Port alternate signal stack to upcoming glibc 2.34emacs-26

* src/sysdep.c (sigsegv_stack): Increase size to 64 KiB and align
it to max_align_t.  This copies from Gnulib’s c-stack.c, and works
around a portability bug in draft glibc 2.34, which no longer
defines SIGSTKSZ when _GNU_SOURCE is defined.

(cherry picked from commit f97e07ea807cc6d38774a3888a15091b20645ac6)

1 files changed, 9 insertions, 1 deletions

diff --git a/src/sysdep.c b/src/sysdep.c
index b2aecc0ddac..1f7f02cb003 100644
--- a/src/sysdep.c
+++ b/src/sysdep.c
@@ -1792,7 +1792,15 @@ handle_arith_signal (int sig)
 
 /* Alternate stack used by SIGSEGV handler below.  */
 
-static unsigned char sigsegv_stack[SIGSTKSZ];
+/* Storage for the alternate signal stack.
+   64 KiB is not too large for Emacs, and is large enough
+   for all known platforms.  Smaller sizes may run into trouble.
+   For example, libsigsegv 2.6 through 2.8 have a bug where some
+   architectures use more than the Linux default of an 8 KiB alternate
+   stack when deciding if a fault was caused by stack overflow.  */
+static max_align_t sigsegv_stack[(64 * 1024
+                                  + sizeof (max_align_t) - 1)
+                                 / sizeof (max_align_t)];
 
 
 /* Return true if SIGINFO indicates a stack overflow.  */