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.

2 files changed, 28 insertions, 1 deletions

diff --git a/bits/confname.h b/bits/confname.h
index 8068bda273..ec6cd07957 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