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 23B19C4167B for ; Thu, 7 Dec 2023 17:58:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1443385AbjLGR6Q (ORCPT ); Thu, 7 Dec 2023 12:58:16 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49422 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235228AbjLGR6L (ORCPT ); Thu, 7 Dec 2023 12:58:11 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 41CCA173C for ; Thu, 7 Dec 2023 09:57:48 -0800 (PST) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 500ABC433C8; Thu, 7 Dec 2023 17:57:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1701971867; bh=he6LQ25WriwrUDDfYMH36GaKEnf82jbzGv13WZYSigA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=l4TN7o1bbCnfchU9RLrHJPvjo+IZ4KULQNSU5zsj3q64CV1bTzf9IP4m/+rGhIOIn oODMJChWCx477zxVUlNsE6IiNamrGVAYLkSOsAW8UWbIDDKQEU0zCFkl5Mk4fYQIts HOo+02W54XWWpeoEuIF0nOJ7f0mHuv24kSBz+ftktQSyzd2VhtGnUmbfYn1oR9I4w+ TEmD0BS8uKH8WiFT3owGtMdsYPLRGGonI2MbEl75vFuxW7WT48v05dZDyYWiqibHTF pdqXCqbnIvyVn+8rcYo+hwaS9FmzBMzKw7Ff6x4+jlp2JrR99IxRJkBwBIDAwizBwj cQ2PSv3eCw8wg== Date: Thu, 7 Dec 2023 18:57:42 +0100 From: Christian Brauner To: Tycho Andersen , Oleg Nesterov Cc: "Eric W . Biederman" , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, Tycho Andersen , Jan Kara , linux-fsdevel@vger.kernel.org, Joel Fernandes Subject: Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders Message-ID: <20231207-weither-autopilot-8daee206e6c5@brauner> References: <20231130163946.277502-1-tycho@tycho.pizza> <20231130173938.GA21808@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 01, 2023 at 09:31:40AM -0700, Tycho Andersen wrote: > On Thu, Nov 30, 2023 at 10:57:01AM -0700, Tycho Andersen wrote: > > On Thu, Nov 30, 2023 at 06:39:39PM +0100, Oleg Nesterov wrote: > > > I think that wake_up_all(wait_pidfd) should have a single caller, > > > do_notify_pidfd(). This probably means it should be shiftef from > > > do_notify_parent() to exit_notify(), I am not sure... > > Indeed, below passes the tests without issue and is much less ugly. So I think I raised that question on another medium already but what does the interaction with de_thread() look like? Say some process creates pidfd for a thread in a non-empty thread-group is created via CLONE_PIDFD. The pidfd_file->private_data is set to struct pid of that task. The task the pidfd refers to later exec's. Once it passed de_thread() the task the pidfd refers to assumes the struct pid of the old thread-group leader and continues. At the same time, the old thread-group leader now assumes the struct pid of the task that just exec'd. So after de_thread() the pidfd now referes to the old thread-group leaders struct pid. Any subsequent operation will fail because the process has already exited. Basically, the pidfd now refers to the old thread-group leader and any subsequent operation will fail even though the task still exists. Conversely, if someone had created a pidfd that referred to the old thread-group leader task then this pidfd will now suddenly refer to the new thread-group leader task for the same reason: the struct pid's were exchanged. So this also means, iiuc, that the pidfd could now be passed to waitid(P_PIFD) to retrieve the status of the old thread-group leader that just got zapped. And for the case where the pidfd referred to the old thread-group leader task you would now suddenly _not_ be able to wait on that task anymore. If these concerns are correct, then I think we need to decide what semantics we want and how to handle this because that's not ok.