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

Kostik Belousov kostikbel at gmail.com
Fri Jan 14 09:03:07 UTC 2011


On Thu, Jan 13, 2011 at 04:34:22PM -0700, Warner Losh wrote:
> On 01/13/2011 13:28, Kostik Belousov wrote:
> >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
> 
> I'm seeing weird core dumps for ssh and friends on i386 on stable/8 from 
> a few days ago.  Could that be related?
Few days ago ? It was in the tree for probably one year.
I very much doubt it, but cannot say anything until you show the backtrace.

Our in-tree gcc masks this by typically doing stack realignment on the
entry into the main().
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