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=-6.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 708C6C433E1 for ; Tue, 7 Jul 2020 17:14:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4CEC1206F6 for ; Tue, 7 Jul 2020 17:14:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728189AbgGGRN4 (ORCPT ); Tue, 7 Jul 2020 13:13:56 -0400 Received: from out02.mta.xmission.com ([166.70.13.232]:33938 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727777AbgGGRNz (ORCPT ); Tue, 7 Jul 2020 13:13:55 -0400 Received: from in01.mta.xmission.com ([166.70.13.51]) by out02.mta.xmission.com with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1jsrAC-0004jJ-2G; Tue, 07 Jul 2020 11:13:48 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1jsrAA-0000lm-TW; Tue, 07 Jul 2020 11:13:47 -0600 From: ebiederm@xmission.com (Eric W. Biederman) To: Christian Brauner Cc: Alexei Starovoitov , linux-kernel@vger.kernel.org, David Miller , Greg Kroah-Hartman , Tetsuo Handa , Kees Cook , Andrew Morton , Alexei Starovoitov , Al Viro , bpf , linux-fsdevel , Daniel Borkmann , Jakub Kicinski , Masahiro Yamada , Gary Lin , Bruno Meneguele , LSM List , Casey Schaufler , Luis Chamberlain , Linus Torvalds References: <87y2o1swee.fsf_-_@x220.int.ebiederm.org> <20200702164140.4468-13-ebiederm@xmission.com> <20200703203021.paebx25miovmaxqt@ast-mbp.dhcp.thefacebook.com> <873668s2j8.fsf@x220.int.ebiederm.org> <20200704155052.kmrest5useyxcfnu@wittgenstein> Date: Tue, 07 Jul 2020 12:09:05 -0500 In-Reply-To: <20200704155052.kmrest5useyxcfnu@wittgenstein> (Christian Brauner's message of "Sat, 4 Jul 2020 17:50:52 +0200") Message-ID: <87mu4bjlqm.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1jsrAA-0000lm-TW;;;mid=<87mu4bjlqm.fsf@x220.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX18xg/9AOxJCx9+kag+rX8lbj8yMBnwD6cc= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v3 13/16] exit: Factor thread_group_exited out of pidfd_poll X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: Christian Brauner writes: > On Fri, Jul 03, 2020 at 04:37:47PM -0500, Eric W. Biederman wrote: >> Alexei Starovoitov writes: >> >> > On Thu, Jul 02, 2020 at 11:41:37AM -0500, Eric W. Biederman wrote: >> >> Create an independent helper thread_group_exited report return true >> >> when all threads have passed exit_notify in do_exit. AKA all of the >> >> threads are at least zombies and might be dead or completely gone. >> >> >> >> Create this helper by taking the logic out of pidfd_poll where >> >> it is already tested, and adding a missing READ_ONCE on >> >> the read of task->exit_state. >> >> >> >> I will be changing the user mode driver code to use this same logic >> >> to know when a user mode driver needs to be restarted. >> >> >> >> Place the new helper thread_group_exited in kernel/exit.c and >> >> EXPORT it so it can be used by modules. >> >> >> >> Signed-off-by: "Eric W. Biederman" >> >> --- >> >> include/linux/sched/signal.h | 2 ++ >> >> kernel/exit.c | 24 ++++++++++++++++++++++++ >> >> kernel/fork.c | 6 +----- >> >> 3 files changed, 27 insertions(+), 5 deletions(-) >> >> >> >> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h >> >> index 0ee5e696c5d8..1bad18a1d8ba 100644 >> >> --- a/include/linux/sched/signal.h >> >> +++ b/include/linux/sched/signal.h >> >> @@ -674,6 +674,8 @@ static inline int thread_group_empty(struct task_struct *p) >> >> #define delay_group_leader(p) \ >> >> (thread_group_leader(p) && !thread_group_empty(p)) >> >> >> >> +extern bool thread_group_exited(struct pid *pid); >> >> + >> >> extern struct sighand_struct *__lock_task_sighand(struct task_struct *task, >> >> unsigned long *flags); >> >> >> >> diff --git a/kernel/exit.c b/kernel/exit.c >> >> index d3294b611df1..a7f112feb0f6 100644 >> >> --- a/kernel/exit.c >> >> +++ b/kernel/exit.c >> >> @@ -1713,6 +1713,30 @@ COMPAT_SYSCALL_DEFINE5(waitid, >> >> } >> >> #endif >> >> >> >> +/** >> >> + * thread_group_exited - check that a thread group has exited >> >> + * @pid: tgid of thread group to be checked. >> >> + * >> >> + * Test if thread group is has exited (all threads are zombies, dead >> >> + * or completely gone). >> >> + * >> >> + * Return: true if the thread group has exited. false otherwise. >> >> + */ >> >> +bool thread_group_exited(struct pid *pid) >> >> +{ >> >> + struct task_struct *task; >> >> + bool exited; >> >> + >> >> + rcu_read_lock(); >> >> + task = pid_task(pid, PIDTYPE_PID); >> >> + exited = !task || >> >> + (READ_ONCE(task->exit_state) && thread_group_empty(task)); >> >> + rcu_read_unlock(); >> >> + >> >> + return exited; >> >> +} >> > >> > I'm not sure why you think READ_ONCE was missing. >> > It's different in wait_consider_task() where READ_ONCE is needed because >> > of multiple checks. Here it's done once. >> >> In practice it probably has no effect on the generated code. But >> READ_ONCE is about telling the compiler not to be clever. Don't use >> tearing loads or stores etc. When all of the other readers are using >> READ_ONCE I just get nervous if we have a case that doesn't. > > That's not true. The only place where READ_ONCE(->exit_state) is used is > in wait_consider_task() and nowhere else. We had that discussion a while > ago where I or someone proposed to simply place a READ_ONCE() around all > accesses to exit_state for the sake of kcsan and we agreed that it's > unnecessary and not to do this. > But it obviously doesn't hurt to have it. There is a larger discussion to be had around the proper handling of exit_state. In this particular case because we are accessing exit_state with only rcu_read_lock protection, because the outcome of the read is about correctness, and because the compiler has nothing else telling it not to re-read exit_state, I believe we actually need the READ_ONCE. At the same time it would take a pretty special compiler to want to reaccess that field in thread_group_exited. I have looked through and I don't find any of the other access of exit_state where the result is about correctness (so that we care) and we don't hold tasklist_lock. But I have removed the necessary wording from the commit comment. There is a much larger discussion to be had about what to do with exit_state, because I think I found about half the accesses were slightly buggy in one form or another. Eric