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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 46E0AC32774 for ; Mon, 22 Aug 2022 22:28:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237976AbiHVW2C convert rfc822-to-8bit (ORCPT ); Mon, 22 Aug 2022 18:28:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42514 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232461AbiHVW2B (ORCPT ); Mon, 22 Aug 2022 18:28:01 -0400 X-Greylist: delayed 4256 seconds by postgrey-1.37 at lindbergh.monkeyblade.net; Mon, 22 Aug 2022 15:27:59 PDT Received: from cloud48395.mywhc.ca (cloud48395.mywhc.ca [173.209.37.211]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 693D1491F2; Mon, 22 Aug 2022 15:27:59 -0700 (PDT) Received: from [45.44.224.220] (port=49006 helo=[192.168.1.179]) by cloud48395.mywhc.ca with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.95) (envelope-from ) id 1oQEn7-0007uE-5u; Mon, 22 Aug 2022 17:17:01 -0400 Message-ID: <61abfb5a517e0ee253b0dc7ba9cd32ebd558bcb0.camel@trillion01.com> Subject: Re: [PATCH 2/2] coredump: Allow coredumps to pipes to work with io_uring From: Olivier Langlois To: "Eric W. Biederman" , Jens Axboe Cc: Pavel Begunkov , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, Alexander Viro , Oleg Nesterov , Linus Torvalds Date: Mon, 22 Aug 2022 17:16:59 -0400 In-Reply-To: <87mtd3rals.fsf_-_@email.froward.int.ebiederm.org> References: <192c9697e379bf084636a8213108be6c3b948d0b.camel@trillion01.com> <9692dbb420eef43a9775f425cb8f6f33c9ba2db9.camel@trillion01.com> <87h7i694ij.fsf_-_@disp2133> <1b519092-2ebf-3800-306d-c354c24a9ad1@gmail.com> <13250a8d-1a59-4b7b-92e4-1231d73cbdda@gmail.com> <878rw9u6fb.fsf@email.froward.int.ebiederm.org> <303f7772-eb31-5beb-2bd0-4278566591b0@gmail.com> <87ilsg13yz.fsf@email.froward.int.ebiederm.org> <8218f1a245d054c940e25142fd00a5f17238d078.camel@trillion01.com> <87y1wnrap0.fsf_-_@email.froward.int.ebiederm.org> <87mtd3rals.fsf_-_@email.froward.int.ebiederm.org> Organization: Trillion01 Inc Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 8BIT User-Agent: Evolution 3.44.3 MIME-Version: 1.0 X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - cloud48395.mywhc.ca X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - trillion01.com X-Get-Message-Sender-Via: cloud48395.mywhc.ca: authenticated_id: olivier@trillion01.com X-Authenticated-Sender: cloud48395.mywhc.ca: olivier@trillion01.com X-Source: X-Source-Args: X-Source-Dir: Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, 2022-07-20 at 11:51 -0500, Eric W. Biederman wrote: > > Now that io_uring like everything else stops for coredumps in > get_signal the code can once again allow any interruptible > condition after coredump_wait to interrupt the coredump. > > Clear TIF_NOTIFY_SIGNAL after coredump_wait, to guarantee that > anything that sets TIF_NOTIFY_SIGNAL before coredump_wait completed > won't cause the coredumps to interrupted. > > With all of the other threads in the process stopped io_uring doesn't > call task_work_add on the thread running do_coredump.  Combined with > the clearing of TIF_NOTIFY_SIGNAL this allows processes that use > io_uring to coredump through pipes. > > Restore dump_interrupted to be a simple call to signal_pending > effectively reverting commit 06af8679449d ("coredump: Limit what can > interrupt coredumps").  At this point only SIGKILL delivered to the > coredumping thread should be able to cause signal_pending to return > true. > > A nice followup would be to find a reliable race free way to modify > task_work_add and probably set_notify_signal to skip setting > TIF_NOTIFY_SIGNAL once it is clear a task will no longer process > signals and other interruptible conditions.  That would allow > TIF_NOTIFY_SIGNAL to be cleared where TIF_SIGPENDING is cleared in > coredump_zap_process. > > To be as certain as possible that this works, I tested this with > commit 1d5f5ea7cb7d ("io-wq: remove worker to owner tw dependency") > reverted.  Which means that not only is TIF_NOTIFY_SIGNAL prevented > from stopping coredumps to pipes, the sequence of stopping threads to > participate in the coredump avoids deadlocks that were possible > previously. > > Signed-off-by: "Eric W. Biederman" > Hi Eric, What is stopping the task calling do_coredump() to be interrupted and call task_work_add() from the interrupt context? This is precisely what I was experiencing last summer when I did work on this issue. My understanding of how async I/O works with io_uring is that the task is added to a wait queue without being put to sleep and when the io_uring callback is called from the interrupt context, task_work_add() is called so that the next time io_uring syscall is invoked, pending work is processed to complete the I/O. So if: 1. io_uring request is initiated AND the task is in a wait queue 2. do_coredump() is called before the I/O is completed IMHO, this is how you end up having task_work_add() called while the coredump is generated. So far, the only way that I have found making sure that this was not happening was to cancel every pending io_uring requests before writing the coredump by calling io_uring_task_cancel(): --- a/fs/coredump.c +++ b/fs/coredump.c @@ -43,7 +43,7 @@ #include #include #include - +#include #include #include #include @@ -561,6 +561,8 @@ need_suid_safe = true; } + io_uring_task_cancel(); + retval = coredump_wait(siginfo->si_signo, &core_state); if (retval < 0) goto fail_creds; Greetings,