linux-kernel - Re: fs: gpf in simple

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Date:	Thu, 06 Mar 2014 11:02:00 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Jan Kara <jack@...e.cz>
CC:	Al Viro <viro@...IV.linux.org.uk>, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: fs: gpf in simple_setattr

On 03/05/2014 07:45 AM, Jan Kara wrote:
> On Tue 04-03-14 19:00:32, Sasha Levin wrote:
>> On 03/03/2014 04:40 PM, Jan Kara wrote:
>>> On Sat 01-03-14 15:05:21, Sasha Levin wrote:
>>>>> ping again?
>>>>>
>>>>> I've been working on it, but don't see an obvious issue.
>>>>>
>>>>> It does look like an access to invalid memory easily doable from
>>>>> userspace, so it should probably get fixed soon...
>>>    Hum, can you maybe dump the name in dentry passed to simple_setattr()? Or
>>> maybe even the whole path using dentry_path() (but not sure if that will
>>> be workable on half-torn-down fs)? Maybe it will give us a hint at which
>>> filesystem to look...
>>
>> It's just garbage, this is why I'm having a hard time making any progress with
>> this bug.
>    OK, but that is strange because we hold a reference to the dentry so
> noone should free it. So dentry->d_name should be valid... Is the rest of
> the dentry also garbage? E.g. does dentry->d_inode still point to the inode
> we call __mark_inode_dirty() on? Is dentry->d_sb == dentry->d_inode->i_sb?
> Also if the inode isn't completely garbage, we can maybe infer something
> from inode->i_op - that should point to some statically allocated
> operations struct so we should be able to guess fs type from that.

It's actually pretty tricky. This issue being a race makes catching it at the right time
difficult.

I've tried catching it in simple_setattr() before calling mark_inode_dirty() by testing
for the poison values inside inode, but they seem to be perfectly fine there and still
show up as bad within mark_inode_dirty().

Then I tried trapping it inside mark_inode_dirty(), but at that point I usually get garbage
inside inode, and have no way to go back to dentry.

Right now I'm just trying to dump everything that goes through simple_setattr() in hopes that
I could easily figure out what went wrong by looking at the log, but that just stops the bug
from reproducing.


Thanks,
Sasha
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