linux-kernel - Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry

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Message-ID: <20110720104721.03a2a2f0@notabene.brown>
Date:	Wed, 20 Jul 2011 10:47:21 +1000
From:	NeilBrown <neilb@...e.de>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry

On Wed, 20 Jul 2011 00:55:15 +0100 Al Viro <viro@...IV.linux.org.uk> wrote:

> On Wed, Jul 20, 2011 at 12:52:16AM +0100, Al Viro wrote:
> > On Wed, Jul 20, 2011 at 12:45:51AM +0100, Al Viro wrote:
> > 
> > > You know what...  I doubt that you want to mess with ->d_seq checks here.
> > > It's definitely not Hugh's bug (unless he has bindings somewhere odd) and
> > > both ->mnt_mountpoint and ->mnt_root are pinned (and we are holding
> > > vfsmount_lock anyway).  *inode assignment too early is a real bug, indeed,
> > > and we want to assign nd->seq if we cross mountpoint as both versions do,
> > > but check just before that is, in the best case, BUG_ON() fodder.  We'd
> > > just found a vfsmount with ->mnt_mountpoint equal to path->dentry; it *can't*
> > > be stale, or we have a really nasty problem anyway.
> > 
> > Kudos to neilb for spotting the pointless check, BTW; and no, his theory
> > that it might be needed since we could race with umount() is wrong - due to
> > vfsmount_lock being held.

Thanks.. but that patch seems to introduce an unused variable "seq".

NeilBrown


> 
> Updated, pushed.  Please, pull from the usual place (
> git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs-2.6.git/ for-linus
> ).
> 
> Shortlog:
> Al Viro (1):
>       Fix cifs_get_root()
> 
> Linus Torvalds (1):
>       vfs: fix race in rcu lookup of pruned dentry
> 
> Diffstat:
>  fs/cifs/cifsfs.c |  100 +++++++++++++++--------------------------------------
>  fs/namei.c       |   10 +++++-
>  2 files changed, 38 insertions(+), 72 deletions(-)

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