linux-kernel - Re: fs/namei.c: Misuse of sequence counts?

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Message-ID: <20141012040142.GB24463@zzz>
Date:	Sat, 11 Oct 2014 23:01:42 -0500
From:	Eric Biggers <ebiggers3@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: fs/namei.c: Misuse of sequence counts?

On Sun, Oct 12, 2014 at 01:12:59AM +0100, Al Viro wrote:
> 
> Huh?  What's to guarantee that dentry hasn't become negative since the
> moment we'd fetched the seqcount?  _That_ is the problem we are dealing
> with here - link_path_walk() relies on nd->inode being non-NULL.

Hmm, I guess that makes sense.  So the code is actually verifying that the inode
is still the inode that was referenced from the current or root directory when
nd->path was set.  But couldn't the problem also be solved by setting nd->inode
directly in the fs->seq retry loops?  (The file descriptor case could be
'nd->inode = file_inode(f.file);'.)  Then there would be no need for the extra
read_seqcount_retry() just for the inode.  The patch you posted looks correct,
but I wonder if this approach would be better.
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