GNU bug report logs - #21004
25.0.50; Questionable call to getrlimit in handle_sigsegv

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Ken Brown <kbrown <at> cornell.edu>
To: bug-gnu-emacs <at> gnu.org
Subject: 25.0.50; Questionable call to getrlimit in handle_sigsegv
Date: Tue, 07 Jul 2015 16:11:53 -0400

There is a call to setrlimit in main() that tries to increase the stack
size.  As a result of this, the value of rlim.rlim_cur in
handle_sigsegv() might exceed the actual stack size.  See Corinna
Vinschen's message at

  https://www.cygwin.com/ml/cygwin/2015-07/msg00092.html

for a more detailed explanation.

Corinna suggests using pthread_getattr_np instead of getrlimit to avoid
this problem, as in the following patch:

diff --git a/src/sysdep.c b/src/sysdep.c
index 91036f0..c49e333 100644
--- a/src/sysdep.c
+++ b/src/sysdep.c
@@ -1625,6 +1625,8 @@ handle_arith_signal (int sig)

#ifdef HAVE_STACK_OVERFLOW_HANDLING

+#include <pthread.h>
+
/* -1 if stack grows down as expected on most OS/ABI variants, 1 
otherwise.  */

static int stack_direction;
@@ -1642,9 +1644,11 @@ handle_sigsegv (int sig, siginfo_t *siginfo, void 
*arg)
too nested calls to mark_object.  No way to survive.  */
if (!gc_in_progress)
{
-      struct rlimit rlim;
+      pthread_attr_t attr;
+      size_t stacksize;

-      if (!getrlimit (RLIMIT_STACK, &rlim))
+      if (!pthread_getattr_np (pthread_self (), &attr)
+         && !pthread_attr_getstacksize (&attr, &stacksize))
{
/* STACK_DANGER_ZONE has to be bigger than 16K on Cygwin, for
reasons explained in
@@ -1657,7 +1661,7 @@ handle_sigsegv (int sig, siginfo_t *siginfo, void 
*arg)
char *beg, *end, *addr;

beg = stack_bottom;
-         end = stack_bottom + stack_direction * rlim.rlim_cur;
+         end = stack_bottom + stack_direction * stacksize;
if (beg > end)
addr = beg, beg = end, end = addr;
addr = (char *) siginfo->si_addr;

Of course, the definition of HAVE_STACK_OVERFLOW_HANDLING would have to
be changed to ensure that pthread_getattr_np exists.

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