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

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Message-ID: <20180409102414.GA23471@infradead.org>
Date:   Mon, 9 Apr 2018 03:24:14 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Pali Rohár <pali.rohar@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 Mon, Apr 09, 2018 at 12:10:09PM +0200, Pali Rohár wrote:
> I do not think it is enough, specially in cases when hard links are in
> use. Example:
> 
> fd = open("/a")
> link("/a", "/b")
> link("/a", "/c")
> unlink("/a")
> 
> And now I want to call funlink for "fd". Which file should be unlinked?
> "/b" or "/c" or none or both?

'/a' as that is what fd refers to.

> 
> 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.

> Note that there is difference which symlink is shown in /proc/<pid>/fd
> between last two examples -- even those two examples results in the same
> state (if there is no race). For last example <fd> points to "/b", for
> previous <fd> points to "/a (deleted)".

Yes, that is fully expected.