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Date:      Thu, 22 Feb 2001 11:48:34 -0800 (PST)
From:      John Baldwin <jhb@FreeBSD.org>
To:        John Baldwin <jhb@FreeBSD.org>
Cc:        cvs-all@FreeBSD.org, cvs-committers@FreeBSD.org
Subject:   RE: cvs commit: src/sys/i386/i386 trap.c
Message-ID:  <XFMail.010222114834.jhb@FreeBSD.org>
In-Reply-To: <200102221935.f1MJZLx89084@freefall.freebsd.org>

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On 22-Feb-01 John Baldwin wrote:
> jhb         2001/02/22 11:35:21 PST
> 
>   Modified files:
>     sys/i386/i386        trap.c 
>   Log:
>   The p_md.md_regs member of proc is used in signal handling to reference
>   the the original trapframe of the syscall, trap, or interrupt that entered
>   the kernel.  Before SMPng, ast's were handled via a psuedo trap at the
>   end of doerti.  With the SMPng commit, ast's were broken out into a
>   separate ast() function that was called from doreti to match the behavior
>   of other architectures.  Unfortunately, when this was done, the
>   p_md.md_regs member of curproc was not updateda in ast(), thus when
>   signals are handled by userret() after an interrupt that returns to
>   userland, we end up using a stale trapframe that will result in the
>   registers from the old trapframe overwriting the real trapframe and
>   smashing all the registers right before we return to usermode.  The saved
>   %cs:%eip from where we were in usermode are saved in the trapframe for
>   example.

The breakage here could only occur if we had an interrupt to userland and that
interrupt ended up sending a signal to the process.  Actually, one realistic
case this might have happended would be with teh SIGPROF and SIGVTALRM signals
set by hardclock().  This could have manifested itself as programs exhibiting
rather errant execution and very strange behavior.  I'm not sure if this could
have crashed the kernel itself or not.  I'm not sure if this could have
resulted in the stack pointer and/or the stack contents getting corrupted as it
apparently seems to be happening for some people right now, but I would
encourage anyone seeing weird bugs on x86 to try this out and see if it helps.
:)

-- 

John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/

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