Date: Fri, 14 Jan 2011 10:28:40 +0200 From: Kostik Belousov <kostikbel@gmail.com> To: Warner Losh <imp@bsdimp.com> Cc: freebsd-hackers@freebsd.org Subject: Re: What does the FreeBSD/i386 ABI say about stack alignment? Message-ID: <20110114082840.GM2518@deviant.kiev.zoral.com.ua> In-Reply-To: <4D2F8BFE.9070607@bsdimp.com> References: <AANLkTikrsHUO3M%2Bfvo0kO%2B3dPq8OHu5L2zBf3fa3jL2x@mail.gmail.com> <20110113202848.GI2518@deviant.kiev.zoral.com.ua> <4D2F8BFE.9070607@bsdimp.com>
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--2DT8m24oj8Onh3Jp Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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:=20 > >>http://www.freebsd.org/cgi/getmsg.cgi?fetch=3D438552+0+/usr/local/www/d= b/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) =3D=3D 0 just b= efore > >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 >=20 > I'm seeing weird core dumps for ssh and friends on i386 on stable/8 from= =20 > 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(). --2DT8m24oj8Onh3Jp Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (FreeBSD) iEYEARECAAYFAk0wCTgACgkQC3+MBN1Mb4hINQCdGcmAJ6uHU1M6NHM6CNWbzVdK 9qIAoJd6sIfWp6vqc2MeaxS4be8KmVYH =EAQ3 -----END PGP SIGNATURE----- --2DT8m24oj8Onh3Jp--
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