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=-2.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, 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 68BC8C43460 for ; Fri, 23 Apr 2021 08:14:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3CC9F61409 for ; Fri, 23 Apr 2021 08:14:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241350AbhDWIPZ (ORCPT ); Fri, 23 Apr 2021 04:15:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44444 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241147AbhDWIPX (ORCPT ); Fri, 23 Apr 2021 04:15:23 -0400 Received: from mail-il1-x136.google.com (mail-il1-x136.google.com [IPv6:2607:f8b0:4864:20::136]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C3F90C061574; Fri, 23 Apr 2021 01:14:45 -0700 (PDT) Received: by mail-il1-x136.google.com with SMTP id y10so18183460ilv.0; Fri, 23 Apr 2021 01:14:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=pTDg6zKpQu7cJZ1DqdWrnVuB9GdhXUrPkPNL5cWVUoU=; b=aE3ECeuajeVMQgcnoBSuf7Z3TeBiQ/lS437h4ybxx+yOsC33xDXSuKmJGkzDfei9OJ V4PjSWxmYMuaaq7cghJDsuCVnHimX4v/dwmpZz4e0CvH4nrgMa0GL07DlXPBTg/PRSmR 0lOYk+tQKEKZuNBpQ39ppzUDjHHO2CDX6k0rD4ja8k75Bxj9Kw2ekPxp4MhjKRAQStp+ ZVxzwITqQhjf01SQ6ZNVPqwyuJ1VDFvYJHqzaYbWYEmdHCw+SpeSY0RWUYPQ98bmb85y KEwmdgAMCYi2I+cTXqR97g26qdyNSm6Yr6dLdHKgTMGYAoQHVF43zMrsUjSCfPdbDx3g JxXw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=pTDg6zKpQu7cJZ1DqdWrnVuB9GdhXUrPkPNL5cWVUoU=; b=KkagjUoDJfpb6k3Gjmq03XWek+On2/C5eMO8lyNNvo26fFZf4la0lHsfvlvvL3emL/ E1Sc3JTmH68e025i0MDy4lJyiL57fWgeOqlPYgkaXOPjetS3IcJGUJxqh2KQOElEyEvj okBVwtzBR0soNIUKpyUo+HFVhWlZLuMZst8vYidoq68cNDPmxZefvwU/Unel2XjFkGpM Lf6/ZLbPz5oJQtsC6nEJbH9cuo+5ohAG+82IjudjTRJEvghYnZHSjD/ajql5gy8RS/5e ItF6iCtIA6i3smkWCx8iQmuOafn53CZTL92TYFsXBpalG9d53s2WN47VIesCOzuimlbS 8uLQ== X-Gm-Message-State: AOAM533v1W5Y/Ti50Pl+fx+mBVo1P8WlV1ixKVxLKuTyr5xCDDniYu/T A5PpjBMlZy7Lieh9Tuvx7zcxpu6PXA56EsLTcq4= X-Google-Smtp-Source: ABdhPJzy/tF0+EBI4DAJ58uPJtkD06A/kUBEYti8E5fIhH/NZGiKAE9RdUrDYa1PyGEBdKVsNB+w25Re+FXgPH33Jx8= X-Received: by 2002:a92:b74a:: with SMTP id c10mr1987086ilm.72.1619165685224; Fri, 23 Apr 2021 01:14:45 -0700 (PDT) MIME-Version: 1.0 References: <20210419132020.ydyb2ly6e3clhe2j@wittgenstein> <20210419135550.GH8706@quack2.suse.cz> <20210419150233.rgozm4cdbasskatk@wittgenstein> <20210421080449.GK8706@quack2.suse.cz> In-Reply-To: From: Amir Goldstein Date: Fri, 23 Apr 2021 11:14:34 +0300 Message-ID: Subject: Re: [PATCH 2/2] fanotify: Add pidfd support to the fanotify API To: Matthew Bobrowski Cc: Jan Kara , Christian Brauner , Linux API , linux-fsdevel Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, Apr 23, 2021 at 11:02 AM Matthew Bobrowski wrote: > > On Fri, Apr 23, 2021 at 10:39:46AM +0300, Amir Goldstein wrote: > > On Fri, Apr 23, 2021 at 2:06 AM Matthew Bobrowski wrote: > > > > > > On Wed, Apr 21, 2021 at 10:04:49AM +0200, Jan Kara wrote: > > > > On Tue 20-04-21 12:36:59, Matthew Bobrowski wrote: > > > > > On Mon, Apr 19, 2021 at 05:02:33PM +0200, Christian Brauner wrote: > > > > > > A general question about struct fanotify_event_metadata and its > > > > > > extensibility model: > > > > > > looking through the code it seems that this struct is read via > > > > > > fanotify_rad(). So the user is expected to supply a buffer with at least > > > > > > > > > > > > #define FAN_EVENT_METADATA_LEN (sizeof(struct fanotify_event_metadata)) > > > > > > > > > > > > bytes. In addition you can return the info to the user about how many > > > > > > bytes the kernel has written from fanotify_read(). > > > > > > > > > > > > So afaict extending fanotify_event_metadata should be _fairly_ > > > > > > straightforward, right? It would essentially the complement to > > > > > > copy_struct_from_user() which Aleksa and I added (1 or 2 years ago) > > > > > > which deals with user->kernel and you're dealing with kernel->user: > > > > > > - If the user supplied a buffer smaller than the minimum known struct > > > > > > size -> reject. > > > > > > - If the user supplied a buffer < smaller than what the current kernel > > > > > > supports -> copy only what userspace knows about, and return the size > > > > > > userspace knows about. > > > > > > - If the user supplied a buffer that is larger than what the current > > > > > > kernel knows about -> copy only what the kernel knows about, zero the > > > > > > rest, and return the kernel size. > > > > > > > > > > > > Extension should then be fairly straightforward (64bit aligned > > > > > > increments)? > > > > > > > > > > You'd think that it's fairly straightforward, but I have a feeling > > > > > that the whole fanotify_event_metadata extensibility discussion and > > > > > the current limitation to do so revolves around whether it can be > > > > > achieved in a way which can guarantee that no userspace applications > > > > > would break. I think the answer to this is that there's no guarantee > > > > > because of <>, so the decision to extend fanotify's feature > > > > > set was done via other means i.e. introduction of additional > > > > > structures. > > > > > > > > There's no real problem extending fanotify_event_metadata. We already have > > > > multiple extended version of that structure in use (see e.g. FAN_REPORT_FID > > > > flag and its effect, extended versions of the structure in > > > > include/uapi/linux/fanotify.h). The key for backward compatibility is to > > > > create extended struct only when explicitely requested by a flag when > > > > creating notification group - and that would be the case here - > > > > FAN_REPORT_PIDFD or how you called it. It is just that extending the > > > > structure means adding 8 bytes to each event and parsing extended structure > > > > is more cumbersome than just fetching s32 from a well known location. > > > > > > > > On the other hand extended structure is self-describing (i.e., you can tell > > > > the meaning of all the fields just from the event you receive) while > > > > reusing 'pid' field means that you have to know how the notification group > > > > was created (whether FAN_REPORT_PIDFD was used or not) to be able to > > > > interpret the contents of the event. Actually I think the self-describing > > > > feature of fanotify event stream is useful (e.g. when application manages > > > > multiple fanotify groups or when fanotify group descriptors are passed > > > > among processes) so now I'm more leaning towards using the extended > > > > structure instead of reusing 'pid' as Christian suggests. I'm sorry for the > > > > confusion. > > > > > > This approach makes sense to me. > > > > > > Jan/Amir, just to be clear, we've agreed to go ahead with the extended > > > struct approach whereby specifying the FAN_REPORT_PIDFD flag will > > > result in an event which includes an additional struct > > > (i.e. fanotify_event_info_pid) alongside the generic existing > > > > struct fanotify_event_info_pidfd? > > Well, yeah? I mean, my line of thought was that we'd also need to > include struct fanotify_event_info_header alongside the event to > provide more meta-information about the additional event you'd expect > to receive when FAN_REPORT_PIDFD is provided, so we'd end up with > something like: > > struct fanotify_event_info_pidfd { > struct fanotify_event_info_header hdr; > __s32 pidfd; > } > > Unless this of course is overbaking it and there's no need to do this? > We need this. I was just pointing out that you wrote fanotify_event_info_pid must have been a typo. Thanks, Amir.