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 C7276C4708F for ; Wed, 2 Jun 2021 06:30:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9F2EC6139A for ; Wed, 2 Jun 2021 06:30:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230087AbhFBGca (ORCPT ); Wed, 2 Jun 2021 02:32:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42214 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229566AbhFBGca (ORCPT ); Wed, 2 Jun 2021 02:32:30 -0400 Received: from mail-pg1-x52d.google.com (mail-pg1-x52d.google.com [IPv6:2607:f8b0:4864:20::52d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A81FBC061574 for ; Tue, 1 Jun 2021 23:30:47 -0700 (PDT) Received: by mail-pg1-x52d.google.com with SMTP id l1so1427512pgm.1 for ; Tue, 01 Jun 2021 23:30:47 -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=RPYw0CT/sE7e6+GkZ/qmyyF9rUrngI3hRJtgYQrtWFE=; b=Kzm2HQvm4SLVEHwYofwWI11Wz/7F3uZcSTifmcRfSJ7aF3dcABFEHDkK94G6NsFlmp tOrEI3zT9kkW8UIRbKWmJX9ChAoHuwLCfexmaFPJehT7EO3Qgir9r5cWpHR818aNwAHh fjC/tqTyepUy3js+8rtvIeyKaf5pEhH4rCBKUROgC+olZLlHSJHGZ33DSeBLBUn9A5NW 59vw75dXMqUEqc23HYwqnWxRq5qVp657RDuZidBDQ2Pi7PQ1ilgYg2K3lpEqawlKGmNx 7qbc73fSDwTFUEDooiLVOFqh3WDyLrSZbdNBAqdsZteFoal9OYJLofR6eKBc2fE+DNyv kKAw== 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=RPYw0CT/sE7e6+GkZ/qmyyF9rUrngI3hRJtgYQrtWFE=; b=RJd5G6CzSZkDE/JS5k+ciiPo2t+3O83cp3zChCghCxAEFK9hGP+gJ4rOVA8KWDeHwj vJngR4wIFG3Y9QgXpNBNPqJBGrW9fdgPY0c1UDRmPgJz3L46ptzvD+EkHO3Fza8zuUJT h0DAR6aED/fmgxghwyTX/MEgNA8hvuJP2wO2zj1Q5c0i3XTjNRMGQCVxxnFVnxTjHtft PnXmsQS8JUgU1Xzu7BwYKktlRIEUMMcztoSG9ZXy97x7Vcqw/4jyitJ2O+64nzT9pNNe 0U+572A9eafLvsdwYGfQR+IGiPJ3PuIJg7In5l+97KUgSl2mKkRwZ0T7F5S2FBrsj09X VqOg== X-Gm-Message-State: AOAM531MY5GtfoSeaWI5s0RxafY0flKmEkgKQA880OvUYaEA95iCSv3k icNdla2JO6ducdkmzvKSpo+WMQ== X-Google-Smtp-Source: ABdhPJy+VDt8FFAwrcaJmi08ISHmoD12/Wv+2m3Nc5cNaalGGdn5FzWYfmIf9fMoNG13TVJWME/8VA== X-Received: by 2002:a63:1c1c:: with SMTP id c28mr6010276pgc.16.1622615446937; Tue, 01 Jun 2021 23:30:46 -0700 (PDT) Received: from google.com ([2401:fa00:9:211:3f5b:c29c:c9af:dde7]) by smtp.gmail.com with ESMTPSA id o22sm3749348pjq.28.2021.06.01.23.30.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Jun 2021 23:30:46 -0700 (PDT) Date: Wed, 2 Jun 2021 16:30:33 +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> <20210526180529.egrtfruccbioe7az@wittgenstein> <20210601114628.f3w33yyca5twgfho@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210601114628.f3w33yyca5twgfho@wittgenstein> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Jun 01, 2021 at 01:46:28PM +0200, Christian Brauner wrote: > On Tue, Jun 01, 2021 at 09:03:26PM +1000, Matthew Bobrowski wrote: > > On Wed, May 26, 2021 at 08:05:29PM +0200, Christian Brauner wrote: > > > On Wed, May 26, 2021 at 09:20:55AM +1000, Matthew Bobrowski wrote: > > > > 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: > > > > > > Ok, I see. You already do have a reference to a struct pid and in that > > > case we should just always return a pidfd to the caller. For > > > pidfd_open() for example we only report an error when > > > find_get_pid() doesn't find a struct pid to refer to. But in your > > > case here you already have a struct pid so I think we should just keep > > > this simple and always return a pidfd to the caller and in fact do > > > burden them with figuring out that the process is gone via > > > pidfd_send_signal() instead of complicating our lives here. > > > > Ah, actually Christian... Before, I go ahead and send through the updated > > series. Given what you've mentioned above I'm working with the assumption > > that you're OK with dropping the pid_has_task() check from pidfd_create() > > [0]. Is that right? > > > > If so, I don't know how I feel about this given that pidfd_create() is now > > to be exposed to the rest of the kernel and the pidfd API, as it stands, > > doesn't support the creation of pidfds for non-thread-group leaders. I > > suppose what I don't want is other kernel subsystems, if any, thinking it's > > OK to call pidfd_create() with an arbitrary struct pid and setting the > > expectation that a fully functional pidfd will be returned. > > > > The way I see it, I think we've got two options here: > > > > 1) Leave the pid_has_task() check within pidfd_create() and perform another > > explicit pid_has_task() check from the fanotify code before calling > > pidfd_create(). If it returns false, we set something like FAN_NOPIDFD > > indicating to userspace that there's no such process when the event was > > created. > > > > 2) Scrap using pidfd_create() all together and implement a fanotify > > specific pidfd creation wrapper which would allow for more > > control. Something along the lines of what you've done in kernel/fork.c > > [1]. Not the biggest fan of this idea just yet given the possibility of > > it leading to an API drift over time. > > > > WDYT? > > Hm, why would you have to drop the pid_has_task() check again? Because of the race that I brielfy decscribed here [0]. The race exists because we perform the pidfd creation during the notification queue processing and not in the event allocation stages (for reasons that Jan has already covered here [1]). So, tl;dr there is the case where the fanotify calls pidfd_create() and the check for pid_has_task() fails because the struct pid that we're hanging onto within an event no longer contains a task of type PIDTYPE_TGID... [0] https://www.spinics.net/lists/linux-api/msg48630.html [1] https://www.spinics.net/lists/linux-api/msg48632.html /M