linux-kernel - Re: Race in ovl_copy

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Message-ID: <CAJfpegtoO6wub5+diEhiVfUmj+ZxeT5wi_6EPxFJgqtXuqraog@mail.gmail.com>
Date:	Tue, 12 Aug 2014 11:37:15 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	David Howells <dhowells@...hat.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-unionfs@...r.kernel.org,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Race in ovl_copy_up()?

On Mon, Aug 11, 2014 at 4:37 PM, David Howells <dhowells@...hat.com> wrote:

> In the for-loop in the middle, if you ascend any levels at all, how are you
> protected from racing with copy-ups taking place on those more rootwards
> dentries?
>
> I can see ovl_copy_up_one() using lock_rename() on the workdir and upperdir,
> but there's no relevant lock on the overlayfs fs.

Locking upperdir should be enough protection.

Lets say we have a race:

A: copy_up(a/b/c/d)
B: copy_up(a/b/e/f)

Both ascend to "b" and try to lock "a".  The one that wins the lock
acquisition will do the copy-up, the other one will just skip it once
it gets the lock.

Thanks,
Miklos
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