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Date:   Tue, 13 Sep 2022 05:29:02 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Miklos Szeredi <mszeredi@redhat.com>
Cc:     Xavier Roche <xavier.roche@algolia.com>,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2] vfs: fix link vs. rename race
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On Tue, Sep 13, 2022 at 03:04:17AM +0100, Al Viro wrote:
> On Mon, Feb 21, 2022 at 09:20:02AM +0100, Miklos Szeredi wrote:
> 
> [digging through the old piles of mail]
> 
> Eyes-watering control flow in do_linkat() aside (it's bound to rot; too
> much of it won't get any regression testing and it's convoluted enough
> to break easily), the main problem I have with that is the DoS potential.
> 
> You have a system-wide lock, and if it's stuck you'll get every damn
> rename(2) stuck as well.  Sure, having it taken only upon the race
> with rename() (or unlink(), for that matter) make it harder to get
> stuck with lock held, but that'll make the problem harder to reproduce
> and debug...

FWIW, how much trouble would we have if link(2) would do the following?

	find the parent of source
	lock it
	look the child up
	verify it's a non-directory
	bump child's i_nlink
		all failure exits past that point decrement child's i_nlink
	unlock the parent
	find the parent of destination
	lock it
	look the destination up
	call vfs_link
	decrement child's i_nlink - vfs_link has bumped it
	unlock the parent of destination

I do realize it can lead to leaked link count on a crash, obviously...