From mboxrd@z Thu Jan  1 00:00:00 1970
From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)
Subject: Re: [PATCH review 5/6] vfs: Test for and handle paths that are
	unreachable from their mnt_root
Date: Mon, 10 Aug 2015 14:34:41 -0500
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To: Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
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Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org> writes:

> On Mon, Aug 03, 2015 at 04:30:22PM -0500, Eric W. Biederman wrote:
>
>> +	if (!is_subdir(nd->path.dentry, mnt->mnt_root))
>> +		return false;
>
> Umm...  What's to protect us from racing with d_move() right here?

is_subdir does the read_seqretry on rename_lock.  Which is enough
to ensure connectivity exists at a single moment in time.

Beyond that the entire path lookup races with d_move, and the code
calls path_connected just after finding the parent directory, which
ensures that in the moment that follow_dotdot is setting nd->dentry
that the original nd->dentry is connected, and by extension the new
as the new one is an ancestor.

Or are you thinking of a different race?

Eric