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

[Peter Blok]

Hi,

I¹m probably missing something but if you require a stack variable to be
aligned why not use a type attribute like __attribute__ ((aligned (8)))

Peter



On 1/14/11 00:34 AM, "Warner Losh" <imp at bsdimp.com> 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?
> 
> Warner
> 
> _______________________________________________
> freebsd-hackers at freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
> To unsubscribe, send any mail to "freebsd-hackers-unsubscribe at freebsd.org"