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Date:   Sat, 13 Oct 2018 10:04:32 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Aleksa Sarai <asarai@suse.de>, Jann Horn <jannh@google.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>, jlayton@kernel.org,
        Bruce Fields <bfields@fieldses.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        David Howells <dhowells@redhat.com>, christian@brauner.io,
        Tycho Andersen <tycho@tycho.ws>,
        David Drysdale <drysdale@google.com>, dev@opencontainers.org,
        containers@lists.linux-foundation.org,
        linux-fsdevel@vger.kernel.org,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH v3 3/3] namei: aggressively check for nd->root escape on
 ".." resolution
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On Sat, Oct 13, 2018 at 07:53:26PM +1100, Aleksa Sarai wrote:

> I didn't know about path_is_under() -- I just checked and it appears to
> not take &rename_lock? From my understanding, in order to protect
> against the rename attack you need to take &rename_lock (or check
> against &rename_lock at least and retry if it changed).
> 
> I could definitely use path_is_under() if you prefer, though I think
> that in this case we'd need to take &rename_lock (right?). Also is there
> a speed issue with taking the write-side of a seqlock when we are just
> reading -- is this more efficient than doing a retry like in __d_path?

???

1) it uses is_subdir(), which does deal with rename_lock
2) what it does is taking mount_lock.lock.  I.e. the same
thing as the second retry in __d_path().  _If_ it shows
up in profiles, we can switch it to read_seqbegin_or_lock(),
but I'd like to see the profiling data first.