linux-ext4 - Re: Oops with ext(3|4) and audit and Xen

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Date:	Mon, 8 Oct 2012 14:45:39 -0700
From:	Peter Moody <pmoody@...gle.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	"Theodore Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: Oops with ext(3|4) and audit and Xen

On Mon, Oct 8, 2012 at 2:40 PM, Eric Sandeen <sandeen@...hat.com> wrote:
> On 10/8/12 4:39 PM, Theodore Ts'o wrote:
>> On Mon, Oct 08, 2012 at 02:08:02PM -0500, Eric Sandeen wrote:
>>> I had suggested this on the other list, but will put it here too, though it
>>> might be a long shot.  If threadinfo gets corrupted, the irqs_enabled()
>>> test might give the wrong answer.
>>>
>>> Peter also mentioned that he had tried putting WARN_ON(irqs_disabled()) at
>>> various places along the stack above and never got it to trip; until after
>>> the BUG_ON() had fired; this makes me think corruption might be a possibility
>>> after all.
>>
>> Well, there is absolutely no place where we disable interrupts in
>> ext3.  In ext4 we do have some spinlock_irqsave/irqresture() calls,
>> but they are tightly bracketed --- and since you can reproduce this
>> with ext3, I think that pretty much exonerates ext4.
>>
>> Hmm.... one possibility might be that it's some XEN-specific paravirt
>> call that happens to be called by ext3/ext4 and which is leaving
>> interrupts disabled on its return due to a missing irqrestore() call?
>>
>> Can you reproduce the problem if you disable XEN and run this on a
>> native system?

Nope, I can't reproduce with this setup (and I've tried a *ton*)

>> What if you run a kernel w/o auditing but under Xen?

Nope, this doesn't trigger it either.

>> Maybe that will allow you to figure out what the critical variable
>> might be?

Yeah, I'm working with the Xen folks to get a test cluster built that
I can test this out on.

>> I'll note that if ext3 or ext4 was playing with interrupts and leaving
>> them disabled, we'd have a huge number of people complaining.  So the
>> question is whether it's something unique to audit, or unique to Xen,
>> or perhaps the combination of the two....

Yeah, I figured if this was something in ext3/4, I would not be the
first person asking about it. I mostly brought it here this morning
because ext2 seemed immune.

> and unique to running a 32-bit binary as well, right?

Yes, this does seem to be required for triggering this.

Cheers,
peter

>>                                       - Ted
>
>



-- 
Peter Moody      Google    1.650.253.7306
Security Engineer  pgp:0xC3410038
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