linux-kernel - [PATCH 1/4] pids: de_thread: don't clear session/pgrp pids for the old leader

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Message-ID: <20080320152816.GA6332@tv-sign.ru>
Date:	Thu, 20 Mar 2008 18:28:17 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/4] pids: de_thread: don't clear session/pgrp pids for the old leader

Based on Eric W. Biederman's idea.

Unless task == current, without tasklist_lock held task_session()/task_pgrp()
can return NULL if the caller races with de_thread() which switches the group
leader.

Change transfer_pid() to not clear old->pids[type].pid for the old leader.
This means that its .pid can point to "nowhere", but this is already true for
sub-threads, and the old leader is not group_leader() any longer. IOW, with
or without this change we can't trust task's special pids unless it is the
group leader.

With this change the following code

	rcu_read_lock();
	task = find_task_by_xxx();
	do_something(task_pgrp(task), task_session(task));
	rcu_read_unlock();

can't race with exec and hit the NULL pid.

Signed-off-by: Oleg Nesterov <oleg@...sign.ru>

--- 25/kernel/pid.c~1_PID_EXEC	2008-03-20 17:00:56.000000000 +0300
+++ 25/kernel/pid.c	2008-03-20 17:22:19.000000000 +0300
@@ -354,7 +354,6 @@ void transfer_pid(struct task_struct *ol
 {
 	new->pids[type].pid = old->pids[type].pid;
 	hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
-	old->pids[type].pid = NULL;
 }

 struct task_struct *pid_task(struct pid *pid, enum pid_type type)

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