From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=9xw0=ZF=vger.kernel.org=linux-m68k-owner@kernel.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
X-Spam-Level: 
X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,
	MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no
	autolearn_force=no version=3.4.0
Received: from mail.kernel.org (mail.kernel.org [198.145.29.99])
	by smtp.lore.kernel.org (Postfix) with ESMTP id 9C00AC43331
	for <linux-m68k@archiver.kernel.org>; Wed, 13 Nov 2019 02:09:58 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [209.132.180.67])
	by mail.kernel.org (Postfix) with ESMTP id 76CC321783
	for <linux-m68k@archiver.kernel.org>; Wed, 13 Nov 2019 02:09:58 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1730713AbfKMCJv (ORCPT <rfc822;linux-m68k@archiver.kernel.org>);
        Tue, 12 Nov 2019 21:09:51 -0500
Received: from zeniv.linux.org.uk ([195.92.253.2]:39438 "EHLO
        ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1727468AbfKMCJt (ORCPT
        <rfc822;linux-m68k@lists.linux-m68k.org>);
        Tue, 12 Nov 2019 21:09:49 -0500
Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux))
        id 1iUi5t-0008S0-DR; Wed, 13 Nov 2019 02:09:17 +0000
Date:   Wed, 13 Nov 2019 02:09:17 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Jeff Layton <jlayton@kernel.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Arnd Bergmann <arnd@arndb.de>,
        David Howells <dhowells@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Jann Horn <jannh@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Eric Biederman <ebiederm@xmission.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Tycho Andersen <tycho@tycho.ws>,
        David Drysdale <drysdale@google.com>,
        Chanho Min <chanho.min@lge.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Aleksa Sarai <asarai@suse.de>,
        containers@lists.linux-foundation.org, linux-alpha@vger.kernel.org,
        linux-api@vger.kernel.org, libc-alpha@sourceware.org,
        linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org,
        linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
        linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org
Subject: Re: [PATCH v15 6/9] namei: LOOKUP_{IN_ROOT,BENEATH}: permit limited
 ".." resolution
Message-ID: <20191113020917.GC26530@ZenIV.linux.org.uk>
References: <20191105090553.6350-1-cyphar@cyphar.com>
 <20191105090553.6350-7-cyphar@cyphar.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20191105090553.6350-7-cyphar@cyphar.com>
User-Agent: Mutt/1.12.1 (2019-06-15)
Sender: linux-m68k-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-m68k.vger.kernel.org>
X-Mailing-List: linux-m68k@vger.kernel.org
Message-ID: <20191113020917.hXIr8dFGdoTjkq9gC0oeqXXOJ2KTY1ARfeYvrlFC7K4@z>

On Tue, Nov 05, 2019 at 08:05:50PM +1100, Aleksa Sarai wrote:

> One other possible alternative (which previous versions of this patch
> used) would be to check with path_is_under() if there was a racing
> rename or mount (after re-taking the relevant seqlocks). While this does
> work, it results in possible O(n*m) behaviour if there are many renames
> or mounts occuring *anywhere on the system*.

BTW, do you realize that open-by-fhandle (or working nfsd, for that matter)
will trigger arseloads of write_seqlock(&rename_lock) simply on d_splice_alias()
bringing disconnected subtrees in contact with parent?