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,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 7B6F2C432BE for ; Tue, 17 Aug 2021 18:24:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5752560F22 for ; Tue, 17 Aug 2021 18:24:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232807AbhHQSYi (ORCPT ); Tue, 17 Aug 2021 14:24:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54060 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231438AbhHQSYg (ORCPT ); Tue, 17 Aug 2021 14:24:36 -0400 Received: from mail-ot1-x32e.google.com (mail-ot1-x32e.google.com [IPv6:2607:f8b0:4864:20::32e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 46D59C0613CF for ; Tue, 17 Aug 2021 11:24:03 -0700 (PDT) Received: by mail-ot1-x32e.google.com with SMTP id w22-20020a056830411600b0048bcf4c6bd9so3320236ott.8 for ; Tue, 17 Aug 2021 11:24:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:from:to:cc:references:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=YD49dMesiuiGIs3lWpMyTY0Byv6nyQNGpd82y5PxhRY=; b=TZftNj8CvCCiy65TKOpUe81x8LO0uA/WTMoxnTmWYwO6ScZee+PySmIdUQQX2KUcND M5nOyP5T3yGjMrccflNjKCTyIp+Ro5MH+Wp6PD4NO93iFLr4BwB8DMG7JvDrGVkDA20X N5VPbETpEfpB54HysxxPxeVqigYjLOUeVKEO2EyWnhKu2uw0YVMMy5KP0IYVP4wlKLXc rL2r/KTXwGlGyG+pXNmLFwYFbHHjj8bWHWQk8MDFr2+Vt6r3HogVdhP4TwbTvF9dNOfS snOaqNTS0jOPeY/sqys3NjA2VzYzWBxpnKd94N/PHqOmriSKtPVwx5eQqMy+CJ6l7Rqt zhwg== 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=YD49dMesiuiGIs3lWpMyTY0Byv6nyQNGpd82y5PxhRY=; b=UxLDzKrFh48rIVZbYR5T1WKBSXeoQAJwpC/W+qI0J936rCr/VJy6X48n3WPZI46TqW kXdgqYValfFfP+WS4XrolkGEx04oL8h0kqOkXxrATUWuKz9S/qKmLlWiELX0fvABZc/U 9cSGZG5hDZvlt48bPHIN8qny+9UgKEgDyrwkzy6Aam99s/jveoxyG5FjYar1HCkwexLY ff2Mc2nNgypNe+29JY848+yQbdJFZV4WGz3dcxdDK6QrrjRWz+Qa/eOp8FczHTRq0f+S qebWDz3zHkhtgC6IDNMY2+QnBnB5318loSmgrP6h7UKjXSGJd+XFcVJsJ08jym6pQBEc ogxA== X-Gm-Message-State: AOAM531p+6tIM7miNzFXtqjYY63psGUbL/WQkZDxgyR0JfwEkwSiPTCl 5sWiZI7o/5yah90it0dEgcKXAQ== X-Google-Smtp-Source: ABdhPJyxH5BHKXrb2DJ+vzqG0rFTy0fjfbuLx8MZLavIT7jDUxTvZj4oPyoO8KVQC3MKokU6oJR9dg== X-Received: by 2002:a05:6830:31ac:: with SMTP id q12mr3647495ots.152.1629224642503; Tue, 17 Aug 2021 11:24:02 -0700 (PDT) Received: from [192.168.1.30] ([207.135.234.126]) by smtp.gmail.com with ESMTPSA id 4sm627873oil.38.2021.08.17.11.24.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 17 Aug 2021 11:24:02 -0700 (PDT) Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps From: Jens Axboe To: Olivier Langlois , Tony Battersby , "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> Message-ID: <24c795c6-4ec4-518e-bf9b-860207eee8c7@kernel.dk> Date: Tue, 17 Aug 2021 12:24:01 -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: <0bc38b13-5a7e-8620-6dce-18731f15467e@kernel.dk> 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 8/17/21 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) -- Jens Axboe