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
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