diff mbox series

V8 [PATCH] sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]

Message ID CAMe9rOrb_0Sf9+8qHH+LmbHAuFg6MRQdwnWTQWmhDrw3GdAuGw@mail.gmail.com
State Superseded
Headers show
Series V8 [PATCH] sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305] | expand

Commit Message

H.J. Lu Nov. 20, 2020, 11:13 p.m. UTC
On Fri, Nov 20, 2020 at 6:12 AM Florian Weimer <fw@deneb.enyo.de> wrote:
>
> * H. J. Lu:
>
> > Can we reach a consensus for _GNU_SOURCE vs _SC_SIGSTKSZ_SOURCE?
> >
> > How about we make _GNU_SOURCE to enable _SC_SIGSTKSZ_SOURCE and
> > check _SC_SIGSTKSZ_SOURCE != 0?  If _GNU_SOURCE triggers compilation
> > error and source codes can't be changed, we can add
> > -D_SC_SIGSTKSZ_SOURCE=0 to disable _SC_SIGSTKSZ_SOURCE.
>
> I think the source code compatibility is much more obscure than the
> other things we broke quite recently, even without deprecation.  Why
> do we add the complexity in this case?  I feel it is disproportional.

Here is the patch with _GNU_SOURCE.  Any comments?

Comments

Carlos O'Donell Jan. 20, 2021, 2:16 p.m. UTC | #1
On 11/20/20 6:13 PM, H.J. Lu via Libc-alpha wrote:
> On Fri, Nov 20, 2020 at 6:12 AM Florian Weimer <fw@deneb.enyo.de> wrote:
>>
>> * H. J. Lu:
>>
>>> Can we reach a consensus for _GNU_SOURCE vs _SC_SIGSTKSZ_SOURCE?
>>>
>>> How about we make _GNU_SOURCE to enable _SC_SIGSTKSZ_SOURCE and
>>> check _SC_SIGSTKSZ_SOURCE != 0?  If _GNU_SOURCE triggers compilation
>>> error and source codes can't be changed, we can add
>>> -D_SC_SIGSTKSZ_SOURCE=0 to disable _SC_SIGSTKSZ_SOURCE.
>>
>> I think the source code compatibility is much more obscure than the
>> other things we broke quite recently, even without deprecation.  Why
>> do we add the complexity in this case?  I feel it is disproportional.
> 
> Here is the patch with _GNU_SOURCE.  Any comments?

(1) New version of patch.
 
This patch no longer compiles due to __cpuid_count errors.

Could you please update this and post a v9?

This is ABI addition for the constants so I want to make sure I review
something that works and is final from your end.

(2) Kernel definition of constant for AT_MINSIGSTKSZ?

We don't have a released kernel for x86 with this constant defined?

The kernel patch was only just updated 5 days ago:
https://sourceware.org/pipermail/libc-alpha/2021-January/121671.html

Doesn't this mean this patch is inappropriate for glibc 2.33?
Florian Weimer Jan. 20, 2021, 3:06 p.m. UTC | #2
* Carlos O'Donell via Libc-alpha:

> (2) Kernel definition of constant for AT_MINSIGSTKSZ?
>
> We don't have a released kernel for x86 with this constant defined?

I think it's in the generic headers.  Don't you see it there?

Thanks,
Florian
Carlos O'Donell Jan. 20, 2021, 3:30 p.m. UTC | #3
On 1/20/21 10:06 AM, Florian Weimer wrote:
> * Carlos O'Donell via Libc-alpha:
> 
>> (2) Kernel definition of constant for AT_MINSIGSTKSZ?
>>
>> We don't have a released kernel for x86 with this constant defined?
> 
> I think it's in the generic headers.  Don't you see it there?

I do not see it there.

I'm at commit 45dfb8a5659ad286c28fa59008271dbc4e5e3f2d on:
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
(which I use for building glibc).

The constant value for AT_MINSIGSTKSZ is not generic.

It is defined by each architecture in per-architecture UAPI headers.

As of today it's only valid for aarch64 (arm64) i.e.
arch/arm64/include/uapi/asm/auxvec.h:#define AT_MINSIGSTKSZ	51	/* stack needed for signal delivery */

The fact that Intel will define it as 51 also is not guaranteed
until a released kernel contains a final value.

It has not been proposed for: include/uapi/linux/auxvec.h, and
so has no generic value that can be used by glibc.

My opinion is that the following needs to happen before we can
adopt this in glibc:

* Get AT_MINSIGSTKSZ into the generic auxvec.h in linux.
* Release a kernel with this value.
* Update glibc to follow with support in the next release.

There is a reason we wait for the kernel to adopt these values
because that's the agreed upon protocol and expectations from
developers on both sides.

What is our next step?
H.J. Lu Jan. 20, 2021, 3:33 p.m. UTC | #4
On Wed, Jan 20, 2021 at 7:30 AM Carlos O'Donell <carlos@redhat.com> wrote:
>
> On 1/20/21 10:06 AM, Florian Weimer wrote:
> > * Carlos O'Donell via Libc-alpha:
> >
> >> (2) Kernel definition of constant for AT_MINSIGSTKSZ?
> >>
> >> We don't have a released kernel for x86 with this constant defined?
> >
> > I think it's in the generic headers.  Don't you see it there?
>
> I do not see it there.
>
> I'm at commit 45dfb8a5659ad286c28fa59008271dbc4e5e3f2d on:
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> (which I use for building glibc).
>
> The constant value for AT_MINSIGSTKSZ is not generic.
>
> It is defined by each architecture in per-architecture UAPI headers.
>
> As of today it's only valid for aarch64 (arm64) i.e.
> arch/arm64/include/uapi/asm/auxvec.h:#define AT_MINSIGSTKSZ     51      /* stack needed for signal delivery */
>
> The fact that Intel will define it as 51 also is not guaranteed
> until a released kernel contains a final value.
>
> It has not been proposed for: include/uapi/linux/auxvec.h, and
> so has no generic value that can be used by glibc.
>
> My opinion is that the following needs to happen before we can
> adopt this in glibc:
>
> * Get AT_MINSIGSTKSZ into the generic auxvec.h in linux.
> * Release a kernel with this value.
> * Update glibc to follow with support in the next release.

It has been in glibc since 2018:

commit 174195409d6a3d639dd28e08cf99d6f2559eca9c
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Aug 20 15:46:26 2018 +0000

    Add NT_VMCOREDD, AT_MINSIGSTKSZ from Linux 4.18 to elf.h.

    This patch adds two new constants from Linux 4.18 to elf.h,
    NT_VMCOREDD and AT_MINSIGSTKSZ.

    Tested for x86_64.

            * elf/elf.c (NT_VMCOREDD): New macro.
            (AT_MINSIGSTKSZ): Likewise.

> There is a reason we wait for the kernel to adopt these values
> because that's the agreed upon protocol and expectations from
> developers on both sides.
>
> What is our next step?
>
> --
> Cheers,
> Carlos.
>
Florian Weimer Jan. 20, 2021, 3:33 p.m. UTC | #5
* Carlos O'Donell:

> On 1/20/21 10:06 AM, Florian Weimer wrote:
>> * Carlos O'Donell via Libc-alpha:
>> 
>>> (2) Kernel definition of constant for AT_MINSIGSTKSZ?
>>>
>>> We don't have a released kernel for x86 with this constant defined?
>> 
>> I think it's in the generic headers.  Don't you see it there?
>
> I do not see it there.
>
> I'm at commit 45dfb8a5659ad286c28fa59008271dbc4e5e3f2d on:
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> (which I use for building glibc).
>
> The constant value for AT_MINSIGSTKSZ is not generic.
>
> It is defined by each architecture in per-architecture UAPI headers.
>
> As of today it's only valid for aarch64 (arm64) i.e.
> arch/arm64/include/uapi/asm/auxvec.h:#define AT_MINSIGSTKSZ	51	/* stack needed for signal delivery */
>
> The fact that Intel will define it as 51 also is not guaranteed
> until a released kernel contains a final value.

But it's in our <elf.h>.

Thanks,
Florian
Carlos O'Donell Jan. 20, 2021, 3:59 p.m. UTC | #6
On 1/20/21 10:33 AM, H.J. Lu wrote:
> It has been in glibc since 2018:
> 
> commit 174195409d6a3d639dd28e08cf99d6f2559eca9c
> Author: Joseph Myers <joseph@codesourcery.com>
> Date:   Mon Aug 20 15:46:26 2018 +0000
> 
>     Add NT_VMCOREDD, AT_MINSIGSTKSZ from Linux 4.18 to elf.h.
> 
>     This patch adds two new constants from Linux 4.18 to elf.h,
>     NT_VMCOREDD and AT_MINSIGSTKSZ.
> 
>     Tested for x86_64.
> 
>             * elf/elf.c (NT_VMCOREDD): New macro.
>             (AT_MINSIGSTKSZ): Likewise.

Thanks.

In which case this patch is wrong?
https://sourceware.org/pipermail/libc-alpha/2021-January/121673.html

We should be putting the constant in the generic headers for use by
all architectures?

Do we all, including Linux kernel developers, agree that this constant
is now effectively ABI for *all* architectures?

My plan now is:
- Request the Intel kernel patch change the generic auxvec.h in linux
  to use a constant value 51 for all arches for AT_MINSIGSTKSZ.
- Review the V9 patch.

Thoughts?
H.J. Lu Jan. 20, 2021, 4:04 p.m. UTC | #7
On Wed, Jan 20, 2021 at 7:59 AM Carlos O'Donell <carlos@redhat.com> wrote:
>
> On 1/20/21 10:33 AM, H.J. Lu wrote:
> > It has been in glibc since 2018:
> >
> > commit 174195409d6a3d639dd28e08cf99d6f2559eca9c
> > Author: Joseph Myers <joseph@codesourcery.com>
> > Date:   Mon Aug 20 15:46:26 2018 +0000
> >
> >     Add NT_VMCOREDD, AT_MINSIGSTKSZ from Linux 4.18 to elf.h.
> >
> >     This patch adds two new constants from Linux 4.18 to elf.h,
> >     NT_VMCOREDD and AT_MINSIGSTKSZ.
> >
> >     Tested for x86_64.
> >
> >             * elf/elf.c (NT_VMCOREDD): New macro.
> >             (AT_MINSIGSTKSZ): Likewise.
>
> Thanks.
>
> In which case this patch is wrong?
> https://sourceware.org/pipermail/libc-alpha/2021-January/121673.html
>
> We should be putting the constant in the generic headers for use by
> all architectures?
>
> Do we all, including Linux kernel developers, agree that this constant
> is now effectively ABI for *all* architectures?
>
> My plan now is:
> - Request the Intel kernel patch change the generic auxvec.h in linux
>   to use a constant value 51 for all arches for AT_MINSIGSTKSZ.

CC Chang.  Sounds good to me.

> - Review the V9 patch.
>
> Thoughts?
>

Sounds reasonable.

Thanks.
diff mbox series

Patch

From 7ce7a0e753edd5fbda65cec987d681c65e86960d 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 or _GNU_SOURCE are 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/bits/sigstack.h                       |  5 ++
 include/bits/sigstksz.h                       |  7 ++
 include/features.h                            | 11 +++
 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/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  | 83 ++++++++++++++++++
 .../sysv/linux/x86/include/bits/sigstack.h    |  5 ++
 sysdeps/x86/cpu-features.c                    |  3 +
 sysdeps/x86/dl-minsigstacksize.h              | 27 ++++++
 23 files changed, 416 insertions(+), 3 deletions(-)
 create mode 100644 bits/sigstksz.h
 create mode 100644 include/bits/sigstack.h
 create mode 100644 include/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/unix/sysv/linux/x86/include/bits/sigstack.h
 create mode 100644 sysdeps/x86/dl-minsigstacksize.h

diff --git a/NEWS b/NEWS
index 0335fb98e5..759a88ce5c 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 or
+  _GNU_SOURCE are 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 15e6d787c9..dd631f50c4 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 = CONSTANT_MINSIGSTKSZ;
+
 int _dl_inhibit_cache;
 
 unsigned int _dl_osversion;
@@ -295,6 +297,9 @@  _dl_aux_init (ElfW(auxv_t) *av)
       case AT_RANDOM:
 	_dl_random = (void *) av->a_un.a_val;
 	break;
+      case AT_MINSIGSTKSZ:
+	_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..47400f2937 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 CONSTANT_MINSIGSTKSZ.  */
+  _Static_assert (__builtin_constant_p (CONSTANT_MINSIGSTKSZ),
+		  "CONSTANT_MINSIGSTKSZ is constant");
+  GLRO(dl_minsigstacksize) = CONSTANT_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/bits/sigstack.h b/include/bits/sigstack.h
new file mode 100644
index 0000000000..eea939f59d
--- /dev/null
+++ b/include/bits/sigstack.h
@@ -0,0 +1,5 @@ 
+#include_next <bits/sigstack.h>
+
+#if !defined _ISOMAC && !defined CONSTANT_MINSIGSTKSZ
+# define CONSTANT_MINSIGSTKSZ MINSIGSTKSZ
+#endif
diff --git a/include/bits/sigstksz.h b/include/bits/sigstksz.h
new file mode 100644
index 0000000000..2ca891e918
--- /dev/null
+++ b/include/bits/sigstksz.h
@@ -0,0 +1,7 @@ 
+/* NB: Don't define MINSIGSTKSZ nor SIGSTKSZ to sysconf (SC_SIGSTKSZ) for
+   glibc build.  IS_IN can only be used when _ISOMAC isn't defined.  */
+#ifdef _ISOMAC
+# include_next <bits/sigstksz.h>
+#elif IS_IN (libsupport)
+# include_next <bits/sigstksz.h>
+#endif
diff --git a/include/features.h b/include/features.h
index f3e62d3362..deaf50928b 100644
--- a/include/features.h
+++ b/include/features.h
@@ -48,6 +48,8 @@ 
    _LARGEFILE64_SOURCE	Additional functionality from LFS for large files.
    _FILE_OFFSET_BITS=N	Select default filesystem interface.
    _ATFILE_SOURCE	Additional *at interfaces.
+   _SC_SIGSTKSZ_SOURCE	Select correct (but non compile-time constant)
+			MINSIGSTKSZ and SIGSTKSZ.
    _GNU_SOURCE		All of the above, plus GNU extensions.
    _DEFAULT_SOURCE	The default set of features (taking precedence over
 			__STRICT_ANSI__).
@@ -94,6 +96,8 @@ 
    __USE_FILE_OFFSET64	Define 64bit interface as default.
    __USE_MISC		Define things from 4.3BSD or System V Unix.
    __USE_ATFILE		Define *at interfaces and AT_* constants for them.
+   __USE_SC_SIGSTKSZ	Define correct (but non compile-time constant)
+			MINSIGSTKSZ and SIGSTKSZ.
    __USE_GNU		Define GNU extensions.
    __USE_FORTIFY_LEVEL	Additional security measures used, according to level.
 
@@ -137,6 +141,7 @@ 
 #undef	__USE_FILE_OFFSET64
 #undef	__USE_MISC
 #undef	__USE_ATFILE
+#undef	__USE_SC_SIGSTKSZ
 #undef	__USE_GNU
 #undef	__USE_FORTIFY_LEVEL
 #undef	__KERNEL_STRICT_NAMES
@@ -213,6 +218,8 @@ 
 # define _DEFAULT_SOURCE	1
 # undef  _ATFILE_SOURCE
 # define _ATFILE_SOURCE	1
+# undef  _SC_SIGSTKSZ_SOURCE
+# define _SC_SIGSTKSZ_SOURCE 1
 #endif
 
 /* If nothing (other than _GNU_SOURCE and _DEFAULT_SOURCE) is defined,
@@ -388,6 +395,10 @@ 
 # define __USE_ATFILE	1
 #endif
 
+#ifdef	_SC_SIGSTKSZ_SOURCE
+# define __USE_SC_SIGSTKSZ	1
+#endif
+
 #ifdef	_GNU_SOURCE
 # define __USE_GNU	1
 #endif
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..8fef4150fa 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, correct (but non compile-time 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 b1da03cafe..38093e2b8a 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -536,6 +536,9 @@  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/bits/sigstksz.h b/sysdeps/unix/sysv/linux/bits/sigstksz.h
new file mode 100644
index 0000000000..926508f2b4
--- /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 defined __USE_SC_SIGSTKSZ && __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 9c5a4cb1a4..22eeac50ca 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..6088bbc99e
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86/dl-minsigstacksize.h
@@ -0,0 +1,83 @@ 
+/* 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/>.  */
+
+/* Emulate AT_MINSIGSTKSZ with XSAVE. */
+
+static inline void
+dl_check_minsigstacksize (const struct cpu_features *cpu_features)
+{
+  /* Return if AT_MINSIGSTKSZ is provide by kernel.  */
+  if (GLRO(dl_minsigstacksize) != 0)
+    return;
+
+  if (cpu_features->basic.max_cpuid >= 0xd
+      && CPU_FEATURES_CPU_P (cpu_features, OSXSAVE))
+    {
+      /* 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;
+    }
+  else
+    {
+      /* NB: Default to a constant MINSIGSTKSZ.  */
+      _Static_assert (__builtin_constant_p (MINSIGSTKSZ),
+		      "MINSIGSTKSZ is constant");
+      GLRO(dl_minsigstacksize) = MINSIGSTKSZ;
+    }
+}
diff --git a/sysdeps/unix/sysv/linux/x86/include/bits/sigstack.h b/sysdeps/unix/sysv/linux/x86/include/bits/sigstack.h
new file mode 100644
index 0000000000..208754c497
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86/include/bits/sigstack.h
@@ -0,0 +1,5 @@ 
+#include_next <bits/sigstack.h>
+
+#ifndef _ISOMAC
+# define CONSTANT_MINSIGSTKSZ 0
+#endif
diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
index f26deba38d..bef6084f5e 100644
--- a/sysdeps/x86/cpu-features.c
+++ b/sysdeps/x86/cpu-features.c
@@ -19,6 +19,7 @@ 
 #include <cpuid.h>
 #include <dl-hwcap.h>
 #include <libc-pointer-arith.h>
+#include <dl-minsigstacksize.h>
 #if IS_IN (libc) && !defined SHARED
 # include <assert.h>
 # include <unistd.h>
@@ -367,6 +368,8 @@  get_common_indices (struct cpu_features *cpu_features,
 		   cpu_features->features[COMMON_CPUID_INDEX_19].cpuid.ebx,
 		   cpu_features->features[COMMON_CPUID_INDEX_19].cpuid.ecx,
 		   cpu_features->features[COMMON_CPUID_INDEX_19].cpuid.edx);
+
+  dl_check_minsigstacksize (cpu_features);
 }
 
 _Static_assert (((index_arch_Fast_Unaligned_Load
diff --git a/sysdeps/x86/dl-minsigstacksize.h b/sysdeps/x86/dl-minsigstacksize.h
new file mode 100644
index 0000000000..959871c970
--- /dev/null
+++ b/sysdeps/x86/dl-minsigstacksize.h
@@ -0,0 +1,27 @@ 
+/* 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 (const struct cpu_features *cpu_features)
+{
+  /* NB: Default to a constant MINSIGSTKSZ.  */
+  _Static_assert (__builtin_constant_p (MINSIGSTKSZ),
+		  "MINSIGSTKSZ is constant");
+  GLRO(dl_minsigstacksize) = MINSIGSTKSZ;
+}
-- 
2.28.0