linux-kernel - Re: Race-free unlinking of directory entries

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Message-ID: <20180409103554.64flwtm5xamhrkkp@pali>
Date:   Mon, 9 Apr 2018 12:35:54 +0200
From:   Pali Rohár <pali.rohar@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jan Kara <jack@...e.cz>
Subject: Re: Race-free unlinking of directory entries

On Monday 09 April 2018 03:24:14 Christoph Hellwig wrote:
> On Mon, Apr 09, 2018 at 12:10:09PM +0200, Pali Rohár wrote:
> > Another example:
> > 
> > fd = open("/a")
> > link("/a", "/b")
> > unlink("/a")
> > 
> > Calling funlink for fd should unlink "/b" or it should fail?
> 
> It should fail, as '/a' doesn't refer to name that is visible in the
> namespace.
> 
> > And another example:
> > 
> > fd = open("/a")
> > rename("/a", "/b")
> > 
> > What should funlink do for fd now?
> 
> remove the directory entry refering to '/b' as that is what fd refers
> to.

Why it should differ in these two cases? Calling /bin/ln /a /b followed by
/bin/rm /a results in the same state as calling /bin/mv /a /b. This is
something which works in POSIX systems.

I think it is strange that new possible funlink call would work only if
external applications uses /bin/mv and would fail if /bin/ln and /bin/rm
are used.

This is reason why I suggested two parameters funlink, it takes fd for
unlinking and pathname which must contain same inode as fd. So when you
call it with fd+"/b" it unlink "/b" without failing.

-- 
Pali Rohár
pali.rohar@...il.com