From: Oleg Nesterov <oleg@redhat.com>
Date: Thu, 2 Apr 2009 23:58:35 +0000 (-0700)
Subject: pids: document task_pgrp/task_session is not safe without tasklist/rcu
X-Git-Url: http://git.cdn.openwrt.org/?a=commitdiff_plain;h=6dda81f4384b94930826eded254d8c16f89a9248;p=openwrt%2Fstaging%2Fblogic.git

pids: document task_pgrp/task_session is not safe without tasklist/rcu

Even if task == current, it is not safe to dereference the result of
task_pgrp/task_session.  We can race with another thread which changes the
special pid via setpgid/setsid.

Document this.  The next 2 patches give an example of the unsafe usage, we
have more bad users.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Louis Rilling <Louis.Rilling@kerlabs.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b47c94e7560b..722dd313bf8a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1489,6 +1489,11 @@ static inline struct pid *task_tgid(struct task_struct *task)
 	return task->group_leader->pids[PIDTYPE_PID].pid;
 }
 
+/*
+ * Without tasklist or rcu lock it is not safe to dereference
+ * the result of task_pgrp/task_session even if task == current,
+ * we can race with another thread doing sys_setsid/sys_setpgid.
+ */
 static inline struct pid *task_pgrp(struct task_struct *task)
 {
 	return task->group_leader->pids[PIDTYPE_PGID].pid;