Re: [OOPS] repeatable 2.4.8-ac7, 2.4.7-ac6 just run xdos

[Brian Gerst <bgerst@didntduck.org>]

Andi Kleen wrote:
> 
> Brian Gerst <bgerst@didntduck.org> writes:
> 
> > >
> > > CPU:    0
> > > EIP:    0010:[<c0180a18>]
> > > Using defaults from ksymoops -t elf32-i386 -a i386
> > > EFLAGS: 00010002
> > > eax: 00001000   ebx: c4562368   ecx: 00000000   edx: 00000001
> > > esi: c4562368   edi: c4a954d4   ebp: 00000001   esp: c6887d88
> > > ds: 008   es: 0000   ss: 0018
> >                 ^^^^
> > Here is your problem.  %es is set to the null segment.  I had my
> > suspicions about the segment reload optimisation in the -ac kernels, and
> > this proves it.  Try backing out the changes to arch/i386/kernel/entry.S
> > and include/asm-i386/hw_irq.h and see if that fixes the problem.
> 
> This patch should fix the problem. One assumption coded into the reload
> optimization is violated by vm86 mode. Please test.

Yes.  What happened here is that %ds and %es were not being updated
atomically.  Under normal operation, this would just leave %es with
USER_DS, which is sufficiently equivalent to KERNEL_DS to not cause a
fault.  Coming out of vm86 mode however forces the data segment
registers to null after saving the real mode values on the stack.  If an
interrupt happened between setting %ds and %es (what are the odds?) then
that assumption would fail and leave %es null, causing the next string
instruction to go boom.  The same fix should be applied to entry.S as
well.

-- 

						Brian Gerst