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

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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
This commit is contained in:
Mike Frysinger 2017-03-15 23:59:31 -07:00
parent 622222846a
commit 21f042c804
2 changed files with 11 additions and 0 deletions
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6
ChangeLog
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@ -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.
2017-04-01 Wladimir van der Laan <laanwj@gmail.com>
[BZ# 21338]

5
sysdeps/unix/sysv/linux/spawni.c
View file

@ -319,6 +319,11 @@ __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);