[HURD, PATCH] Fix stack switching at initialization time

[Ludovic Courtès]

Hello,

The attached patch fixes stack switching in init-first.c when cthreads
are used (this is for code compiled with a cross-GCC 4.5.1 for
i586-pc-gnu; Debian GNU/Hurd uses an older GCC and is not affected.)

With this compiler, ‘data[-1]’ does not point to the return address.
Thus, ‘switch_stacks’ would never be called, which breaks TLS.

It seems to me that using ‘data[-1]’ here was fragile in the presence of
nested functions, inlining, and tail-call optimizations.

Thanks,
Ludo’.

>From efa6d7e310b1d8918054222483f100d43525ba66 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Fri, 22 Jul 2011 15:46:11 +0200
Subject: [PATCH] Hurd: Fix stack switching at initialization time when using
 cthreads.

---
 ChangeLog                           |    7 +++++++
 sysdeps/mach/hurd/i386/init-first.c |    4 ++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 606bc22..6239986 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2011-07-22  Ludovic Courtès  <ludo@gnu.org>
+
+       * sysdeps/mach/hurd/i386/init-first.c (init): Use
+       `__builtin_return_address' and `__builtin_frame_address' instead
+       of making assumptions about the location of the return address
+       relative to DATA.
+
 2010-04-09  Ulrich Drepper  <drepper@redhat.com>
 
        * nscd/aicache.c (addhstaiX): Correct passing memory to address
diff --git a/sysdeps/mach/hurd/i386/init-first.c 
b/sysdeps/mach/hurd/i386/init-first.c
index 7d93638..09af037 100644
--- a/sysdeps/mach/hurd/i386/init-first.c
+++ b/sysdeps/mach/hurd/i386/init-first.c
@@ -257,8 +257,8 @@ init (int *data)
       /* Push the user code address on the top of the new stack.  It will
         be the return address for `init1'; we will jump there with NEWSP
         as the stack pointer.  */
-      *--newsp = data[-1];
-      data[-1] = (int) &switch_stacks;
+      *--newsp = __builtin_return_address (0);
+      * ((void **) __builtin_frame_address (0) + 1) = &switch_stacks;
       /* Force NEWSP into %eax and &init1 into %ecx, which are not restored
         by function return.  */
       asm volatile ("# a %0 c %1" : : "a" (newsp), "c" (&init1));
-- 
1.7.6