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 4AC8CC2D0C0 for ; Sat, 21 Dec 2019 13:53:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E625821D7E for ; Sat, 21 Dec 2019 13:53:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726909AbfLUNxw (ORCPT ); Sat, 21 Dec 2019 08:53:52 -0500 Received: from mout.kundenserver.de ([212.227.126.134]:34285 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726339AbfLUNxv (ORCPT ); Sat, 21 Dec 2019 08:53:51 -0500 Received: from mail-qk1-f182.google.com ([209.85.222.182]) by mrelayeu.kundenserver.de (mreue012 [212.227.15.129]) with ESMTPSA (Nemesis) id 1MG90u-1iS9zd45cK-00GcBo; Sat, 21 Dec 2019 14:53:50 +0100 Received: by mail-qk1-f182.google.com with SMTP id z14so8770952qkg.9; Sat, 21 Dec 2019 05:53:49 -0800 (PST) X-Gm-Message-State: APjAAAWfxpQ8Ni+qs3L+IUdUwrTRHzu3mWhdoGW7koLU/Ww4oZjx4mXp Pv4W022I6m7YZW8XzfEqOeJ5whABEWSq4ZbKby4= X-Google-Smtp-Source: APXvYqzdkKB4eWJLyIMGbY84AVCwTo394eD7uI1YKdpNDtFn1NdJbNSgdawpABz0czREJyOLKl+WVti5eWx7OSdPMjA= X-Received: by 2002:a37:a8d4:: with SMTP id r203mr18290266qke.394.1576936428655; Sat, 21 Dec 2019 05:53:48 -0800 (PST) MIME-Version: 1.0 References: <20191218235459.GA17271@ircssh-2.c.rugged-nimbus-611.internal> <20191219103525.yqb5f4pbd2dvztkb@wittgenstein> <20191220043510.r5h6wvsp2p5glyjv@yavin.dot.cyphar.com> In-Reply-To: <20191220043510.r5h6wvsp2p5glyjv@yavin.dot.cyphar.com> From: Arnd Bergmann Date: Sat, 21 Dec 2019 13:53:32 +0000 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v4 2/5] pid: Add PIDFD_IOCTL_GETFD to fetch file descriptors from processes To: Aleksa Sarai Cc: Sargun Dhillon , Christian Brauner , Oleg Nesterov , Florian Weimer , "linux-kernel@vger.kernel.org" , Linux Containers , Linux API , Linux FS-devel Mailing List , Tycho Andersen , Jann Horn , Andy Lutomirski , Al Viro , Gian-Carlo Pascutto , =?UTF-8?Q?Emilio_Cobos_=C3=81lvarez?= , Jed Davis Content-Type: text/plain; charset="UTF-8" X-Provags-ID: V03:K1:X8kFLzryBpbh2nWdfRj5h+/1qG5+N7nbLw/cmfzzvQLHYnAls2Q d7j2+cIsRkKagtxMjLivXjhrxEVnGAoKGTQKqUFq+G50mobe0iGvLffdmhj3hkybOAPghmm Lh2GF1BIKZwDw/wNGEpCIeRB0yFG17cFYrb6KsIF1ApGmVsU0uah6DG3mQyZsPj7D+9/RRw emKlBwYnVM0VP/4+pIV4g== X-UI-Out-Filterresults: notjunk:1;V03:K0:WID3Qm7SKJs=:tBvWt3bGwwDfbxhLW4vA33 6H3b6vNIHT2JSKV2JaTOUZVnQTXz0Sm2DNJEQXilnfpIIJLTS6DP/zoKvwT8Z+fSS5qU+oM1l NFW3uEFBOzMHMZxR+xsP7WHbHx2Z28CjzNb/SJd2xwedBKKPTRb9hzr0YFFm37KH9g1fr0zLQ aKS7hv5D5K0c7ai97gV2DFlUO67GhyPe+j1Otwol2BzzhccOrKuE2X2NFx+26n2ywYVx4BCl1 vTazP0iloGeb8yETb+TFgmjd1g+gV7ktLsCtCoM8l8VZjxC90Ey5FOJMst/FfCPDzjyYzRugn L/UXiFtsmzJrRNhU1806BcPm1S6N/Rasgp5OwE0EkMlssqiDqhae/G1dxWrnZTEC6yZrCgSdz vPqHi7QzeBisD7v5SesyiLKl9/Koz8LPawRj5fGF/U+mLQN70UvQ4egH6Z8ADfBsmW3GC1NSw eIL9kg07vbhhnm36XELhbsj3dwIgZOrFPUb12Zd71pUiLorfaBThwJN5S9rplMDHMx5aZ1pGW RI3irOewibSRX3ppsaTaBNFptfw1BdsING7XVreaY/9A0AH/cIef6cs9Qt3I+AT7qL30bEEt0 G2mApF7h8r9rcg0YMIjF6VxDtU/CogUP8p8DeRVtShEd1ZSTAqPNEC8y07RYz2jPlmc35/vUf dc+shbuMXmQvU6MDNNMPVSpAke/+uRoZPTopjR9M3hSFlaRM6bLuqqIyeP+ieguqs+XyawgZY TACRR32lKW2WFAr0d0jiV4eNuMZcMH5m2UNnuYcXPZb7GCLftJ3m/btRD9kDj7OsXFzOrfLyC YbqTl4P0yib0iKqfF0+TXP+0B2FbxasRtU9gm/O2cXY12WmjFIV9gYSMmNefmdKUFaO9+d9rV 5hjjLXdrVkUG81St/LuQ== Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 20, 2019 at 4:35 AM Aleksa Sarai wrote: > > On 2019-12-19, Sargun Dhillon wrote: > > On Thu, Dec 19, 2019 at 2:35 AM Christian Brauner > > wrote: > > > I guess this is the remaining question we should settle, i.e. what do we > > > prefer. > > > I still think that adding a new syscall for this seems a bit rich. On > > > the other hand it seems that a lot more people agree that using a > > > dedicated syscall instead of an ioctl is the correct way; especially > > > when it touches core kernel functionality. I mean that was one of the > > > takeaways from the pidfd API ioctl-vs-syscall discussion. > > > > > > A syscall is nicer especially for core-kernel code like this. > > > So I guess the only way to find out is to try the syscall approach and > > > either get yelled and switch to an ioctl() or have it accepted. > > > > > > What does everyone else think? Arnd, still in favor of a syscall I take > > > it. Oleg, you had suggested a syscall too, right? Florian, any > > > thoughts/worries on/about this from the glibc side? > > > > > > Christian > > > > My feelings towards this are that syscalls might pose a problem if we > > ever want to extend this API. Of course we can have a reserved > > "flags" field, and populate it later, but what if we turn out to need > > a proper struct? I already know we're going to want to add one > > around cgroup metadata (net_cls), and likely we'll want to add > > a "steal" flag as well. As Arnd mentioned earlier, this is trivial to > > fix in a traditional ioctl environment, as ioctls are "cheap". How > > do we feel about potentially adding a pidfd_getfd2? Or are we > > confident that reserved flags will save us? > > If we end up making this a syscall, then we can re-use the > copy_struct_from_user() API to make it both extensible and compatible in > both directions. I wasn't aware that this was frowned upon for ioctls > (sorry for the extra work) but there are several syscalls which use this > model for extendability (clone3, openat2, sched_setattr, > perf_events_open) so there shouldn't be any such complaints for a > syscall which is extensible. I would still not do it for syscalls, although for other reasons: - in an ioctl, it's better to come up with a new command code if you have a larger structure - in a system call, it's best to pass all arguments as individual registers, the only time we use indirect data structures is when there are more than six arguments. Arnd