linux-kernel - Re: fs/attr.c:notify_change locking warning.

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Message-ID: <20131016070528.GB18721@infradead.org>
Date:	Wed, 16 Oct 2013 00:05:28 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Dave Jones <davej@...hat.com>, xfs@....sgi.com,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: fs/attr.c:notify_change locking warning.

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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