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,URIBL_BLOCKED,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 35BBCC47087 for ; Tue, 25 May 2021 23:21:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1B7E06141C for ; Tue, 25 May 2021 23:21:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231174AbhEYXWk (ORCPT ); Tue, 25 May 2021 19:22:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54720 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229790AbhEYXWj (ORCPT ); Tue, 25 May 2021 19:22:39 -0400 Received: from mail-pg1-x530.google.com (mail-pg1-x530.google.com [IPv6:2607:f8b0:4864:20::530]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8675DC061574 for ; Tue, 25 May 2021 16:21:08 -0700 (PDT) Received: by mail-pg1-x530.google.com with SMTP id q15so23983097pgg.12 for ; Tue, 25 May 2021 16:21:08 -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=N9pRtcpwEgIj6sC0tHqwYGJBlDf6VjtQs2mfFfUfl6g=; b=eM5f6HsI/GmdQGh0ezMqwI/wS6K88p5V+4+nkKfgg7z9bCJVxfabyrOhNmdw00kB33 tCk9CY+4PJEK82DpKSgc/nd/RIW7ux57thwYKIx29lVMv+6qOXK8DT3ufc79lKKWA8vR 9Mc/u7M7h1ZmFQzmKYjHQSTnxQpIOZBC0Mp3Fu0/LDUIlheg42ubaR+aV9VlDKemejyv iAcpZ8Zxl7NKRsUvQe/IjPsINfN2wzvmp4JAezHdvy9SeN802OHAWjHZLIpxTANGwLeL /uucrr9tIwGKayjHp7CRnA0eWe9b1P36Fsgip83lycGpNuwzqEPLqPa9i/AHF2txImAS uZwg== 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=N9pRtcpwEgIj6sC0tHqwYGJBlDf6VjtQs2mfFfUfl6g=; b=Xq8F2ArVvKM9HZTzSeKda0Q+nV1qhJkjQJ+zZZyxHQFhfeqkjt0+ssjHFNe8G4MYu+ 5CWj8q7or7CY2zvrAmo27weB75SMakCkh5u3jfDrA4aanJqbHpuSq+fsKalO4HxgjT0Y a5X28kDpD5XgM+0O38lqgLILcPSKxtBqIXLBpemFvF1iv6Q6dzo3ioqjzPvFZbMbl2mK RayIdUYEZXyKDQiB1mU4/KPoB6fHEKgt1C/Yb9JlkZpQfJtXfr1/3bk/5RG3/Obg1Mv7 ODTHYwzPSh4drX9uq/3dU+JhvRgbSV0c3LHtGWjRvIDb0Y21vgDSVR3+LQ7XsfLL1uEe nH5w== X-Gm-Message-State: AOAM531bh7wATiZsnD1a9P7ijk6UDm2J3GCtQRommxt3uPD/++Dkf5N0 sVaZiXY8F3UPoK0Y1WjQe98EgQ== X-Google-Smtp-Source: ABdhPJzTaM1dRCFm8Mp7aPTQuQDnu9Zc4/Lrm8xw3bO2GrfQex+U+F4LYKC8FIWuLHWi9Xykyn29xQ== X-Received: by 2002:a65:48c2:: with SMTP id o2mr21347245pgs.376.1621984867810; Tue, 25 May 2021 16:21:07 -0700 (PDT) Received: from google.com ([2401:fa00:9:211:a122:6bc0:d8f6:9eea]) by smtp.gmail.com with ESMTPSA id k20sm119872pgl.72.2021.05.25.16.21.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 May 2021 16:21:06 -0700 (PDT) Date: Wed, 26 May 2021 09:20:55 +1000 From: Matthew Bobrowski To: Christian Brauner Cc: Jan Kara , amir73il@gmail.com, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org Subject: Re: [PATCH 0/5] Add pidfd support to the fanotify API Message-ID: References: <20210520135527.GD18952@quack2.suse.cz> <20210521104056.GG18952@quack2.suse.cz> <20210524084746.GB32705@quack2.suse.cz> <20210525103133.uctijrnffehlvjr3@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210525103133.uctijrnffehlvjr3@wittgenstein> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, May 25, 2021 at 12:31:33PM +0200, Christian Brauner wrote: > On Mon, May 24, 2021 at 10:47:46AM +0200, Jan Kara wrote: > > On Sat 22-05-21 09:32:36, Matthew Bobrowski wrote: > > > On Fri, May 21, 2021 at 12:40:56PM +0200, Jan Kara wrote: > > > > On Fri 21-05-21 20:15:35, Matthew Bobrowski wrote: > > > > > On Thu, May 20, 2021 at 03:55:27PM +0200, Jan Kara wrote: > > > > > There's one thing that I'd like to mention, and it's something in > > > > > regards to the overall approach we've taken that I'm not particularly > > > > > happy about and I'd like to hear all your thoughts. Basically, with > > > > > this approach the pidfd creation is done only once an event has been > > > > > queued and the notification worker wakes up and picks up the event > > > > > from the queue processes it. There's a subtle latency introduced when > > > > > taking such an approach which at times leads to pidfd creation > > > > > failures. As in, by the time pidfd_create() is called the struct pid > > > > > has already been reaped, which then results in FAN_NOPIDFD being > > > > > returned in the pidfd info record. > > > > > > > > > > Having said that, I'm wondering what the thoughts are on doing pidfd > > > > > creation earlier on i.e. in the event allocation stages? This way, the > > > > > struct pid is pinned earlier on and rather than FAN_NOPIDFD being > > > > > returned in the pidfd info record because the struct pid has been > > > > > already reaped, userspace application will atleast receive a valid > > > > > pidfd which can be used to check whether the process still exists or > > > > > not. I think it'll just set the expectation better from an API > > > > > perspective. > > > > > > > > Yes, there's this race. OTOH if FAN_NOPIDFD is returned, the listener can > > > > be sure the original process doesn't exist anymore. So is it useful to > > > > still receive pidfd of the dead process? > > > > > > Well, you're absolutely right. However, FWIW I was approaching this > > > from two different angles: > > > > > > 1) I wanted to keep the pattern in which the listener checks for the > > > existence/recycling of the process consistent. As in, the listener > > > would receive the pidfd, then send the pidfd a signal via > > > pidfd_send_signal() and check for -ESRCH which clearly indicates > > > that the target process has terminated. > > > > > > 2) I didn't want to mask failed pidfd creation because of early > > > process termination and other possible failures behind a single > > > FAN_NOPIDFD. IOW, if we take the -ESRCH approach above, the > > > listener can take clear corrective branches as what's to be done > > > next if a race is to have been detected, whereas simply returning > > > FAN_NOPIDFD at this stage can mean multiple things. > > > > > > Now that I've written the above and keeping in mind that we'd like to > > > refrain from doing anything in the event allocation stages, perhaps we > > > could introduce a different error code for detecting early process > > > termination while attempting to construct the info record. WDYT? > > > > Sure, I wouldn't like to overengineer it but having one special fd value for > > "process doesn't exist anymore" and another for general "creating pidfd > > failed" looks OK to me. > > FAN_EPIDFD -> "creation failed" > FAN_NOPIDFD -> "no such process" Yes, I was thinking something along the lines of this... With the approach that I've proposed in this series, the pidfd creation failure trips up in pidfd_create() at the following condition: if (!pid || !pid_has_task(pid, PIDTYPE_TGID)) return -EINVAL; Specifically, the following check: !pid_has_task(pid, PIDTYPE_TGID) In order to properly report either FAN_NOPIDFD/FAN_EPIDFD to userspace, AFAIK I'll have to do one of either two things to better distinguish between why the pidfd creation had failed: 1) Implement an additional check in pidfd_create() that effectively checks whether provided pid still holds reference to a struct pid that isn't in the process of being cleaned up. If it is being cleaned up, then return something like -ESRCH instead of -EINVAL so that the caller, in this case fanotify, can check and set FAN_NOPIDFD if -ESRCH is returned from pidfd_create(). I definitely don't feel as though returning -ESRCH from the !pid_has_task(pid, PIDTYPE_TGID) would be appropriate. In saying that, I'm not aware of a helper by which would allow us to perform such an in-flight check? Perhaps something needs to be introduced here, IDK... 2) Refrain from performing any further changes to pidfd_create() i.e. as proposed in option 1), and manually perform the pidfd creation from some kind of new fanotify helper, as suggested by you here [0]. However, I'm not convinved that I like this approach as we may end up slowly drifting away from pidfd creation semantics over time. [0] https://www.spinics.net/lists/linux-fsdevel/msg195556.html /M