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

[Warner Losh]

On 01/14/2011 01:28, Kostik Belousov wrote:
> 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().

I tend to think you are right...  The backtrace doesn't have an aligned 
instruction to worry about.  I'll rebuild to make sure it is all sane...

Warner