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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 599B4C04E87 for ; Wed, 15 May 2019 15:19:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 31F2C2084E for ; Wed, 15 May 2019 15:19:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728074AbfEOPTX (ORCPT ); Wed, 15 May 2019 11:19:23 -0400 Received: from mx1.redhat.com ([209.132.183.28]:26961 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726581AbfEOPTW (ORCPT ); Wed, 15 May 2019 11:19:22 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 1942D301A3E1; Wed, 15 May 2019 15:19:22 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.159]) by smtp.corp.redhat.com (Postfix) with SMTP id AC9925D9C0; Wed, 15 May 2019 15:19:16 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Wed, 15 May 2019 17:19:19 +0200 (CEST) Date: Wed, 15 May 2019 17:19:13 +0200 From: Oleg Nesterov To: Christian Brauner Cc: jannh@google.com, viro@zeniv.linux.org.uk, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, arnd@arndb.de, dhowells@redhat.com, akpm@linux-foundation.org, cyphar@cyphar.com, ebiederm@xmission.com, elena.reshetova@intel.com, keescook@chromium.org, luto@amacapital.net, luto@kernel.org, tglx@linutronix.de, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH 1/2] pid: add pidfd_open() Message-ID: <20190515151912.GE18892@redhat.com> References: <20190515100400.3450-1-christian@brauner.io> <20190515143857.GB18892@redhat.com> <20190515144927.f2yxyi6w6lhn3xx7@brauner.io> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190515144927.f2yxyi6w6lhn3xx7@brauner.io> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Wed, 15 May 2019 15:19:22 +0000 (UTC) Sender: linux-parisc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-parisc@vger.kernel.org On 05/15, Christian Brauner wrote: > > On Wed, May 15, 2019 at 04:38:58PM +0200, Oleg Nesterov wrote: > > > > it seems that you can do a single check > > > > tsk = pid_task(p, PIDTYPE_TGID); > > if (!tsk) > > ret = -ESRCH; > > > > this even looks more correct if we race with exec changing the leader. > > The logic here being that you can only reach the thread_group leader > from struct pid if PIDTYPE_PID == PIDTYPE_TGID for this struct pid? Not exactly... it is not that PIDTYPE_PID == PIDTYPE_TGID for this pid, struct pid has no "type" or something like this. The logic is that pid->tasks[PIDTYPE_XXX] is the list of task which use this pid as "XXX" type. For example, clone(CLONE_THREAD) creates a pid which has a single non- empty list, pid->tasks[PIDTYPE_PID]. This pid can't be used as TGID or SID. So if pid_task(PIDTYPE_TGID) returns non-NULL we know that this pid was used for a group-leader, see copy_process() which does if (thread_group_leader(p)) attach_pid(p, PIDTYPE_TGID); If we race with exec which changes the leader pid_task(TGID) can return the old leader. We do not care, but this means that we should not check thread_group_leader(). Oleg.