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Re: inode_permission NULL pointer dereference in 3.13-rc1

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: inode_permission NULL pointer dereference in 3.13-rc1
From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date: Sun, 24 Nov 2013 15:27:58 +0000
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20131124140413.GA19271@xxxxxxxxxxxxx>
References: <20131124140413.GA19271@xxxxxxxxxxxxx>
Sender: Al Viro <viro@xxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Sun, Nov 24, 2013 at 06:04:13AM -0800, Christoph Hellwig wrote:
> Seems I can reproduce this by doing a full xfstests run and then
> shutting down the VM.  Doesn't seem to happen with the XFS tree
> which is still based on 3.12-rc1.

may_lookup() with LOOKUP_RCU in nd->flags and NULL nd->inode, by
the look of it...  Interesting.

AFAICS, path_init() initializes nd->inode on all success exits.
As for the places where we reassign nd->inode, we have the following:
fs/namei.c:681: nd->inode = nd->path.dentry->d_inode;
        nd_jump_link(), we would've already left RCU mode
fs/namei.c:856:         nd->inode = nd->path.dentry->d_inode;
        follow_link() - ditto.
fs/namei.c:1160:        nd->inode = nd->path.dentry->d_inode;
        follow_dotdot_rcu()
fs/namei.c:1257:        nd->inode = nd->path.dentry->d_inode;
        follow_dotdot() - not in RCU mode
fs/namei.c:1547:        nd->inode = inode;
        walk_component() - inode can't be NULL here, variable has
been explicitly compared with NULL a few lines prior
fs/namei.c:1825:                nd->inode = inode;
        path_init() with LOOKUP_ROOT; not from stat(2) and it would've
already oopsed with NULL inode anyway.
fs/namei.c:1892:        nd->inode = nd->path.dentry->d_inode;
        path_init()
fs/namei.c:3006:        nd->inode = inode;
        do_last(), not from stat(2) and no may_lookup() done past that
point anyway.
fs/namei.c:3077:        nd->inode = dir->d_inode;
        do_last(), not from stat(2)

which seems to leave two candidates - follow_dotdot_rcu() and path_init(),
both setting nd->inode to nd->path.dentry->d_inode...

Could you try to reproduce it with something like
        if (read_seqretry(&mount_lock, nd->m_seq))
                goto failed;
slapped before the success exit in follow_dotdot_rcu(), just to see if
we are hitting some races with umount here?

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