libgcc patch committed: Avoid hooks in split-stack code

[Ian Lance Taylor]

The split-stack code invokes mmap and munmap with a limited amount of
stack space.  That is fine when the functions just make a system call,
but it's not fine when programs use LD_PRELOAD or linker tricks to add
hooks to mmap/munmap.  Those hooks may use too much stack space,
leading to an obscure program crash.

Avoid that at least on GNU/Linux by calling __mmap/__munmap instead.

Bootstrapped and ran Go and split-stack tests on x86_64-pc-linux-gnu.
Committed to mainline.

Ian

2020-04-03  Ian Lance Taylor  <iant@golang.org>

* generic-morestack.c: On GNU/Linux use __mmap/__munmap rather
than mmap/munmap, to avoid hooks.
-------------- next part --------------
diff --git a/libgcc/generic-morestack.c b/libgcc/generic-morestack.c
index c26dba1ae4a..bb9f67a7366 100644
--- a/libgcc/generic-morestack.c
+++ b/libgcc/generic-morestack.c
@@ -53,6 +53,23 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 
 #include "generic-morestack.h"
 
+/* Some systems use LD_PRELOAD or similar tricks to add hooks to
+   mmap/munmap.  That breaks this code, because when we call mmap
+   there is enough stack space for the system call but there is not,
+   in general, enough stack space to run a hook.  At least when using
+   glibc on GNU/Linux we can avoid the problem by calling __mmap and
+   __munmap.  */
+
+#ifdef __gnu_linux__
+
+extern void *__mmap (void *, size_t, int, int, int, off_t);
+extern int __munmap (void *, size_t);
+
+#define mmap __mmap
+#define munmap __munmap
+
+#endif /* defined(__gnu_linux__) */
+
 typedef unsigned uintptr_type __attribute__ ((mode (pointer)));
 
 /* This file contains subroutines that are used by code compiled with