Date: Thu, 13 Jan 2011 16:34:22 -0700 From: Warner Losh <imp@bsdimp.com> To: freebsd-hackers@FreeBSD.org Subject: Re: What does the FreeBSD/i386 ABI say about stack alignment? Message-ID: <4D2F8BFE.9070607@bsdimp.com> In-Reply-To: <20110113202848.GI2518@deviant.kiev.zoral.com.ua> References: <AANLkTikrsHUO3M%2Bfvo0kO%2B3dPq8OHu5L2zBf3fa3jL2x@mail.gmail.com> <20110113202848.GI2518@deviant.kiev.zoral.com.ua>
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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
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