linux-kernel - Re: 2.6.34-rc4 : OOPS in unmap

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Message-ID: <20100414160710.GB3167@redhat.com>
Date:	Wed, 14 Apr 2010 12:07:10 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Parag Warudkar <parag.lkml@...il.com>,
	linux-kernel@...r.kernel.org, Haren Myneni <hbabu@...ibm.com>,
	kexec@...ts.infradead.org
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: 2.6.34-rc4 : OOPS in unmap_vma

On Wed, Apr 14, 2010 at 05:22:31PM +0200, Borislav Petkov wrote:
> From: Linus Torvalds <torvalds@...ux-foundation.org>
> Date: Wed, Apr 14, 2010 at 07:32:08AM -0700
> 
> Hi Linus,
> 
> > On Wed, 14 Apr 2010, Borislav Petkov wrote:
> > > 
> > > hmm, it doesn't look like it. Your code translates to something like
> > > 
> > >    0:   b8 00 00 00 00          mov    $0x0,%eax
> > >    5:   80 ff ff                cmp    $0xff,%bh
> > >    8:   ff 48 21                decl   0x21(%rax)
> > >    b:   45 80 48 8b 45          rex.RB orb    $0x45,-0x75(%r8)
> > >   10:   80 48 ff c8             orb    $0xc8,-0x1(%rax)
> > 
> > There's a large constant (0xffffff8000000000) in there at the beginning, 
> > and the disassembly hasn't found the start of the next instruction very 
> > cleanly. The same is true at the end: another large constant is cut off in 
> > the middle. 
> > 
> > The byte just before the dumped instruction stream is almost certainly 
> > '48h', and the last byte of the last constant is 0xff, and the disassembly 
> > ends up being:
> > 
> >    0:	48 b8 00 00 00 00 80 	mov    $0xffffff8000000000,%rax
> >    7:	ff ff ff 
> >    a:	48 21 45 80          	and    %rax,-0x80(%rbp)
> >    e:	48 8b 45 80          	mov    -0x80(%rbp),%rax
> >   12:	48 ff c8             	dec    %rax
> >   15:	48 3b 85 40 ff ff ff 	cmp    -0xc0(%rbp),%rax
> >   1c:	48 8b 85 50 ff ff ff 	mov    -0xb0(%rbp),%rax
> >   23:	48 0f 42 7d 80       	cmovb  -0x80(%rbp),%rdi
> >   28:	48 89 7d 80          	mov    %rdi,-0x80(%rbp)
> >   2c:*	48 8b 38             	mov    (%rax),%rdi     <-- trapping instruction
> >   2f:	48 85 ff             	test   %rdi,%rdi
> >   32:	0f 84 f5 04 00 00    	je     0x52d
> >   38:	48 b8 fb 0f 00 00 00 	mov    $0xffffc00000000ffb,%rax
> >   3f:	c0 ff ff 
> > 
> > But yes, you found the right spot (that 0xffffff8000000000 constant is 
> > -549755813888 decimal):
> 
> Right, the decodecode output looked kinda strange to me and I tried
> to match the instruction order and find the location. But yeah, now
> that I'm looking at show_registers(), we don't start dumping on precise
> instruction boundary but simply 64 bytes in the default case. No time
> for an instruction decoder along that path :).
> 
> > > which I could correlate with what I get here (comments added):
> > 
> > Yup. Close enough. Btw, it's often good to look at both the *.s code _and_ 
> > the *.lst code. If you do "make mm/memory.lst", you'll find those big 
> > constants easily, and then you'll see the code this way:
> 
> [..]
> 
> ok, I can't say that I'm a linux newbie but the .lst code is new to me.
> Damn, and I thought I knew it all :)
> 
> > > so it looks like it tries to find a page table rooted at that address
> > > but the pointer value of 0000000000002203 is bogus.
> > 
> > Yes, it does look like some strange page table corruption, doesn't look 
> > anon_vma related at all. It's intriguing that it started happening now, 
> > though, so..
> 
> Well, Parag said something about kexec kernel so it is definitely
> interesting what he means there - a kexec-enabled kernel or is this the
> "second" kernel his machine kexec'd into after a previous failure. I
> think this could clarify the situation a bit.

FWIW, Just a data point. I pulled in latest kernel and I can boot it
through BIOS as well as kexec boot on my x86_64 box.

Vivek
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