Date: Thu, 15 Nov 2001 19:09:40 -0500 (EST) From: Daniel Eischen <eischen@pcnet1.pcnet.com> To: John Baldwin <jhb@FreeBSD.org> Cc: hackers@FreeBSD.org, freebsd-ports@FreeBSD.org, marcus@marcuscom.com, Maxim Sobolev <sobomax@FreeBSD.org> Subject: Re: Using bit 21 of EFLAGS in user-mode [was: Re: sigreturn: efl Message-ID: <Pine.SUN.3.91.1011115185612.23017A-100000@pcnet1.pcnet.com> In-Reply-To: <XFMail.011115150549.jhb@FreeBSD.org>
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On Thu, 15 Nov 2001, John Baldwin wrote:
> On 15-Nov-01 Daniel Eischen wrote:
> > On Thu, 15 Nov 2001, Maxim Sobolev wrote:
> >> 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
> >
> > Does it need to keep bit 21 of EFLAGS flipped, or can libpng
> > set it back and keep knowledge that CPUID is supported? Or
> > does that bit need to remain set for CPUID to work?
>
> It needs to be able to change it. If you can change the value of the bit (done
> by pushf ; pop %eax ; mov %eax,%ebx ; xor $PSL_ID,%eax ; push %eax ; popf ;
> pushf ; pop %eax ; compare bit PSL_ID of eax ebx to see if they match).
> The problem is if a signal comes in during the middle of that bit toggling due
> to a profiling timer. I think the problem may be that it uses a sequence that
> leaves the bit set, thus the kernel freaks out thinking that the user has
> changed a kernel only flag. The solution is Maxim's patch to make the kernel
> not care about the flag (which it shouldn't since cpuid is not a privileged
> instruction).
I just thought perhaps libpng could do something like:
static int init_done = 0;
static int cpuid_supported = 0;
...
if (init_done == 0) {
block_all_sigs();
cpuid_supported = check_cpuid();
init_done = 1;
unblock_sigs();
}
But if it always needs to change the bit, I guess the above doesn't help.
--
Dan Eischen
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