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 A278EC43460 for ; Fri, 23 Apr 2021 07:39:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 56129613B0 for ; Fri, 23 Apr 2021 07:39:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240603AbhDWHke (ORCPT ); Fri, 23 Apr 2021 03:40:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36672 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229993AbhDWHkd (ORCPT ); Fri, 23 Apr 2021 03:40:33 -0400 Received: from mail-io1-xd2d.google.com (mail-io1-xd2d.google.com [IPv6:2607:f8b0:4864:20::d2d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DF68FC061574; Fri, 23 Apr 2021 00:39:57 -0700 (PDT) Received: by mail-io1-xd2d.google.com with SMTP id s16so42835061iog.9; Fri, 23 Apr 2021 00:39:57 -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=0eFyP4CBoxR2zXMawnsNjKh1kmbyyJeUuqoBmNoBQrk=; b=jXOvDIseSI6lYeNT1dQ+Eaxpj7kkknv1mqtIzkyca0mPEwtkdpt4frIuvCj+cFLkGr 06qjvVSxiJJSw3D82uCmUXJnrCxtekwO8Erjh1e+HMxVtOS1BMddtln1ORexxX5W6p9B ChH2sdjIr3c6QzePfK395g2F4iZmmouqx7CDG4bPMe9DzbQaXU6YO6rNBvu/hX3EyouP +ELllLDL26y0Vpkv9pkPzfNWnivzVRzIJLRSIkLTKHqFjMxkKQHFJL+FXU0WKaKMXfVZ CQDDlz8GvkjNBDPhzGzNyyKH9g6it6cgr8LeIs9ZC0JNcMbKCE2YHfl2Qpmmy2bvklwj crwQ== 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=0eFyP4CBoxR2zXMawnsNjKh1kmbyyJeUuqoBmNoBQrk=; b=l1l2iFanEb3ybl1pcQCYHVULW/prOc1b5L15Fi0KJZksvJe/8WB4eLW900+4L3fImg Fz2q+Mnu1eaSs8vdkH4tLIKHUtISYJKkClyrSzkAzvsaxYDekFpWdNhulS5stybVotlC QkDi94BAQpBFcsiWZTYwX7vHN6iUdi6gJz54i28ADTTOErkdEYqbGnuIOV5pGx2uVZr0 ZQLqtZFqDs7IJl9armkVIEXptq8m5Z2KuCdQqAnWiAgA5n0ZA+SLsFk/rOzW0OrKn9bG wCtwesuQ1PvgPHnS5cjOc1wUmVKEpyRP+KS1wa7ddLBfVOVF8U5AH0W2iFxXbWPSBqe8 y55g== X-Gm-Message-State: AOAM533nzZIXswr6v6/7nYCNqVc/WgFIdq99S44vnZGVto8LRTrl7+ZX xSvx3QxM2A/+hxvAZao34ZoLYifYNnIWnCrrAUc= X-Google-Smtp-Source: ABdhPJyzm6DHxbShrKljxWmaSfmTEQlzg5urevalIrK5Yt6eAjbI6CJrDAwayZFCiIULDo+9OjNYasu9vhHjdBXjk+8= X-Received: by 2002:a05:6602:58d:: with SMTP id v13mr2281525iox.64.1619163597209; Fri, 23 Apr 2021 00:39:57 -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 10:39:46 +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 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? > fanotify_event_metadata (also ensuring that pid has been > provided). Events will be provided to userspace applications just like > when specifying FAN_REPORT_FID, correct? > Yes. Thanks, Amir.