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

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Message-ID: <20180409101009.6kxd47jeowef7wt2@pali>
Date:   Mon, 9 Apr 2018 12:10:09 +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 02:59:19 Christoph Hellwig wrote:
> On Mon, Apr 09, 2018 at 09:42:41AM +0200, Pali Rohár wrote:
> > Hi! I would like to remind this my older email about race free
> > unlinking. Is there any plan to provide such support?
> 
> Given that in Linux we know which directory entry a given fd refers to,
> we shouldn't even need a new syscall, but simply support the
> AT_EMPTY_PATH flag for funlinkat.

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?

Another example:

fd = open("/a")
link("/a", "/b")
unlink("/a")

Calling funlink for fd should unlink "/b" or it should fail?

And another example:

fd = open("/a")
rename("/a", "/b")

What should funlink do for fd now?

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)".

-- 
Pali Rohár
pali.rohar@...il.com