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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham 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 91A24C43603 for ; Mon, 16 Dec 2019 10:28:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6ED3C20725 for ; Mon, 16 Dec 2019 10:28:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727465AbfLPK2L (ORCPT ); Mon, 16 Dec 2019 05:28:11 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:47034 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727099AbfLPK2L (ORCPT ); Mon, 16 Dec 2019 05:28:11 -0500 Received: from [79.140.120.2] (helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1ignbc-0006mW-DJ; Mon, 16 Dec 2019 10:28:00 +0000 Date: Mon, 16 Dec 2019 11:27:59 +0100 From: Christian Brauner To: qiwuchen55@gmail.com Cc: peterz@infradead.org, mingo@kernel.org, oleg@redhat.com, kernel-team@android.com, linux-kernel@vger.kernel.org, chenqiwu@xiaomi.com Subject: Re: [PATCH v2] kernel/exit: do panic earlier to get coredump if global init task exit Message-ID: <20191216102758.4aasl4pzoz5rm4vx@wittgenstein> References: <1576466324-6067-1-git-send-email-qiwuchen55@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1576466324-6067-1-git-send-email-qiwuchen55@gmail.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 16, 2019 at 11:18:44AM +0800, qiwuchen55@gmail.com wrote: > From: chenqiwu > > When global init task get a chance to be killed, panic will happen in > later calling steps by do_exit()->exit_notify()->forget_original_parent() > ->find_child_reaper() if all init threads have exited. > > However, it's hard to extract the coredump of init task from a kernel > crashdump, since exit_mm() has released its mm before panic. In order > to get the backtrace of init task in userspace, it's better to do panic > earlier at the beginning of exitting route. > > It's worth noting that we must take case of a multi-threaded init exitting > issue. We need the test for is_global_init() && group_dead to ensure that > it is all of init threads exiting and not just the current thread. > > Signed-off-by: chenqiwu Just a comment and a nit I can fixup myself. Reviewed-by: Christian Brauner Oleg, wdyt? > --- > changes in v2: > - using is_global_init() && group_dead as panic condition. > - move up group_dead = atomic_dec_and_test(&tsk->signal->live). > - add comment for this change in do_exit(). > --- > kernel/exit.c | 13 ++++++++----- > 1 file changed, 8 insertions(+), 5 deletions(-) > > diff --git a/kernel/exit.c b/kernel/exit.c > index bcbd598..33364c8 100644 > --- a/kernel/exit.c > +++ b/kernel/exit.c > @@ -517,10 +517,6 @@ static struct task_struct *find_child_reaper(struct task_struct *father, > } > > write_unlock_irq(&tasklist_lock); > - if (unlikely(pid_ns == &init_pid_ns)) { > - panic("Attempted to kill init! exitcode=0x%08x\n", > - father->signal->group_exit_code ?: father->exit_code); > - } This made me queasy at first but from what I can see this is safe to remove. We could've only gotten here if: task == child_reaper && !find_alive_thread() && pid_ns == init_pid_ns that should be equivalent to the new is_global_init(tsk) && group_dead check, i.e. I think there's no condition we might loose by removing this check. > > list_for_each_entry_safe(p, n, dead, ptrace_entry) { > list_del_init(&p->ptrace_entry); > @@ -728,6 +724,14 @@ void __noreturn do_exit(long code) > panic("Attempted to kill the idle task!"); > > /* > + * If all threads of global init have exited, do panic imeddiately Nit: s/imeddiately/immediately/ but I can fix this up when applying. > + * to get the coredump to find any clue for init task in userspace. > + */ > + group_dead = atomic_dec_and_test(&tsk->signal->live); > + if (unlikely(is_global_init(tsk) && group_dead)) > + panic("Attempted to kill init! exitcode=0x%08lx\n", code); > + > + /* > * If do_exit is called because this processes oopsed, it's possible > * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before > * continuing. Amongst other possible reasons, this is to prevent > @@ -764,7 +768,6 @@ void __noreturn do_exit(long code) > if (tsk->mm) > sync_mm_rss(tsk->mm); > acct_update_integrals(tsk); > - group_dead = atomic_dec_and_test(&tsk->signal->live); > if (group_dead) { > #ifdef CONFIG_POSIX_TIMERS > hrtimer_cancel(&tsk->signal->real_timer); > -- > 1.9.1 >