linux-kernel - Re: [PATCH 5/7] overlay filesystem (inode.c bad error path)

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Date:	Sat, 21 May 2011 00:26:47 -0400
From:	Erez Zadok <ezk@....cs.sunysb.edu>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	"viro@...IV.linux.org.uk" <viro@...IV.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"nbd@...nwrt.org" <nbd@...nwrt.org>,
	"neilb@...e.de" <neilb@...e.de>,
	"hramrach@...trum.cz" <hramrach@...trum.cz>,
	"jordipujolp@...il.com" <jordipujolp@...il.com>
Subject: Re: [PATCH 5/7] overlay filesystem (inode.c bad error path)

On May 20, 2011, at 10:17 AM, Miklos Szeredi wrote:

> Miklos Szeredi <miklos@...redi.hu> writes:
> 
>> Erez Zadok <ezk@....cs.sunysb.edu> writes:
>> 
>>> I tried your overlayfs.v9 git repo w/ racer, using two separate ext3
>>> filesystems (one for lowerdir and another for upperdir).  I got the
>>> WARN_ON in ovl_permission to trigger within about 10 minutes of
>>> testing.  Looking at the code, I see a problem in returning w/o
>>> cleaning up an dput-ing the alias dentry.  Simple patch enclosed
>>> below.
>> 
>> Hmm, thanks.  The more interesting question is: why does that WARN_ON
>> trigger?  I'll look into it.
> 
> I think I found the cause of all the bug and oopsen you are seeing.
> 
> Overlayfs expects upper and lower dentries to be always positive, it
> never stores negative dentries there, there's no point, instead it
> stores NULL.
> 
> There are basically two ways a positive dentry can become negative:
> 
> A) dentry becomes unhashed with d_count == 0
> 
> B) d_delete with d_count == 1
> 
> Case A is not possible in our case, since overlayfs keeps a ref on the
> upper/lower dentries for the lifetime of the overlayfs dentry.
> 
> Case B is however possible, since no extra ref is taken before calling
> vfs_unlink/vfs_rmdir.  So it looks like this is being triggered.
> 
> This is easy to solve, just grab a ref to upperdentry before
> unlink/rmdir.  Equivalent is if we grab an extra reference from the
> start.  The below patch does this.
> 
> With the patch I can't trigger the bugs anymore.
> 
> Erez, could you please also check if reverting your patches and applying
> this one fixes all the bugs?

Done: reverted my ovl patches and applied yours.  And indeed, the NULL oopses don't show up after 10 minutes of running.  Just to be sure, I ran overlayfs on top of ext2, ext3, xfs, reiserfs, and tmps — no oopses.  And I wasn't able to reproduce the WARN_ON in ovl_permission.

Are you interested in testing overlayfs on top of NFS at this time? You need xattr support right? Stacking over NFS takes extra effort, esp. b/c of the pesky silly deleted files and needing a nameidata everywhere (but I managed after a while to get Wrapfs working stably over nfs3).

> Thanks,
> Miklos
> 
> commit 9192816148e2c6b1d610226b1fc1c04c36216370
> Author: Miklos Szeredi <mszeredi@...e.cz>
> Date:   Fri May 20 16:07:34 2011 +0200
> 
>    ovl: don't allow upperdentry to go negative
[...]

Do you commit such patches to a master branch, the v9 branch, or some other one; I'd like to be able to git pull

Thanks,
Erez.

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