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 C6FE5C432BE for ; Mon, 16 Aug 2021 13:15:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B0D1163305 for ; Mon, 16 Aug 2021 13:15:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238338AbhHPNQE (ORCPT ); Mon, 16 Aug 2021 09:16:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45860 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239769AbhHPNKL (ORCPT ); Mon, 16 Aug 2021 09:10:11 -0400 Received: from mail-wr1-x429.google.com (mail-wr1-x429.google.com [IPv6:2a00:1450:4864:20::429]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D52C8C0363C7; Mon, 16 Aug 2021 06:06:54 -0700 (PDT) Received: by mail-wr1-x429.google.com with SMTP id k29so23513670wrd.7; Mon, 16 Aug 2021 06:06:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:from:to:cc:references:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=wvgUJiUW1naT+DW3zwINBggYbB2TOutmBqo2wsEseRc=; b=RVvoxPfmnySYBMjI9Cezd0eFLQChKRbGoquGQX5qUMaWQmbVUAuxX47eS8VxSteK2B SlO9hiqXPPNn+f9E57VKNQgofCUnd93kmzOEOHBz7YD7g+3n4MAjTs75vi4SKFSpD0lR gyvE70oDZ3JoCW16C8lV1fOBWBlNG4pKlk6+9kP0qTE7kKoqERZVOzoiHIi+zFNG3Lqt nqgXCNqRHo0G3nrNZBQUnsGfzFdgiaMIuTQKCGLtmV2RXqgFZvj/CvPHuXlQ/fgzFs1p fVROVCa6MY2a42zj3ysZVkHqo01J9OO9rZZouu6hayzG4Tkl+rgjWpr4JTrRvICJ3wA6 p10g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:from:to:cc:references:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=wvgUJiUW1naT+DW3zwINBggYbB2TOutmBqo2wsEseRc=; b=QZh0iO8XdrC6nwk8562F1ixCtm/XK3t9MTloLKEAl30ChaQLkERwgpIOXF/owZf6Gd GRUDChiBFXKBnLquQKiRHIE6PZ1GIIgHFLKbzC1sIM7YdtGXY4dZomg2GXGlLcz5msJs xIoaqSb0DuxRCfzg0TsdU9/ba1BeLb36CkpuvRMW/JZ6/12SCT1fIuWr1wyauWCJnYZw 4ZAw5ptaUSJ9tgpANlnBTbDLW6wEClS10qaBTMK+p8u6nL+EdoIJ96n25vJ8mqG4RSJH CgCTP6+zA7owdtKQALPN2AUsIrRdMytxyjYO0sHtNpDIQJF0RlUmtITO0d7mD2+vtfrM jzzg== X-Gm-Message-State: AOAM530rI7bwUjNZIAXeONiiBwvBkDuFqmvY5SElXAdtxhI/2sOqy28Y FcP8idQRQba9kjZ3e8gu99Q= X-Google-Smtp-Source: ABdhPJzKZKjj43V9XRQbHEbi60duXuEN1+dtYpRGC1ZWcSynKPZSLcKV+hwZra34/G6oAiqsZQuscA== X-Received: by 2002:a5d:4442:: with SMTP id x2mr18266693wrr.60.1629119213515; Mon, 16 Aug 2021 06:06:53 -0700 (PDT) Received: from [192.168.8.197] ([85.255.233.12]) by smtp.gmail.com with ESMTPSA id p4sm13354694wrq.81.2021.08.16.06.06.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 16 Aug 2021 06:06:53 -0700 (PDT) Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps From: Pavel Begunkov 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> Message-ID: Date: Mon, 16 Aug 2021 14:06:22 +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: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/16/21 2:02 PM, Pavel Begunkov wrote: > 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. Apologies for this draft rumbling... 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, e.g. in 5.15 will be only requests operating on io_uring, and used during normal exit. Agree that the names may be not too descriptive. >> >> 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