Date: Thu, 15 Nov 2001 23:56:13 EET From: Maxim Sobolev <sobomax@FreeBSD.org> To: marcus@marcuscom.com Cc: freebsd-ports@FreeBSD.org, hackers@FreeBSD.org Subject: Using bit 21 of EFLAGS in user-mode [was: Re: sigreturn: eflags creash (fixed!)] Message-ID: <200111152156.XAA79972@ipcard.iptcom.net> In-Reply-To: <20011115145029.G47613-200000@shumai.marcuscom.com>
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On Thu, 15 Nov 2001 14:56:31 -0500 (EST), Joe Clarke wrote:
>
> I learned about this by reading through some of the -hackers archives.
> One person complained of similar errors trying to get xine to work on
> FreeBSD. Removing the MMX detection code fixed it. I remembered libpng
> also used MMX, so I removed the pnggccrd.c source, and voila!
>
> Based on core dumps, strace output, and a lot of code surfing, this makes
> sense to me. Basically, any png-dependent app's thread that runs longer
> than what ITIMER_PROF is set to gets hit with a SIGPROF. When that
> happens, things context switch. eflags must have been corrupted by the
> MMX code, thus sigreturn() bombs out, and causes uthread_kern to die as
> well. Here's what strace looks like when balsa tries to read a 33 MB
> mailbox:
>
> 74202 sigreturn(0x81f2c64
>
> When this happens, strace politely dies with a bus error.
>
> Thanks for testing this, Maxim. Hopefully someone can find the problem
> and fix it for good.
That explains... After a quick glance at png code I found that
the only place where EFLAGS is altered is CPUID code, where
the library flips bit 21 of EFLAGS in order to ensure that the
CPUID instruction is supported (otherwise it will get SIGILL
on older processors). Unfortunately, for some reason FreeBSB
kernel considers bit 21 of EFLAGS as one that should not be
altered in the user mode, thus making it illegal to use standard
user-mode processor-detection routines based around that bit.
AFAIK, it is a bug in FreeBSD, because there is actually nothing
wrong with altering bit 21 in the user mode - it doesn't have
any side effects and pretty much any of the currently available
on the i386 OSes allows it.
Therefore, I would like to ask you to test attached patch and
if it works and there are no other objections I would like to
commit it shortly. To test the patch, you need to recompile
kernel with patch applied, reboot, recompile/reinstall png with
MXX support turned on and try to run Nautilus. Please let me know
if it helped or not.
Thanks!
-Maxim
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