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,URIBL_BLOCKED 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 F386BC2BA1A for ; Thu, 9 Apr 2020 03:17:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C9EE02087E for ; Thu, 9 Apr 2020 03:17:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726545AbgDIDRR (ORCPT ); Wed, 8 Apr 2020 23:17:17 -0400 Received: from relay9-d.mail.gandi.net ([217.70.183.199]:45873 "EHLO relay9-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726534AbgDIDRR (ORCPT ); Wed, 8 Apr 2020 23:17:17 -0400 X-Originating-IP: 50.39.163.217 Received: from localhost (50-39-163-217.bvtn.or.frontiernet.net [50.39.163.217]) (Authenticated sender: josh@joshtriplett.org) by relay9-d.mail.gandi.net (Postfix) with ESMTPSA id A55BDFF806; Thu, 9 Apr 2020 03:17:11 +0000 (UTC) Date: Wed, 8 Apr 2020 20:17:08 -0700 From: Josh Triplett To: Aleksa Sarai Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, io-uring@vger.kernel.org, linux-arch@vger.kernel.org, Alexander Viro , Arnd Bergmann , Jens Axboe Subject: Re: [PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation Message-ID: <20200409031708.GC6149@localhost> References: <90bf6fd43343ca862e7f61b0834baf2bdbd0e24c.1586321767.git.josh@joshtriplett.org> <20200408120040.mtkqmymfazrv3lqk@yavin.dot.cyphar.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200408120040.mtkqmymfazrv3lqk@yavin.dot.cyphar.com> Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Wed, Apr 08, 2020 at 10:00:40PM +1000, Aleksa Sarai wrote: > On 2020-04-07, Josh Triplett wrote: > > Some applications want to prevent the usual "lowest available fd" > > allocation from allocating certain file descriptors. For instance, they > > may want to prevent allocation of a closed fd 0, 1, or 2 other than via > > dup2/dup3, or reserve some low file descriptors for other purposes. > > > > Add a prctl to increase the minimum fd and return the previous minimum. > > > > System calls that allocate a specific file descriptor, such as > > dup2/dup3, ignore this minimum. > > > > exec resets the minimum fd, to prevent one program from interfering with > > another program's expectations about fd allocation. > > Why is it implemented as an "increase the value" interface? It feels > like this is meant to avoid some kind of security trap (with a library > reducing the value) but it means that if you want to temporarily raise > the minimum fd number it's not possible (without re-exec()ing yourself, > which is hardly a fun thing to do). > > Then again, this might've been discussed before and I missed it... It was: the previous version was a "get" and "set" interface. That interface didn't allow for the possibility that something else in the process had already set a minimum. This new atomic increase interface (which also serves as a "get" interface if you pass 0) makes it possible for a userspace library to reserve a range. (You have no guarantee about previously allocated descriptors in that range, but you know that no *new* automatically allocated descriptors will appear in that range, which suffices; userspace can do the rest.) - Josh Triplett