From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753393AbbKPSpW (ORCPT ); Mon, 16 Nov 2015 13:45:22 -0500 Received: from mail-yk0-f169.google.com ([209.85.160.169]:35929 "EHLO mail-yk0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751546AbbKPSpT (ORCPT ); Mon, 16 Nov 2015 13:45:19 -0500 Date: Mon, 16 Nov 2015 13:45:16 -0500 From: Tejun Heo To: Oleg Nesterov Cc: Jan Kratochvil , Pedro Alves , Andrey Ryabinin , Roland McGrath , LKML Subject: Re: ptrace() hangs on attempt to seize/attach stopped & frozen task Message-ID: <20151116184516.GJ18894@mtj.duckdns.org> References: <5640B7F2.70406@virtuozzo.com> <20151109185506.GA22744@redhat.com> <20151109180207.GA28507@mtj.duckdns.org> <20151110202017.GA2976@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151110202017.GA2976@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Oleg. Sorry about the delay. On Tue, Nov 10, 2015 at 09:20:17PM +0100, Oleg Nesterov wrote: > > We simply need to reimplement cgroup freezer so that its userland > > visible state is well defined (most likely jobctl stop). Right now, > > it's allowing userland to trigger "stuck somewhere in the kernel" > > condition, so interactions with frozen tasks are naturally broken. > > I agree, the freezer is not perfect, and it needs changes. > > Still I think this needs a fix in ptrace code. At least we should not > wait in TASK_UNINTERRUPTIBLE state. > > And perhaps we can simply remove this logic? I forgot why do we hide this > STOPPED -> RUNNING -> TRACED transition from the attaching thread. But the > vague feeling tells me that we discussed this before and perhaps it was me > who suggested to avoid the user-visible change when you introduced this > transition... Heh, it was too long ago for me to remember much. :) > Anyway, now I do not understand why do we want to hide it. Lets consider > the following "test-case", > > void test(int pid) > { > kill(pid, SIGSTOP); > waitpid(pid, NULL, WSTOPPED); > > ptrace(PTRACE_ATTACH-OR-PTRACE_SEIZE, pid, 0,0); > > assert(ptrace(PTRACE_DETACH, pid, 0,0) == 0); > } > > Yes, it will fail if we remove JOBCTL_TRAPPING. But it can equally fail > if SIGCONT comes before ATTACH, so perhaps we do not really care? > > Jan, Pedro, do you think the patch below can break gdb somehow? With this > patch you can never assume that waitpid(WNOHANG) or ptrace(WHATEVER) will > succeed right after PTRACE_ATTACH/PTRACE_SEIZE, even if you know that the > tracee was TASK_STOPPED before attach. > > Tejun, do you see any reason to keep JOBCTL_TRAPPING? Hmmm... It's nasty tho. We're breaking a guaranteed userland behavior to mask a deficiency (IMHO it's an outright bug) in a different subsystem. The problem here is that cgroup-frozen threads become un-runnable on a running system and it doesn't make sense to me to work around that from all the affected places rather than fixing it at the source especially if that involves breaking a known supported userland behavior. This isn't different from the frozen processes failing to respond to SIGKILL. I'd be a lot more comfortable stating that cgroup freezer is currently broken rather than diddling with subtle ptrace semantics. Thanks. -- tejun