From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932096Ab3KVU1V (ORCPT ); Fri, 22 Nov 2013 15:27:21 -0500 Received: from fgwmail5.fujitsu.co.jp ([192.51.44.35]:60899 "EHLO fgwmail5.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754477Ab3KVU1T (ORCPT ); Fri, 22 Nov 2013 15:27:19 -0500 Message-ID: <528FBE22.5030208@jp.fujitsu.com> Date: Fri, 22 Nov 2013 15:27:14 -0500 From: KOSAKI Motohiro User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.1.1 MIME-Version: 1.0 To: oleg@redhat.com, akpm@linux-foundation.org CC: kosaki.motohiro@jp.fujitsu.com, viro@ZenIV.linux.org.uk, keescook@chromium.org, mhocko@suse.cz, snanda@chromium.org, dserrg@gmail.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/4] check_unsafe_exec: kill the dead -EAGAIN and clear_in_exec logic References: <20131122175442.GA31453@redhat.com> In-Reply-To: <20131122175442.GA31453@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org (11/22/2013 12:54 PM), Oleg Nesterov wrote: > fs_struct->in_exec == T means that this ->fs is used by a single > process (thread group), and one of the treads does do_execve(). > > To avoid the mt-exec races this code has the following complications: > > 1. check_unsafe_exec() returns -EBUSY if ->in_exec was > already set by another thread. > > 2. do_execve_common() records "clear_in_exec" to ensure > that the error path can only clear ->in_exec if it was > set by current. > > However, after 9b1bf12d5d51 "signals: move cred_guard_mutex from > task_struct to signal_struct" we do not need these complications: > > 1. We can't race with our sub-thread, this is called under > per-process ->cred_guard_mutex. And we can't race with > another CLONE_FS task, we already checked that this fs > is not shared. > > We can remove the dead -EAGAIN logic. > > 2. "out_unmark:" in do_execve_common() is either called > under ->cred_guard_mutex, or after de_thread() which > kills other threads, so we can't race with sub-thread > which could set ->in_exec. And if ->fs is shared with > another process ->in_exec should be false anyway. > > We can clear in_exec unconditionally. > > This also means that check_unsafe_exec() can be void. > > Signed-off-by: Oleg Nesterov I have found no problem in this patch. However, I have a very basic question. Why do we need to keep fs->in_exec? If I understand correctly, it is needed for retrieving fork() and exec() race in the same process. If it is correct, can't we move it it to signal->in_exec? It seems to match signal->cred_guard_mutex and _I_ can easily understand what the code want. In the other words, currently we have no protection against making new thread when p->fs is shared w/ another process and I have no idea why multi process sharing influence multi thread behavior. I am not expert in this area and I may overlook something. Please correct me if I am silly.