From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: 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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 9C8F2C33CB7 for ; Sat, 18 Jan 2020 18:09:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7EDBC21D56 for ; Sat, 18 Jan 2020 18:09:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726974AbgARSJz (ORCPT ); Sat, 18 Jan 2020 13:09:55 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:51438 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726490AbgARSJz (ORCPT ); Sat, 18 Jan 2020 13:09:55 -0500 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1issXV-00B1Vm-HC; Sat, 18 Jan 2020 18:09:41 +0000 Date: Sat, 18 Jan 2020 18:09:41 +0000 From: Al Viro To: Aleksa Sarai Cc: Jeff Layton , "J. Bruce Fields" , Shuah Khan , Florian Weimer , David Laight , Christian Brauner , quae@daurnimator.com, dev@opencontainers.org, containers@lists.linux-foundation.org, libc-alpha@sourceware.org, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v3 0/2] openat2: minor uapi cleanups Message-ID: <20200118180941.GT8904@ZenIV.linux.org.uk> References: <20200115144831.GJ8904@ZenIV.linux.org.uk> <20200118120800.16358-1-cyphar@cyphar.com> <20200118152833.GS8904@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200118152833.GS8904@ZenIV.linux.org.uk> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jan 18, 2020 at 03:28:33PM +0000, Al Viro wrote: > #work.openat2 updated, #for-next rebuilt and force-pushed. There's > a massive update of #work.namei as well, also pushed out; not in > #for-next yet, will post the patch series for review later today. BTW, looking through that code again, how could this static bool legitimize_root(struct nameidata *nd) { /* * For scoped-lookups (where nd->root has been zeroed), we need to * restart the whole lookup from scratch -- because set_root() is wrong * for these lookups (nd->dfd is the root, not the filesystem root). */ if (!nd->root.mnt && (nd->flags & LOOKUP_IS_SCOPED)) return false; possibly trigger? The only things that ever clean ->root.mnt are 1) failing legitimize_path(nd, &nd->root, nd->root_seq) in legitimize_root() itself. If *ANY* legitimize_path() has failed, we are through - RCU pathwalk is given up. In particular, if you look at the call chains leading to legitimize_root(), you'll see that it's called by unlazy_walk() or unlazy_child() and failure has either of those buggger off immediately. The same goes for their callers; fail any of those and we are done; the very next thing that will be done with that nameidata is going to be terminate_walk(). We don't look at its fields, etc. - just return to the top level ASAP and call terminate_walk() on it. Which is where we run into if (nd->flags & LOOKUP_ROOT_GRABBED) { path_put(&nd->root); nd->flags &= ~LOOKUP_ROOT_GRABBED; } paired with setting LOOKUP_ROOT_GRABBED just before the attempt to legitimize in legitimize_root(). The next thing *after* terminate_walk() is either path_init() or the end of life for that struct nameidata instance. This is really, really fundamental for understanding the whole thing - a failure of unlazy_walk/unlazy_child means that we are through with that attempt. 2) complete_walk() doing if (!(nd->flags & (LOOKUP_ROOT | LOOKUP_IS_SCOPED))) nd->root.mnt = NULL; Can't happen with LOOKUP_IS_SCOPED in flags, obviously. 3) path_init(). Where it's followed either by leaving through if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) { .... } (and LOOKUP_IS_SCOPED includes LOOKUP_IN_ROOT) or with a failure exit (no calls of *anything* but terminate_walk() after that or with if (flags & LOOKUP_IS_SCOPED) { nd->root = nd->path; ... and that makes damn sure nd->root.mnt is not NULL. And neither of the LOOKUP_IS_SCOPED bits ever gets changed in nd->flags - they remain as path_init() has set them. The same, BTW, goes for the check you've added in the beginning of set_root() - set_root() is called only with NULL nd->root.mnt (trivial to prove) and that is incompatible with LOOKUP_IS_SCOPED. I'm kinda-sorta OK with having WARN_ON() there for a while, but IMO the check in the beginning of legitimize_root() should go away - this kind of defensive programming only makes harder to reason about the behaviour of the entire thing. And fs/namei.c is too convoluted as it is...