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Re: fs/attr.c:notify_change locking warning.

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: fs/attr.c:notify_change locking warning.
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 16 Oct 2013 21:26:51 +1100
Cc: Dave Jones <davej@xxxxxxxxxx>, xfs@xxxxxxxxxxx, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20131016070528.GB18721@xxxxxxxxxxxxx>
References: <20131005005210.GA25773@xxxxxxxxxx> <20131005031918.GL4446@dastard> <20131015201905.GA7509@xxxxxxxxxxxxx> <20131015213618.GU4446@dastard> <20131016070528.GB18721@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Oct 16, 2013 at 12:05:28AM -0700, Christoph Hellwig wrote:
> On Wed, Oct 16, 2013 at 08:36:18AM +1100, Dave Chinner wrote:
> > Sure, but file_remove_suid() doesn't actually modify any VFS inode
> > structures until we process the flags and the modifications within
> > ->setattr, which in XFS are all done under the XFS_ILOCK_EXCL via
> > xfs_setattr_mode(). i.e. both the VFS and XFS inodes S*ID bits are
> > removed only under XFS_ILOCK_EXCL....
> 
> It can set S_NOSEC after calling into ->setattr at least.
> 
> > Hence I see no point in adding extra serialisation via the i_mutex
> > to this path when we can just do something like:
> > 
> >     killsuid = should_remove_suid(file->f_path.dentry);
> >     if (killsuid) {
> >             struct iattr    newattr;
> > 
> >             newattr.ia_valid = ATTR_FORCE | killsuid;
> >             error = xfs_setattr_nonsize(ip, &newattr, 0);
> >             if (error)
> >                     return error;
> >     }
> 
> We'd still need all the other magic in file_remove_suid, which I don't
> actually quite undersdtand fully yet.

The killpriv calls? I couldn't find anything that implemented those
security hooks nor any documentation about it, so I'm pretty much
clueless about it. FWIW, ocfs2 doesn't implement them, either....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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