?

I think this is:

  <https://sourceware.org/bugzilla/show_bug.cgi?id=23293>

It's been open for two years, so it's probably fair to say that it is
not trivial to sort out.  Purely empirically speaking.

Thanks,
Florian
Dave Martin Oct. 19, 2020, 4:30 p.m. UTC | #3
On Wed, Oct 14, 2020 at 08:07:37PM +0200, Florian Weimer via Libc-alpha wrote:
> * Dave Martin via Libc-alpha:
> 
> > (Random, unrelated question ^^ is the missing "0x" on AT_HWCAP
> > and AT_NOTELF intentional?  Are we just stuck with those now?)
> 
> I think it would make sense to file a bug in Bugzilla for that.

OK

> > Also, I discovered while trying to test my version of this that invoking
> > ld.so explicitly messes up the auxv passed to the target program,
> > probably as a side-effect of mungeing the args and environment.  This
> > leads to getauxval () returning garbage in the target program.
> >
> > Should I throw that in bugzilla, or is it likely to be trivial to sort
> > out?
> 
> I think this is:
> 
>   <https://sourceware.org/bugzilla/show_bug.cgi?id=23293>
> 
> It's been open for two years, so it's probably fair to say that it is
> not trivial to sort out.  Purely empirically speaking.

Ah, so long as it's not been forgotten about.

I think I bugged Szabolcs about this when I first hit it, which probably
explains where that ticket came from.

Cheers
---Dave
diff mbox series

Patch

From 8f6a13de0049eec9c68e72d98fef12507c7f5af3 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Fri, 25 Sep 2020 13:10:08 -0700
Subject: [PATCH] sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]

Add _SC_MINSIGSTKSZ for the minimum signal stack size derived from
AT_MINSIGSTKSZ, which is the minimum number of bytes of free stack
space required in order to gurantee successful, non-nested handling
of a single signal whose handler is an empty function, and _SC_SIGSTKSZ
which is the suggested minimum number of bytes of stack space required
for a signal stack.

If AT_MINSIGSTKSZ isn't available, sysconf (_SC_MINSIGSTKSZ) returns
MINSIGSTKSZ.  On Linux/x86 with XSAVE, the signal frame used by kernel
is composed of the following areas and laid out as:

 ------------------------------
 | alignment padding          |
 ------------------------------
 | xsave buffer               |
 ------------------------------
 | fsave header (32-bit only) |
 ------------------------------
 | siginfo + ucontext         |
 ------------------------------

Compute AT_MINSIGSTKSZ value as size of xsave buffer + size of fsave
header (32-bit only) + size of siginfo and ucontext + alignment padding.

If _SC_SIGSTKSZ_SOURCE is defined, MINSIGSTKSZ and SIGSTKSZ are redefined
as

/* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ).  */
 # undef SIGSTKSZ
 # define SIGSTKSZ sysconf (_SC_SIGSTKSZ)

/* Minimum stack size for a signal handler: SIGSTKSZ.  */
 # undef MINSIGSTKSZ
 # define MINSIGSTKSZ SIGSTKSZ

Compilation will fail if the source assumes constant MINSIGSTKSZ or
SIGSTKSZ.

The reason for not simply increasing the kernel's MINSIGSTKSZ #define
(apart from the fact that it is rarely used, due to glibc's shadowing
definitions) was that userspace binaries will have baked in the old
value of the constant and may be making assumptions about it.

For example, the type (char [MINSIGSTKSZ]) changes if this #define
changes.  This could be a problem if an newly built library tries to
memcpy() or dump such an object defined by and old binary.
Bounds-checking and the stack sizes passed to things like sigaltstack()
and makecontext() could similarly go wrong.
---
 NEWS                                          |  5 ++
 bits/confname.h                               |  8 +-
 bits/sigstksz.h                               | 21 +++++
 elf/dl-support.c                              |  5 ++
 elf/dl-sysdep.c                               |  9 ++
 include/features.h                            | 10 +++
 manual/conf.texi                              | 21 +++++
 manual/creature.texi                          |  6 ++
 posix/sysconf.c                               |  3 +
 signal/Makefile                               |  5 +-
 signal/signal.h                               |  1 +
 signal/tst-minsigstksz-5.c                    | 84 +++++++++++++++++++
 sysdeps/generic/ldsodefs.h                    |  3 +
 sysdeps/unix/sysv/linux/Makefile              |  8 ++
 sysdeps/unix/sysv/linux/bits/sigstksz.h       | 33 ++++++++
 .../unix/sysv/linux/ia64/sysconf-sigstksz.h   | 27 ++++++
 sysdeps/unix/sysv/linux/sysconf-sigstksz.h    | 38 +++++++++
 sysdeps/unix/sysv/linux/sysconf.c             |  9 ++
 .../unix/sysv/linux/x86/dl-minsigstacksize.h  | 77 +++++++++++++++++
 sysdeps/x86/cpu-features.c                    | 16 ++--
 sysdeps/x86/dl-minsigstacksize.h              | 23 +++++
 21 files changed, 404 insertions(+), 8 deletions(-)
 create mode 100644 bits/sigstksz.h
 create mode 100644 signal/tst-minsigstksz-5.c
 create mode 100644 sysdeps/unix/sysv/linux/bits/sigstksz.h
 create mode 100644 sysdeps/unix/sysv/linux/ia64/sysconf-sigstksz.h
 create mode 100644 sysdeps/unix/sysv/linux/sysconf-sigstksz.h
 create mode 100644 sysdeps/unix/sysv/linux/x86/dl-minsigstacksize.h
 create mode 100644 sysdeps/x86/dl-minsigstacksize.h

diff --git a/NEWS b/NEWS
index e84c39aeb1..a7995f8c6a 100644
--- a/NEWS
+++ b/NEWS
@@ -9,6 +9,11 @@  Version 2.33
 
 Major new features:
 
+* Add _SC_MINSIGSTKSZ and _SC_SIGSTKSZ.  When _SC_SIGSTKSZ_SOURCE is
+  defined, MINSIGSTKSZ and SIGSTKSZ are no longer constant on Linux.
+  MINSIGSTKSZ is redefined to sysconf(_SC_MINSIGSTKSZ) and SIGSTKSZ
+  is redefined to sysconf (_SC_SIGSTKSZ).
+
 * The dynamic linker accepts the --argv0 argument and provides opportunity
   to change argv[0] string.
 
diff --git a/bits/confname.h b/bits/confname.h
index 5dc8215093..451d3eb636 100644
--- a/bits/confname.h
+++ b/bits/confname.h
@@ -525,8 +525,14 @@  enum
 
     _SC_THREAD_ROBUST_PRIO_INHERIT,
 #define _SC_THREAD_ROBUST_PRIO_INHERIT	_SC_THREAD_ROBUST_PRIO_INHERIT
-    _SC_THREAD_ROBUST_PRIO_PROTECT
+    _SC_THREAD_ROBUST_PRIO_PROTECT,
 #define _SC_THREAD_ROBUST_PRIO_PROTECT	_SC_THREAD_ROBUST_PRIO_PROTECT
+
+    _SC_MINSIGSTKSZ,
+#define	_SC_MINSIGSTKSZ			_SC_MINSIGSTKSZ
+
+    _SC_SIGSTKSZ
+#define	_SC_SIGSTKSZ			_SC_SIGSTKSZ
   };
 
 /* Values for the NAME argument to `confstr'.  */
diff --git a/bits/sigstksz.h b/bits/sigstksz.h
new file mode 100644
index 0000000000..5535d873b5
--- /dev/null
+++ b/bits/sigstksz.h
@@ -0,0 +1,21 @@ 
+/* Definition of MINSIGSTKSZ.  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/>.  */
+
+#ifndef _SIGNAL_H
+# error "Never include <bits/sigstksz.h> directly; use <signal.h> instead."
+#endif
diff --git a/elf/dl-support.c b/elf/dl-support.c
index afbc94df54..18ac0bdd44 100644
--- a/elf/dl-support.c
+++ b/elf/dl-support.c
@@ -136,6 +136,8 @@  void (*_dl_init_static_tls) (struct link_map *) = &_dl_nothread_init_static_tls;
 
 size_t _dl_pagesize = EXEC_PAGESIZE;
 
+size_t _dl_minsigstacksize = MINSIGSTKSZ;
+
 int _dl_inhibit_cache;
 
 unsigned int _dl_osversion;
@@ -294,6 +296,9 @@  _dl_aux_init (ElfW(auxv_t) *av)
       case AT_RANDOM:
 	_dl_random = (void *) av->a_un.a_val;
 	break;
+      case AT_MINSIGSTKSZ:
+	GLRO(dl_minsigstacksize) = av->a_un.a_val;
+	break;
       DL_PLATFORM_AUXV
       }
   if (seen == 0xf)
diff --git a/elf/dl-sysdep.c b/elf/dl-sysdep.c
index 854570821c..2b1bce8cf5 100644
--- a/elf/dl-sysdep.c
+++ b/elf/dl-sysdep.c
@@ -117,6 +117,11 @@  _dl_sysdep_start (void **start_argptr,
   user_entry = (ElfW(Addr)) ENTRY_POINT;
   GLRO(dl_platform) = NULL; /* Default to nothing known about the platform.  */
 
+  /* NB: Default to a constant MINSIGSTKSZ.  */
+  _Static_assert (__builtin_constant_p (MINSIGSTKSZ),
+		  "MINSIGSTKSZ is constant");
+  GLRO(dl_minsigstacksize) = MINSIGSTKSZ;
+
   for (av = GLRO(dl_auxv); av->a_type != AT_NULL; set_seen (av++))
     switch (av->a_type)
       {
@@ -181,6 +186,9 @@  _dl_sysdep_start (void **start_argptr,
       case AT_RANDOM:
 	_dl_random = (void *) av->a_un.a_val;
 	break;
+      case AT_MINSIGSTKSZ:
+	GLRO(dl_minsigstacksize) = av->a_un.a_val;
+	break;
       DL_PLATFORM_AUXV
       }
 
@@ -308,6 +316,7 @@  _dl_show_auxv (void)
 	  [AT_SYSINFO_EHDR - 2] =	{ "SYSINFO_EHDR:      0x", hex },
 	  [AT_RANDOM - 2] =		{ "RANDOM:            0x", hex },
 	  [AT_HWCAP2 - 2] =		{ "HWCAP2:            0x", hex },
+	  [AT_MINSIGSTKSZ - 2] =	{ "MINSIGSTKSZ        ", dec },
 	  [AT_L1I_CACHESIZE - 2] =	{ "L1I_CACHESIZE:     ", dec },
 	  [AT_L1I_CACHEGEOMETRY - 2] =	{ "L1I_CACHEGEOMETRY: 0x", hex },
 	  [AT_L1D_CACHESIZE - 2] =	{ "L1D_CACHESIZE:     ", dec },
diff --git a/include/features.h b/include/features.h
index f3e62d3362..38b528e027 100644
--- a/include/features.h
+++ b/include/features.h
@@ -55,6 +55,8 @@ 
    _FORTIFY_SOURCE	Add security hardening to many library functions.
 			Set to 1 or 2; 2 performs stricter checks than 1.
 
+   _SC_SIGSTKSZ_SOURCE	Select non-constant MINSIGSTKSZ and SIGSTKSZ.
+
    _REENTRANT, _THREAD_SAFE
 			Obsolete; equivalent to _POSIX_C_SOURCE=199506L.
 
@@ -96,6 +98,7 @@ 
    __USE_ATFILE		Define *at interfaces and AT_* constants for them.
    __USE_GNU		Define GNU extensions.
    __USE_FORTIFY_LEVEL	Additional security measures used, according to level.
+   __USE_SC_SIGSTKSZ	Define non-constant MINSIGSTKSZ and SIGSTKSZ.
 
    The macros `__GNU_LIBRARY__', `__GLIBC__', and `__GLIBC_MINOR__' are
    defined by this file unconditionally.  `__GNU_LIBRARY__' is provided
@@ -139,6 +142,7 @@ 
 #undef	__USE_ATFILE
 #undef	__USE_GNU
 #undef	__USE_FORTIFY_LEVEL
+#undef	__USE_SC_SIGSTKSZ
 #undef	__KERNEL_STRICT_NAMES
 #undef	__GLIBC_USE_ISOC2X
 #undef	__GLIBC_USE_DEPRECATED_GETS
@@ -407,6 +411,12 @@ 
 # define __USE_FORTIFY_LEVEL 0
 #endif
 
+#ifdef	_SC_SIGSTKSZ_SOURCE
+# define __USE_SC_SIGSTKSZ	1
+#else
+# define __USE_SC_SIGSTKSZ	0
+#endif
+
 /* The function 'gets' existed in C89, but is impossible to use
    safely.  It has been removed from ISO C11 and ISO C++14.  Note: for
    compatibility with various implementations of <cstdio>, this test
diff --git a/manual/conf.texi b/manual/conf.texi
index f959b00bb6..ba9847aaa4 100644
--- a/manual/conf.texi
+++ b/manual/conf.texi
@@ -913,6 +913,27 @@  Inquire about the parameter corresponding to @code{NL_SETMAX}.
 @item _SC_NL_TEXTMAX
 @standards{X/Open, unistd.h}
 Inquire about the parameter corresponding to @code{NL_TEXTMAX}.
+
+@item _SC_MINSIGSTKSZ
+@standards{GNU, unistd.h}
+Inquire about the minimum number of bytes of free stack space required
+in order to guarantee successful, non-nested handling of a single signal
+whose handler is an empty function.
+
+@item _SC_SIGSTKSZ
+@standards{GNU, unistd.h}
+Inquire about the suggested minimum number of bytes of stack space
+required for a signal stack.
+
+This is not guaranteed to be enough for any specific purpose other than
+the invocation of a single, non-nested, empty handler, but nonetheless
+should be enough for basic scenarios involving simple signal handlers
+and very low levels of signal nesting (say, 2 or 3 levels at the very
+most).
+
+This value is provided for developer convenience and to ease migration
+from the legacy @code{SIGSTKSZ} constant.  Programs requiring stronger
+guarantees should avoid using it if at all possible.
 @end vtable
 
 @node Examples of Sysconf
diff --git a/manual/creature.texi b/manual/creature.texi
index be5050468b..21c6d3d5d0 100644
--- a/manual/creature.texi
+++ b/manual/creature.texi
@@ -257,6 +257,12 @@  various library functions.  If defined to @math{2}, even stricter
 checks are applied.
 @end defvr
 
+@defvr Macro _SC_SIGSTKSZ_SOURCE
+@standards{GNU, (none)}
+If this macro is defined, non-constant MINSIGSTKSZ and SIGSTKSZ are
+defined.
+@end defvr
+
 @defvr Macro _REENTRANT
 @defvrx Macro _THREAD_SAFE
 @standards{Obsolete, (none)}
diff --git a/posix/sysconf.c b/posix/sysconf.c
index ba303384c1..ca7833e6c4 100644
--- a/posix/sysconf.c
+++ b/posix/sysconf.c
@@ -266,6 +266,9 @@  __sysconf (int name)
     case _SC_XOPEN_REALTIME:
     case _SC_XOPEN_REALTIME_THREADS:
 
+    case _SC_MINSIGSTKSZ:
+    case _SC_SIGSTKSZ:
+
       break;
     }
 
diff --git a/signal/Makefile b/signal/Makefile
index 2ec3ddd74f..641d30582d 100644
--- a/signal/Makefile
+++ b/signal/Makefile
@@ -31,7 +31,8 @@  headers := signal.h sys/signal.h \
 	   bits/types/sigevent_t.h bits/types/siginfo_t.h \
 	   bits/types/sigset_t.h bits/types/sigval_t.h \
 	   bits/types/stack_t.h bits/types/struct_sigstack.h \
-	   bits/types/__sigval_t.h bits/signal_ext.h
+	   bits/types/__sigval_t.h bits/signal_ext.h \
+	   bits/sigstksz.h
 
 routines	:= signal raise killpg \
 		   sigaction sigprocmask kill \
@@ -48,7 +49,7 @@  routines	:= signal raise killpg \
 tests		:= tst-signal tst-sigset tst-sigsimple tst-raise tst-sigset2 \
 		   tst-sigwait-eintr tst-sigaction \
 		   tst-minsigstksz-1 tst-minsigstksz-2 tst-minsigstksz-3 \
-		   tst-minsigstksz-3a tst-minsigstksz-4 \
+		   tst-minsigstksz-3a tst-minsigstksz-4 tst-minsigstksz-5 \
 		   tst-sigisemptyset
 
 include ../Rules
diff --git a/signal/signal.h b/signal/signal.h
index effe3d698f..0311eb2a66 100644
--- a/signal/signal.h
+++ b/signal/signal.h
@@ -312,6 +312,7 @@  extern int siginterrupt (int __sig, int __interrupt) __THROW
   __attribute_deprecated_msg__ ("Use sigaction with SA_RESTART instead");
 
 # include <bits/sigstack.h>
+# include <bits/sigstksz.h>
 # include <bits/ss_flags.h>
 
 /* Alternate signal handler stack interface.
diff --git a/signal/tst-minsigstksz-5.c b/signal/tst-minsigstksz-5.c
new file mode 100644
index 0000000000..d657d2f4e6
--- /dev/null
+++ b/signal/tst-minsigstksz-5.c
@@ -0,0 +1,84 @@ 
+/* Test of signal delivery on an alternate stack with MINSIGSTKSZ size.
+   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 <signal.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <support/check.h>
+#include <support/support.h>
+
+static volatile sig_atomic_t handler_run;
+
+static void
+handler (int signo)
+{
+  /* Clear a bit of on-stack memory.  */
+  volatile char buffer[256];
+  for (size_t i = 0; i < sizeof (buffer); ++i)
+    buffer[i] = 0;
+  handler_run = 1;
+}
+
+int
+do_test (void)
+{
+  size_t stack_buffer_size = 64 * 1024 * 1024;
+  void *stack_buffer = xmalloc (stack_buffer_size);
+  void *stack_end = stack_buffer + stack_buffer_size;
+  memset (stack_buffer, 0xCC, stack_buffer_size);
+
+  void *stack_bottom = stack_buffer + (stack_buffer_size + MINSIGSTKSZ) / 2;
+  void *stack_top = stack_bottom + MINSIGSTKSZ;
+  stack_t stack =
+    {
+      .ss_sp = stack_bottom,
+      .ss_size = MINSIGSTKSZ,
+    };
+  if (sigaltstack (&stack, NULL) < 0)
+    FAIL_RET ("sigaltstack: %m\n");
+
+  struct sigaction act =
+    {
+      .sa_handler = handler,
+      .sa_flags = SA_ONSTACK,
+    };
+  if (sigaction (SIGUSR1, &act, NULL) < 0)
+    FAIL_RET ("sigaction: %m\n");
+
+  if (kill (getpid (), SIGUSR1) < 0)
+    FAIL_RET ("kill: %m\n");
+
+  if (handler_run != 1)
+    FAIL_RET ("handler did not run\n");
+
+  for (void *p = stack_buffer; p < stack_bottom; ++p)
+    if (*(unsigned char *) p != 0xCC)
+      FAIL_RET ("changed byte %ld bytes below configured stack\n",
+		(long) (stack_bottom - p));
+  for (void *p = stack_top; p < stack_end; ++p)
+    if (*(unsigned char *) p != 0xCC)
+      FAIL_RET ("changed byte %ld bytes above configured stack\n",
+		(long) (p - stack_top));
+
+  free (stack_buffer);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 382eeb9be0..5ce847aeac 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -528,6 +528,9 @@  struct rtld_global_ro
   /* Cached value of `getpagesize ()'.  */
   EXTERN size_t _dl_pagesize;
 
+  /* Cached value of `sysconf (_SC_MINSIGSTKSZ)'.  */
+  EXTERN size_t _dl_minsigstacksize;
+
   /* Do we read from ld.so.cache?  */
   EXTERN int _dl_inhibit_cache;
 
diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index d760debf40..a17ca28de8 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -190,6 +190,9 @@  sysdep_routines += ntp_gettime ntp_gettimex
 endif
 
 ifeq ($(subdir),signal)
+# Compile tst-minsigstksz-5.c with _SC_SIGSTKSZ_SOURCE.
+CFLAGS-tst-minsigstksz-5.c += -D_SC_SIGSTKSZ_SOURCE
+
 tests-special += $(objpfx)tst-signal-numbers.out
 # Depending on signal.o* is a hack.  What we actually want is a dependency
 # on signal.h and everything it includes.  That's impractical to write
@@ -228,6 +231,11 @@  ifeq ($(subdir),sunrpc)
 sysdep_headers += nfs/nfs.h
 endif
 
+ifeq ($(subdir),support)
+# Compile xsigstack.c with _SC_SIGSTKSZ_SOURCE.
+CFLAGS-xsigstack.c += -D_SC_SIGSTKSZ_SOURCE
+endif
+
 ifeq ($(subdir),termios)
 sysdep_headers += termio.h
 endif
diff --git a/sysdeps/unix/sysv/linux/bits/sigstksz.h b/sysdeps/unix/sysv/linux/bits/sigstksz.h
new file mode 100644
index 0000000000..cd5b3cc895
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/bits/sigstksz.h
@@ -0,0 +1,33 @@ 
+/* Definition of MINSIGSTKSZ and SIGSTKSZ.  Linux 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/>.  */
+
+#ifndef _SIGNAL_H
+# error "Never include <bits/sigstksz.h> directly; use <signal.h> instead."
+#endif
+
+#if __USE_SC_SIGSTKSZ
+# include <unistd.h>
+
+/* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ).  */
+# undef SIGSTKSZ
+# define SIGSTKSZ sysconf (_SC_SIGSTKSZ)
+
+/* Minimum stack size for a signal handler: SIGSTKSZ.  */
+# undef MINSIGSTKSZ
+# define MINSIGSTKSZ SIGSTKSZ
+#endif
diff --git a/sysdeps/unix/sysv/linux/ia64/sysconf-sigstksz.h b/sysdeps/unix/sysv/linux/ia64/sysconf-sigstksz.h
new file mode 100644
index 0000000000..7e5ceba151
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/ia64/sysconf-sigstksz.h
@@ -0,0 +1,27 @@ 
+/* sysconf_sigstksz ().  Linux/ia64 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/>.  */
+
+/* Return sysconf (_SC_SIGSTKSZ).  */
+
+static long int
+sysconf_sigstksz (void)
+{
+  _Static_assert (__builtin_constant_p (SIGSTKSZ),
+		  "SIGSTKSZ is constant");
+  return SIGSTKSZ;
+}
diff --git a/sysdeps/unix/sysv/linux/sysconf-sigstksz.h b/sysdeps/unix/sysv/linux/sysconf-sigstksz.h
new file mode 100644
index 0000000000..64d450b22c
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/sysconf-sigstksz.h
@@ -0,0 +1,38 @@ 
+/* sysconf_sigstksz ().  Linux 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/>.  */
+
+/* Return sysconf (_SC_SIGSTKSZ).  */
+
+static long int
+sysconf_sigstksz (void)
+{
+  long int minsigstacksize = GLRO(dl_minsigstacksize);
+  assert (minsigstacksize != 0);
+  _Static_assert (__builtin_constant_p (MINSIGSTKSZ),
+		  "MINSIGSTKSZ is constant");
+  if (minsigstacksize < MINSIGSTKSZ)
+    minsigstacksize = MINSIGSTKSZ;
+  /* MAX (MINSIGSTKSZ, sysconf (_SC_MINSIGSTKSZ)) * 4.  */
+  long int sigstacksize = minsigstacksize * 4;
+  /* Return MAX (SIGSTKSZ, sigstacksize).  */
+  _Static_assert (__builtin_constant_p (SIGSTKSZ),
+		  "SIGSTKSZ is constant");
+  if (sigstacksize < SIGSTKSZ)
+    sigstacksize = SIGSTKSZ;
+  return sigstacksize;
+}
diff --git a/sysdeps/unix/sysv/linux/sysconf.c b/sysdeps/unix/sysv/linux/sysconf.c
index 7958a74164..d1d86df3fb 100644
--- a/sysdeps/unix/sysv/linux/sysconf.c
+++ b/sysdeps/unix/sysv/linux/sysconf.c
@@ -16,6 +16,7 @@ 
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <stdlib.h>
@@ -26,6 +27,7 @@ 
 #include <sys/param.h>
 #include <not-cancel.h>
 #include <ldsodefs.h>
+#include <sysconf-sigstksz.h>
 
 /* Legacy value of ARG_MAX.  The macro is now not defined since the
    actual value varies based on the stack size.  */
@@ -75,6 +77,13 @@  __sysconf (int name)
       }
       break;
 
+    case _SC_MINSIGSTKSZ:
+      assert (GLRO(dl_minsigstacksize) != 0);
+      return GLRO(dl_minsigstacksize);
+
+    case _SC_SIGSTKSZ:
+      return sysconf_sigstksz ();
+
     default:
       break;
     }
diff --git a/sysdeps/unix/sysv/linux/x86/dl-minsigstacksize.h b/sysdeps/unix/sysv/linux/x86/dl-minsigstacksize.h
new file mode 100644
index 0000000000..d2dc436572
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86/dl-minsigstacksize.h
@@ -0,0 +1,77 @@ 
+/* Emulate AT_MINSIGSTKSZ.  Linux/x86 version.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   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 <unistd.h>
+
+/* Emulate AT_MINSIGSTKSZ with XSAVE. */
+
+static inline void
+dl_check_minsigstacksize (void)
+{
+  /* NB: Default to a constant MINSIGSTKSZ.  */
+  _Static_assert (__builtin_constant_p (MINSIGSTKSZ),
+		  "MINSIGSTKSZ is constant");
+  /* Return if AT_MINSIGSTKSZ is provide by kernel.  */
+  if (GLRO(dl_minsigstacksize) != MINSIGSTKSZ)
+    return;
+
+  /* Emulate AT_MINSIGSTKSZ.  In Linux kernel, the signal frame data
+     with XSAVE is composed of the following areas and laid out as:
+     ------------------------------
+     | alignment padding          |
+     ------------------------------
+     | xsave buffer               |
+     ------------------------------
+     | fsave header (32-bit only) |
+     ------------------------------
+     | siginfo + ucontext         |
+     ------------------------------
+ */
+
+  unsigned int sigframe_size;
+
+#ifdef __x86_64__
+  /* NB: sizeof(struct rt_sigframe) + 8-byte return address in Linux
+     kernel.  */
+  sigframe_size = 440 + 8;
+#else
+  /* NB: sizeof(struct sigframe_ia32) + sizeof(struct fregs_state)) +
+     4-byte return address + 3 * 4-byte arguments in Linux kernel.  */
+  sigframe_size = 736 + 112 + 4 + 3 * 4;
+#endif
+
+  /* Add 15 bytes to align the stack to 16 bytes.  */
+  sigframe_size += 15;
+
+  /* Make the space before xsave buffer multiple of 16 bytes.  */
+  sigframe_size = ALIGN_UP (sigframe_size, 16);
+
+  /* Add (64 - 16)-byte padding to align xsave buffer at 64 bytes.  */
+  sigframe_size += 64 - 16;
+
+  unsigned int eax, ebx, ecx, edx;
+  __cpuid_count (0xd, 0, eax, ebx, ecx, edx);
+
+  /* Add the size of xsave buffer.  */
+  sigframe_size += ebx;
+
+  /* Add the size of FP_XSTATE_MAGIC2.  */
+#define FP_XSTATE_MAGIC2 0x46505845U
+  sigframe_size += sizeof (FP_XSTATE_MAGIC2);
+
+  GLRO(dl_minsigstacksize) = sigframe_size;
+}
diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
index 286ff96771..c80ab05cbd 100644
--- a/sysdeps/x86/cpu-features.c
+++ b/sysdeps/x86/cpu-features.c
@@ -20,6 +20,7 @@ 
 #include <cpu-features.h>
 #include <dl-hwcap.h>
 #include <libc-pointer-arith.h>
+#include <dl-minsigstacksize.h>
 
 #if HAVE_TUNABLES
 # define TUNABLE_NAMESPACE cpu
@@ -350,11 +351,16 @@  get_common_indices (struct cpu_features *cpu_features,
     }
 
   if (cpu_features->basic.max_cpuid >= 0xd)
-    __cpuid_count (0xd, 1,
-		   cpu_features->features[COMMON_CPUID_INDEX_D_ECX_1].cpuid.eax,
-		   cpu_features->features[COMMON_CPUID_INDEX_D_ECX_1].cpuid.ebx,
-		   cpu_features->features[COMMON_CPUID_INDEX_D_ECX_1].cpuid.ecx,
-		   cpu_features->features[COMMON_CPUID_INDEX_D_ECX_1].cpuid.edx);
+    {
+      /* NB: OSXSAVE has been set in COMMON_CPUID_INDEX_1 above.  */
+      if (CPU_FEATURES_CPU_P (cpu_features, OSXSAVE))
+	dl_check_minsigstacksize ();
+      __cpuid_count (0xd, 1,
+		     cpu_features->features[COMMON_CPUID_INDEX_D_ECX_1].cpuid.eax,
+		     cpu_features->features[COMMON_CPUID_INDEX_D_ECX_1].cpuid.ebx,
+		     cpu_features->features[COMMON_CPUID_INDEX_D_ECX_1].cpuid.ecx,
+		     cpu_features->features[COMMON_CPUID_INDEX_D_ECX_1].cpuid.edx);
+    }
 
   if (cpu_features->basic.max_cpuid >= 0x19)
     __cpuid_count (0x19, 0,
diff --git a/sysdeps/x86/dl-minsigstacksize.h b/sysdeps/x86/dl-minsigstacksize.h
new file mode 100644
index 0000000000..da2e40ffd4
--- /dev/null
+++ b/sysdeps/x86/dl-minsigstacksize.h
@@ -0,0 +1,23 @@ 
+/* Emulate AT_MINSIGSTKSZ.  Generic x86 version.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   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/>.  */
+
+/* Emulate AT_MINSIGSTKSZ with XSAVE. */
+
+static inline void
+dl_check_minsigstacksize (void)
+{
+}
-- 
2.26.2