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=-13.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL 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 DBA5BC433B4 for ; Fri, 23 Apr 2021 08:03:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A436F6120F for ; Fri, 23 Apr 2021 08:03:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241577AbhDWIDx (ORCPT ); Fri, 23 Apr 2021 04:03:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41708 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241655AbhDWIDd (ORCPT ); Fri, 23 Apr 2021 04:03:33 -0400 Received: from mail-pl1-x634.google.com (mail-pl1-x634.google.com [IPv6:2607:f8b0:4864:20::634]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3896CC06138C for ; Fri, 23 Apr 2021 01:02:57 -0700 (PDT) Received: by mail-pl1-x634.google.com with SMTP id n10so13698384plc.0 for ; Fri, 23 Apr 2021 01:02:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=Zkwqr4ii+y2yCGvUW33yVCSAvzQ6+dlaSRw7HFJQvbc=; b=C7XvTy5eOHk1xRblYxz2nk9GJKaPr9grYTNBEnjJ78gjE0Jher/h6PYLIG97mzJ2Mh aDaMaZCAqGtN26ZOlsX/dtvMJO8v5zDrT008M8Wol1gzyyhNvraaFRZWHhB0UehW37Bv 8jDkTEdjWZ54Hnru7DXbmzrDV65bTfTxFbyido3TDPYH9jJISzl71epy0HjRtiUaT6pK nC/B/AWBS7q2WfF1WZn4dHgvW9phnkq9EyuvEP1Z780TiTBPQJ1rjv7HwdQzxgCr80dL c4kAW2pbpJk9KaUTBtcPdFIwiXzoQiLK1xS45tGno+T7FKLg7BkeQLL9ppLLZ3cp6OnB 0YFw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=Zkwqr4ii+y2yCGvUW33yVCSAvzQ6+dlaSRw7HFJQvbc=; b=XeOFiqjFw6+FqhAh/DC/DicvJ5rE2rOhcC9Lvv1+q51YiOb93vp2J50a32R2rWLH5f km5Zw0LgNVvmin/jR6xqw1I+3gYjWtML2YbtzEEZl0s/eLeR6QqwG2t5ZOF9IAYEarPp laYAqF8Eao3He3nOPTS5h1gJ+QUUBPO2Rc/tYHYdqTcqBDouBe1ZmllAxnwj5vzRLA43 884rgmEaL/81srlZFCJ3mEpVSuiI45ljr0InrQjNpp19riQUBypt3g6z3i6V9OEtZ6Vb hmNDDk83a3wuYZzdq6YvfdAvCWwELnDlVl7GcecC4Wj94HvjswyaX7i5vJbgU6R7ZTu5 jc7g== X-Gm-Message-State: AOAM532o9zS66xC9zIfo4+NxnuOYp6Cyj1flHimG6MdACYi8Bh1PQ0yq 4L4fJOfJsJ1G/bNWvesI/HIXep670OM8sShu X-Google-Smtp-Source: ABdhPJxWew8WyY5y3xI1CHVk9SRXGkAs3qTR9Yn8Dsk066v8X/hJuKqYnLIQVkQRB22GAow8wbwNkA== X-Received: by 2002:a17:902:8308:b029:e9:d69:a2f with SMTP id bd8-20020a1709028308b02900e90d690a2fmr2855049plb.20.1619164976557; Fri, 23 Apr 2021 01:02:56 -0700 (PDT) Received: from google.com ([2401:fa00:9:211:686a:2391:ed27:7821]) by smtp.gmail.com with ESMTPSA id h11sm3968099pjs.52.2021.04.23.01.02.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Apr 2021 01:02:55 -0700 (PDT) Date: Fri, 23 Apr 2021 18:02:44 +1000 From: Matthew Bobrowski To: Amir Goldstein Cc: Jan Kara , Christian Brauner , Linux API , linux-fsdevel Subject: Re: [PATCH 2/2] fanotify: Add pidfd support to the fanotify API Message-ID: References: <20210419132020.ydyb2ly6e3clhe2j@wittgenstein> <20210419135550.GH8706@quack2.suse.cz> <20210419150233.rgozm4cdbasskatk@wittgenstein> <20210421080449.GK8706@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org 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? /M