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=-8.0 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,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 78A1CC4338F for ; Mon, 16 Aug 2021 13:03:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5153A63290 for ; Mon, 16 Aug 2021 13:03:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235857AbhHPNDp (ORCPT ); Mon, 16 Aug 2021 09:03:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44420 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235519AbhHPNDl (ORCPT ); Mon, 16 Aug 2021 09:03:41 -0400 Received: from mail-wr1-x434.google.com (mail-wr1-x434.google.com [IPv6:2a00:1450:4864:20::434]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CB386C061764; Mon, 16 Aug 2021 06:03:09 -0700 (PDT) Received: by mail-wr1-x434.google.com with SMTP id f5so23452107wrm.13; Mon, 16 Aug 2021 06:03:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=to:cc:references:from:subject:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=4ey74r9h5B4FsleU6ivv3WvBZFvF09kXQTJf5FkLGKg=; b=kiowXfPOnvNLxiobCImqWlbPdEaYpfeeS7uTkrbVyfLPq2Ww2H2+rjHOU4oKlIBEH2 gMX7ddNGCAbi2fRmWxquoAxbIJwq/cewfhkyBpQ5e0tACbl1YU/oWO1v6E4kWZFLQxI2 jJbLXt3KRjTjjhgDCHGH/xL1ArHmtZAXvpKlkeiPDqdHd6rjjThMz9sGpDsZUuYrOCdg rZZTI6Mm4ogmvEK1oi3Om9S4bK8WJGqt7wA5sFiGMPWvb91tQ8xRg4tHbPPpoFngBbfq jva28tvoUANUIDGuQg1j2PB+5dFLRSi1BxgL0XvMozIh2KjWUJ+vH4J0nP/2eBRNznta rkVQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:to:cc:references:from:subject:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=4ey74r9h5B4FsleU6ivv3WvBZFvF09kXQTJf5FkLGKg=; b=gcq2Cjir1Z9q68JJaUQO9At7WTeUpqin7mw7Z/J3ctwN3EgNDVt2aHuwOKE3MxIFv2 dynWKmnF68yqdzKNW5oxGxKr5cpi2OP9Lyoh51XvlTFsy0PUh+bU0PKV32nnfH0zAfYe LW8dBWc1VhHq5hH6gMWHGZc7sXTOsemoQ8cq3xDyuBMMigAUYcMfDk9Rpm2S1aRyzK+o qFo3IWfTXyG6GLzBCi0HCexklKhkgQJcmhuu8iWjPMDOsk+fHE9MShEesEQ6TFR82ilO EWB8FLSIGCYTvQT8jJI+H66jO7cFHsMwKjWM1GRLeVN7lHRcdopwMyeBdU4iplDi76aG e7aA== X-Gm-Message-State: AOAM530Yxr5VV2R+ejR66zVjgtYTDggGqJurg91/FynLDSzk+ye0KzDz AwqzAZY+GKzLMqmDrjN+P/w= X-Google-Smtp-Source: ABdhPJzFHwwFH6z6Bs4J55RWCb/F7PeZ8LNy8QhQCco09FhXUf52kwHWLCmhkL7pQSLENuUV93k4Gw== X-Received: by 2002:a5d:574d:: with SMTP id q13mr18764483wrw.425.1629118988423; Mon, 16 Aug 2021 06:03:08 -0700 (PDT) Received: from [192.168.8.197] ([85.255.233.12]) by smtp.gmail.com with ESMTPSA id q75sm11125774wme.40.2021.08.16.06.03.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 16 Aug 2021 06:03:01 -0700 (PDT) To: Olivier Langlois , Jens Axboe , Tony Battersby , "Eric W. Biederman" , Oleg Nesterov Cc: Linus Torvalds , Linux Kernel Mailing List , linux-fsdevel , io-uring , Alexander Viro References: <198e912402486f66214146d4eabad8cb3f010a8e.camel@trillion01.com> <87eeda7nqe.fsf@disp2133> <87pmwt6biw.fsf@disp2133> <87czst5yxh.fsf_-_@disp2133> <87y2bh4jg5.fsf@disp2133> <87sg1p4h0g.fsf_-_@disp2133> <20210614141032.GA13677@redhat.com> <87pmwmn5m0.fsf@disp2133> <4d93d0600e4a9590a48d320c5a7dd4c54d66f095.camel@trillion01.com> <8af373ec-9609-35a4-f185-f9bdc63d39b7@cybernetics.com> <9d194813-ecb1-2fe4-70aa-75faf4e144ad@kernel.dk> From: Pavel Begunkov Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps Message-ID: Date: Mon, 16 Aug 2021 14:02:30 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/15/21 9:42 PM, Olivier Langlois wrote: [...] > When I have first encountered the issue, the very first thing that I > did try was to create a simple test program that would synthetize the > problem. > > After few time consumming failed attempts, I just gave up the idea and > simply settle to my prod program that showcase systematically the > problem every time that I kill the process with a SEGV signal. > > In a nutshell, all the program does is to issue read operations with > io_uring on a TCP socket on which there is a constant data stream. > > Now that I have a better understanding of what is going on, I think > that one way that could reproduce the problem consistently could be > along those lines: > > 1. Create a pipe > 2. fork a child > 3. Initiate a read operation on the pipe with io_uring from the child > 4. Let the parent kill its child with a core dump generating signal. > 5. Write something in the pipe from the parent so that the io_uring > read operation completes while the core dump is generated. > > I guess that I'll end up doing that if I cannot fix the issue with my > current setup but here is what I have attempted so far: > > 1. Call io_uring_files_cancel from do_coredump > 2. Same as #1 but also make sure that TIF_NOTIFY_SIGNAL is cleared on > returning from io_uring_files_cancel > > Those attempts didn't work but lurking in the io_uring dev mailing list > is starting to pay off. I thought that I did reach the bottom of the > rabbit hole in my journey of understanding io_uring but the recent > patch set sent by Hao Xu > > https://lore.kernel.org/io-uring/90fce498-968e-6812-7b6a-fdf8520ea8d9@kernel.dk/T/#t > > made me realize that I still haven't assimilated all the small io_uring > nuances... > > Here is my feedback. From my casual io_uring code reader point of view, > it is not 100% obvious what the difference is between > io_uring_files_cancel and io_uring_task_cancel As you mentioned, io_uring_task_cancel() cancels and waits for all requests submitted by current task, used in exec() and SQPOLL because of potential races. io_uring_task_cancel() cancels only selected ones and io_uring_files_cancel() cancels and waits only some specific requests that we absolutely have to, e.g. in 5.15 it'll be only requests referencing the ring itself. It's used on normal task exit. io_uring_task_cancel() cancels and waits all requests submitted by current task, used on exec() because of races. As you mentioned > > It seems like io_uring_files_cancel is cancelling polls only if they > have the REQ_F_INFLIGHT flag set. > > I have no idea what an inflight request means and why someone would > want to call io_uring_files_cancel over io_uring_task_cancel. > > I guess that if I was to meditate on the question for few hours, I > would at some point get some illumination strike me but I believe that > it could be a good idea to document in the code those concepts for > helping casual readers... > > Bottomline, I now understand that io_uring_files_cancel does not cancel > all the requests. Therefore, without fully understanding what I am > doing, I am going to replace my call to io_uring_files_cancel from > do_coredump with io_uring_task_cancel and see if this finally fix the > issue for good. > > What I am trying to do is to cancel pending io_uring requests to make > sure that TIF_NOTIFY_SIGNAL isn't set while core dump is generated. > > Maybe another solution would simply be to modify __dump_emit to make it > resilient to TIF_NOTIFY_SIGNAL as Eric W. Biederman originally > suggested. > > or maybe do both... > > Not sure which approach is best. If someone has an opinion, I would be > curious to hear it. > > Greetings, > > -- Pavel Begunkov