What does the FreeBSD/i386 ABI say about stack alignment?

[Kostik Belousov]

On Thu, Jan 13, 2011 at 12:19:00PM -0500, Ryan Stone wrote:
> I've been trying to get an application compiled with gcc 4.5.1 running
> on FreeBSD 8.1, but it's been crashing during startup with a SIGBUS.
> It turns out that the problem is that gcc is issuing SSE
> instructions(in my case, a movdqa) that assume that the stack will be
> aligned to a 16-byte boundary.  It seems that Linux/i386 guarantees
> this, and I worry that gcc has extended this assumption to all i386
> architectures.  I'm assuming that FreeBSD doesn't make any such
> promises based on the fact that I'm getting crashes.
> 
> There does seem to be a flag (-mstackrealign) that you can set to
> force gcc to align the stack to what it wants, but that pessimizes the
> generated code a bit.  Some googling would seem to indicate that
> -mpreferred-stack-boundary won't always handle this problem correctly.
> 
> Any ideas?  My inclination, at least for our local source tree here at
> $WORK, would be to accommodate gcc and guarantee the stack alignment
> that it wants rather than pessimize our application.  It seems we have
> an old local patch/hack in our FreeBSD 6.1 tree(apparently based on
> this: http://www.freebsd.org/cgi/getmsg.cgi?fetch=438552+0+/usr/local/www/db/text/2000/freebsd-current/20000507.freebsd-current).
>  I believe that this patch is the reason why we haven't seen the
> problem when running on 6.1, but the patch doesn't seem to work
> anymore on 8.1.

Look at lib/csu/i386-elf/crt1_s.S, we align stack on startup.
My understanding is that the requirement is (%esp & 0xf) == 0 just before
the call to the function. And we are off by 4 (this is my fault).

Please give this a try.

diff --git a/lib/csu/i386-elf/crt1_s.S b/lib/csu/i386-elf/crt1_s.S
index d7ed0a2..17ac0e3 100644
--- a/lib/csu/i386-elf/crt1_s.S
+++ b/lib/csu/i386-elf/crt1_s.S
@@ -42,6 +42,7 @@ _start:
 	.cfi_def_cfa_register %ebp
 	andl	$0xfffffff0,%esp # align stack
 	leal	8(%ebp),%eax
+	subl	$4,%esp
 	pushl	%eax		# argv
 	pushl	4(%ebp)		# argc
 	pushl	%edx		# rtld cleanup
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