posix_spawn: use a larger min stack for -fstack-check [BZ #21253]

When glibc is built with -fstack-check, trying to use posix_spawn can
lead to segfaults due to gcc internally probing stack memory too far.
The new spawn API will allocate a minimum of 1 page, but the stack
checking logic might probe a couple of pages.  When it tries to walk
them, everything falls apart.

The gcc internal docs [1] state the default interval checking is one
page.  Which means we need two pages (the current one, and the next
probed).  No target currently defines it larger.

Further, it mentions that the default minimum stack size needed to
recover from an overflow is 4/8KiB for sjlj or 8/12KiB for others.
But some Linux targets (like mips and ppc) go up to 16KiB (and some
non-Linux targets go up to 24KiB).

Let's create each child with a minimum of 32KiB slack space to support
them all, and give us future breathing room.

No test is added as existing ones crash.  Even a simple call is
enough to trigger the problem:
	char *argv[] = { "/bin/ls", NULL };
	posix_spawn(NULL, "/bin/ls", NULL, NULL, argv, NULL);

[1] https://gcc.gnu.org/onlinedocs/gcc-6.3.0/gccint/Stack-Checking.html

(cherry picked from commit 21f042c804835d1f7a4a8e06f2c93ca35a182042)

diff --git a/ChangeLog b/ChangeLog
index dfa9537d9019371b2e7eccdad5e146b1b08d3cf7..9047f652fe16205de8905b9d978ff70277e7dd22 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2017-04-03  Mike Frysinger  <vapier@gentoo.org>
+
+       [BZ #21253]
+       * sysdeps/unix/sysv/linux/spawni.c (__spawnix): Increase argv_size
+       slack space by 32KiB.
+
 2017-03-31  Slava Barinov  <v.barinov@samsung.com>
 
        [BZ #21289]

diff --git a/sysdeps/unix/sysv/linux/spawni.c b/sysdeps/unix/sysv/linux/spawni.c
index c946120c3443106603e20b112d9204f085fe6e1f..b5f20a710bc580075a4423f271d820f5eb6b1dcd 100644 (file)
--- a/sysdeps/unix/sysv/linux/spawni.c
+++ b/sysdeps/unix/sysv/linux/spawni.c
@@ -326,6 +326,11 @@ __spawnix (pid_t * pid, const char *file,
 
   /* Add a slack area for child's stack.  */
   size_t argv_size = (argc * sizeof (void *)) + 512;
+  /* We need at least a few pages in case the compiler's stack checking is
+     enabled.  In some configs, it is known to use at least 24KiB.  We use
+     32KiB to be "safe" from anything the compiler might do.  Besides, the
+     extra pages won't actually be allocated unless they get used.  */
+  argv_size += (32 * 1024);
   size_t stack_size = ALIGN_UP (argv_size, GLRO(dl_pagesize));
   void *stack = __mmap (NULL, stack_size, prot,
                        MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);