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.

3 files changed, 23 insertions, 0 deletions

diff --git a/include/bits/sigstack.h b/include/bits/sigstack.h
new file mode 100644
index 0000000..eea939f
--- /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 0000000..2ca891e
--- /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 c6ff971..eb97470 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