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=-12.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 573C8C4338F for ; Tue, 17 Aug 2021 19:59:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2C0D760F22 for ; Tue, 17 Aug 2021 19:59:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233179AbhHQUAa (ORCPT ); Tue, 17 Aug 2021 16:00:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48028 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229649AbhHQUAa (ORCPT ); Tue, 17 Aug 2021 16:00:30 -0400 Received: from mail-pl1-x62c.google.com (mail-pl1-x62c.google.com [IPv6:2607:f8b0:4864:20::62c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E70B3C0613CF for ; Tue, 17 Aug 2021 12:59:56 -0700 (PDT) Received: by mail-pl1-x62c.google.com with SMTP id e15so286066plh.8 for ; Tue, 17 Aug 2021 12:59:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=YCnH7wri8A6gFqECGkimfx1ed8a4h8yMk5Vm8xazQvs=; b=nI4PvMPvMnvKjr2nAJKkODyq6WiYPG2iE7OWohesnqxngEtPAsvmKfIpMYPX9fe1OA eAjeEGnx6kSp6nWBcfSeJqAfAhKcDqWoE6V5LSDLI2TIkY/ZTcE08w35DSsluZNGZEDg tiUvXy/084wz+7hFBNkOFKu2ouTVMF2qy8hLXbqooM+kAbSNStURYVOVFBu+Lu215ONH tdvNSg/TD0U3hTHdOhgfw9NHjfWHjRePthCGAU6VvcK51D0Ezo5npQo1kLYiHRaa6r9W mZU+YJD4hN2BGg7gK1YI/SMlpJkJ6STFRFNAyBHD/YWbXB6OaXFp/DAkaB1fowv8H8/U M1gQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=YCnH7wri8A6gFqECGkimfx1ed8a4h8yMk5Vm8xazQvs=; b=ruIScW9eE5w9SsOWTb7gF1jClAGU2rdALG8RfHhQ+xzAzdnsihKaalTW87rhITAHqE g0u05fpFg5PdlMvW/yjfBWB6N2ZpuIzS6SpjOHpzHd1Nx1Q/kJ1oG/UmDgfeaJ7ve9yf gBbNIQqcjtEv9zVLCHro8u0BeT4yoz+0nwnzN/s+fdT5zW6CgbiEL2E1b7mn/n9HZT6b 3hugra7qIHONVFYUNaprqBXkJYbayd+tfGKbk1i5pNz0RKd9ley1bTD3eptpZb2ZWa7M 3dIze4b/zQSOqFfDAgP7CP4XxIpf5bCBBr7InKDxKQlIpvCidh4/bV9miJiD0oeD/gla u/gg== X-Gm-Message-State: AOAM530ikCUpliG8kOTTI/LvL1Ek86UqGed30sAeTrsW2FiQGc8C+sZz 4QyQwnd7vpHxHYpakuDSu1lZrA== X-Google-Smtp-Source: ABdhPJzSp0orehkN5BzAPPfc1O23yXzXGOwbwnrGmYKz1PmxIelrFdA1gda52sdVbhgC569/T0gVQg== X-Received: by 2002:a17:902:7681:b0:12d:8f52:3d55 with SMTP id m1-20020a170902768100b0012d8f523d55mr4040763pll.70.1629230396092; Tue, 17 Aug 2021 12:59:56 -0700 (PDT) Received: from [192.168.1.116] ([66.219.217.159]) by smtp.gmail.com with ESMTPSA id y12sm4398483pgl.65.2021.08.17.12.59.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 17 Aug 2021 12:59:55 -0700 (PDT) Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps To: Tony Battersby , Olivier Langlois , "Eric W. Biederman" , Oleg Nesterov Cc: Linus Torvalds , Linux Kernel Mailing List , linux-fsdevel , io-uring , Alexander Viro , "Pavel Begunkov>" 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> <0bc38b13-5a7e-8620-6dce-18731f15467e@kernel.dk> <24c795c6-4ec4-518e-bf9b-860207eee8c7@kernel.dk> <05c0cadc-029e-78af-795d-e09cf3e80087@cybernetics.com> From: Jens Axboe Message-ID: Date: Tue, 17 Aug 2021 13:59:54 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <05c0cadc-029e-78af-795d-e09cf3e80087@cybernetics.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 8/17/21 1:29 PM, Tony Battersby wrote: > On 8/17/21 2:24 PM, Jens Axboe wrote: >> On 8/17/21 12:15 PM, Jens Axboe wrote: >>> On 8/15/21 2:42 PM, Olivier Langlois wrote: >>>> On Wed, 2021-08-11 at 19:55 -0600, Jens Axboe wrote: >>>>> On 8/10/21 3:48 PM, Tony Battersby wrote: >>>>>> On 8/5/21 9:06 AM, Olivier Langlois wrote: >>>>>>> Hi all, >>>>>>> >>>>>>> I didn't forgot about this remaining issue and I have kept thinking >>>>>>> about it on and off. >>>>>>> >>>>>>> I did try the following on 5.12.19: >>>>>>> >>>>>>> diff --git a/fs/coredump.c b/fs/coredump.c >>>>>>> index 07afb5ddb1c4..614fe7a54c1a 100644 >>>>>>> --- a/fs/coredump.c >>>>>>> +++ b/fs/coredump.c >>>>>>> @@ -41,6 +41,7 @@ >>>>>>> #include >>>>>>> #include >>>>>>> #include >>>>>>> +#include >>>>>>> >>>>>>> #include >>>>>>> #include >>>>>>> @@ -625,6 +626,8 @@ void do_coredump(const kernel_siginfo_t >>>>>>> *siginfo) >>>>>>> need_suid_safe = true; >>>>>>> } >>>>>>> >>>>>>> + io_uring_files_cancel(current->files); >>>>>>> + >>>>>>> retval = coredump_wait(siginfo->si_signo, &core_state); >>>>>>> if (retval < 0) >>>>>>> goto fail_creds; >>>>>>> -- >>>>>>> 2.32.0 >>>>>>> >>>>>>> with my current understanding, io_uring_files_cancel is supposed to >>>>>>> cancel everything that might set the TIF_NOTIFY_SIGNAL. >>>>>>> >>>>>>> I must report that in my testing with generating a core dump >>>>>>> through a >>>>>>> pipe with the modif above, I still get truncated core dumps. >>>>>>> >>>>>>> systemd is having a weird error: >>>>>>> [ 2577.870742] systemd-coredump[4056]: Failed to get COMM: No such >>>>>>> process >>>>>>> >>>>>>> and nothing is captured >>>>>>> >>>>>>> so I have replaced it with a very simple shell: >>>>>>> $ cat /proc/sys/kernel/core_pattern >>>>>>>> /home/lano1106/bin/pipe_core.sh %e %p >>>>>>> ~/bin $ cat pipe_core.sh >>>>>>> #!/bin/sh >>>>>>> >>>>>>> cat > /home/lano1106/core/core.$1.$2 >>>>>>> >>>>>>> BFD: warning: /home/lano1106/core/core.test.10886 is truncated: >>>>>>> expected core file size >= 24129536, found: 61440 >>>>>>> >>>>>>> I conclude from my attempt that maybe io_uring_files_cancel is not >>>>>>> 100% >>>>>>> cleaning everything that it should clean. >>>>>>> >>>>>>> >>>>>>> >>>>>> I just ran into this problem also - coredumps from an io_uring >>>>>> program >>>>>> to a pipe are truncated. But I am using kernel 5.10.57, which does >>>>>> NOT >>>>>> have commit 12db8b690010 ("entry: Add support for TIF_NOTIFY_SIGNAL") >>>>>> or >>>>>> commit 06af8679449d ("coredump: Limit what can interrupt coredumps"). >>>>>> Kernel 5.4 works though, so I bisected the problem to commit >>>>>> f38c7e3abfba ("io_uring: ensure async buffered read-retry is setup >>>>>> properly") in kernel 5.9. Note that my io_uring program uses only >>>>>> async >>>>>> buffered reads, which may be why this particular commit makes a >>>>>> difference to my program. >>>>>> >>>>>> My io_uring program is a multi-purpose long-running program with many >>>>>> threads. Most threads don't use io_uring but a few of them do. >>>>>> Normally, my core dumps are piped to a program so that they can be >>>>>> compressed before being written to disk, but I can also test writing >>>>>> the >>>>>> core dumps directly to disk. This is what I have found: >>>>>> >>>>>> *) Unpatched 5.10.57: if a thread that doesn't use io_uring triggers >>>>>> a >>>>>> coredump, the core file is written correctly, whether it is written >>>>>> to >>>>>> disk or piped to a program, even if another thread is using io_uring >>>>>> at >>>>>> the same time. >>>>>> >>>>>> *) Unpatched 5.10.57: if a thread that uses io_uring triggers a >>>>>> coredump, the core file is truncated, whether written directly to >>>>>> disk >>>>>> or piped to a program. >>>>>> >>>>>> *) 5.10.57+backport 06af8679449d: if a thread that uses io_uring >>>>>> triggers a coredump, and the core is written directly to disk, then >>>>>> it >>>>>> is written correctly. >>>>>> >>>>>> *) 5.10.57+backport 06af8679449d: if a thread that uses io_uring >>>>>> triggers a coredump, and the core is piped to a program, then it is >>>>>> truncated. >>>>>> >>>>>> *) 5.10.57+revert f38c7e3abfba: core dumps are written correctly, >>>>>> whether written directly to disk or piped to a program. >>>>> That is very interesting. Like Olivier mentioned, it's not that actual >>>>> commit, but rather the change of behavior implemented by it. Before >>>>> that >>>>> commit, we'd hit the async workers more often, whereas after we do the >>>>> correct retry method where it's driven by the wakeup when the page is >>>>> unlocked. This is purely speculation, but perhaps the fact that the >>>>> process changes state potentially mid dump is why the dump ends up >>>>> being >>>>> truncated? >>>>> >>>>> I'd love to dive into this and try and figure it out. Absent a test >>>>> case, at least the above gives me an idea of what to try out. I'll see >>>>> if it makes it easier for me to create a case that does result in a >>>>> truncated core dump. >>>>> >>>> Jens, >>>> >>>> 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 >>>> >>>> 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. >>> It does indeed sound like it's TIF_NOTIFY_SIGNAL that will trigger some >>> signal_pending() and cause an interruption of the core dump. Just out of >>> curiosity, what is your /proc/sys/kernel/core_pattern set to? If it's >>> set to some piped process, can you try and set it to 'core' and see if >>> that eliminates the truncation of the core dumps for your case? >> And assuming that works, then I suspect this one would fix your issue >> even with a piped core dump: >> >> diff --git a/fs/coredump.c b/fs/coredump.c >> index 07afb5ddb1c4..852737a9ccbf 100644 >> --- a/fs/coredump.c >> +++ b/fs/coredump.c >> @@ -41,6 +41,7 @@ >> #include >> #include >> #include >> +#include >> >> #include >> #include >> @@ -603,6 +604,7 @@ void do_coredump(const kernel_siginfo_t *siginfo) >> }; >> >> audit_core_dumps(siginfo->si_signo); >> + io_uring_task_cancel(); >> >> binfmt = mm->binfmt; >> if (!binfmt || !binfmt->core_dump) >> > FYI, I tested kernel 5.10.59 + backport 06af8679449d + the patch above > with my io_uring program. The coredump locked up even when writing the > core file directly to disk; the zombie process could not be killed with > "kill -9". Unfortunately I can't test with newer kernels without > spending some time on it, and I am too busy with other stuff right now. That sounds like 5.10-stable is missing some of the cancelation backports, and your setup makes the cancelation stall because of that. Need to go over the 11/12/13 fixes and ensure that we've got everything we need for those stable versions, particularly 5.10. > My io_uring program does async buffered reads > (io_uring_prep_read()/io_uring_prep_readv()) from a raw disk partition > (no filesystem). One thread submits I/Os while another thread calls > io_uring_wait_cqe() and processes the completions. To trigger the > coredump, I added an intentional abort() in the thread that submits I/Os > after running for a second. OK, so that one is also using task_work for the retry based async buffered reads, so it makes sense. Maybe a temporary work-around is to use 06af8679449d and eliminate the pipe based coredump? -- Jens Axboe