linux-kernel - Re: [PATCH] autofs4: fix race in unhashed dentry code

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Date:	Thu, 12 Apr 2007 02:00:12 +0800
From:	Ian Kent <raven@...maw.net>
To:	Jeff Mahoney <jeffm@...e.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH] autofs4: fix race in unhashed dentry code

On Thu, 2007-04-12 at 01:49 +0800, Ian Kent wrote:
> On Wed, 2007-04-11 at 11:05 -0400, Jeff Mahoney wrote:
> >  Commit f50b6f8691cae2e0064c499dd3ef3f31142987f0 introduced a
> >  race in autofs4 between autofs_lookup_unhashed() and
> >  autofs_dentry_release().
> > 
> >  autofs_dentry_release() ends up clearing the ->dentry and ->inode
> >  members of autofs_info before removing it from the rehash list. The
> >  list is protected by the rehash lock in both functions, but
> >  since autofs_dentry_release() starts tearing the autofs_info struct
> >  down before removing it from the list, autofs_lookup_unhashed() can
> >  get a autofs_info with a NULL dentry.
> > 
> >  This patch moves the clearing of ->dentry and ->inode after the removal
> >  from the rehash list.
> 
> Oh .. excellent, I had a bug report but I just couldn't see it for
> looking.

Maybe I've been a bit hasty with the celebration.
It looks like I've got a bigger locking problem here.
If autofs4_dentry_release waits on the rehash lock and
autofs4_lookup_unhashed reclaims it then the info struct and the dentry
go away unconditionally as the release is called just prior to freeing
the dentry memory, right?.

But you've certainly identified the problem with it.
Let me think about it a little while.

> 
> You've made my day.
> Thanks heaps
> Ian
> 
> > 
> > Signed-off-by: Jeff Mahoney <jeffm@...e.com>
> Acked-by: Ian Kent <raven@...maw.net>
> 
> > 
> > ---
> > 
> >  fs/autofs4/root.c |    6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > --- a/fs/autofs4/root.c	2007-04-11 09:41:44.000000000 -0400
> > +++ b/fs/autofs4/root.c	2007-04-11 10:54:37.000000000 -0400
> > @@ -470,9 +470,6 @@ void autofs4_dentry_release(struct dentr
> >  	if (inf) {
> >  		struct autofs_sb_info *sbi = autofs4_sbi(de->d_sb);
> >  
> > -		inf->dentry = NULL;
> > -		inf->inode = NULL;
> > -
> >  		if (sbi) {
> >  			spin_lock(&sbi->rehash_lock);
> >  			if (!list_empty(&inf->rehash))
> > @@ -480,6 +477,9 @@ void autofs4_dentry_release(struct dentr
> >  			spin_unlock(&sbi->rehash_lock);
> >  		}
> >  
> > +		inf->dentry = NULL;
> > +		inf->inode = NULL;
> > +
> >  		autofs4_free_ino(inf);
> >  	}
> >  }
> > 

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