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

[Oleg Nesterov <oleg@tv-sign.ru>]

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@tv-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)