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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: fs/attr.c:notify_change locking warning.
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Wed, 16 Oct 2013 00:05:28 -0700
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Dave Jones <davej@xxxxxxxxxx>, xfs@xxxxxxxxxxx, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20131015213618.GU4446@dastard>
References: <20131005005210.GA25773@xxxxxxxxxx> <20131005031918.GL4446@dastard> <20131015201905.GA7509@xxxxxxxxxxxxx> <20131015213618.GU4446@dastard>
User-agent: Mutt/1.5.21 (2010-09-15)
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.

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