Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751179AbWHPQkF (ORCPT ); Wed, 16 Aug 2006 12:40:05 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751184AbWHPQkF (ORCPT ); Wed, 16 Aug 2006 12:40:05 -0400 Received: from filfla-vlan276.msk.corbina.net ([213.234.233.49]:18597 "EHLO screens.ru") by vger.kernel.org with ESMTP id S1751179AbWHPQkD (ORCPT ); Wed, 16 Aug 2006 12:40:03 -0400 Date: Thu, 17 Aug 2006 01:03:53 +0400 From: Oleg Nesterov To: "Eric W. Biederman" Cc: Andrew Morton , linux-kernel@vger.kernel.org, containers@lists.osdl.org Subject: Re: [PATCH 5/7] pid: Implement pid_nr Message-ID: <20060816210353.GA628@oleg> References: <1155666193751-git-send-email-ebiederm@xmission.com> <20060816181950.GA472@oleg> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1642 Lines: 49 On 08/16, Eric W. Biederman wrote: > Oleg Nesterov writes: > > > On 08/15, Eric W. Biederman wrote: > >> > >> +static inline pid_t pid_nr(struct pid *pid) > >> +{ > >> + pid_t nr = 0; > >> + if (pid) > >> + nr = pid->nr; > >> + return nr; > >> +} > > > > I think this is not safe, you need rcu locks here or the caller should > > do some locking. > > > > Let's look at f_getown() (PATCH 7/7). What if original task which was > > pointed by ->f_owner.pid has gone, another thread does fcntl(F_SETOWN), > > and pid_nr() takes a preemtion after 'if (pid)'? In this case 'pid->nr' > > may follow a freed memory. > > This isn't an rcu reference. I hold a hard reference count on > the pid entry. So this should be safe. -static void f_modown(struct file *filp, unsigned long pid, +static void f_modown(struct file *filp, struct pid *pid, enum pid_type type, uid_t uid, uid_t euid, int force) { write_lock_irq(&filp->f_owner.lock); if (force || !filp->f_owner.pid) { - filp->f_owner.pid = pid; + put_pid(filp->f_owner.pid); This 'put_pid()' can actually free 'struct pid' if the task/group has already gone away. Another thread doing f_getown() can access a freed memory, no? > What is an rcu reference is going from struct pid to the task > it points to. Yes, you are right... But I'd say it is going form task to pid :) Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/