linux-kernel - [PATCH 1/2] creds: kill __task_cred()->task_is

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Date:	Tue, 20 Sep 2011 16:39:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"Serge E. Hallyn" <serge.hallyn@...onical.com>,
	lkml <linux-kernel@...r.kernel.org>, richard@....at,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tejun Heo <tj@...nel.org>
Subject: [PATCH 1/2] creds: kill __task_cred()->task_is_dead() check

>From 8f92054e commit:

    Instead, add the following validation condition:

        task->exit_state >= 0

    to permit the access if the target task is dead and therefore unable to change
    its own credentials.

OK, but afaics currently this can only help wait_task_zombie() which
calls __task_cred() without rcu lock.

Remove this validation and change wait_task_zombie() to use task_uid()
instead. This means we do rcu_read_lock() only to shut up the lockdep,
but we already do the same in, say, wait_task_stopped().

Unfortunately, we can't kill task_is_dead() right now, it has already
found the users in drivers/staging/, and I bet the usage is wrong.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---

 include/linux/cred.h |    3 +--
 kernel/exit.c        |    2 +-
 2 files changed, 2 insertions(+), 3 deletions(-)

--- 3.1/include/linux/cred.h~1_kill_task_is_dead	2011-09-20 16:28:22.000000000 +0200
+++ 3.1/include/linux/cred.h	2011-09-20 16:28:47.000000000 +0200
@@ -284,8 +284,7 @@ static inline void put_cred(const struct
 #define __task_cred(task)						\
 	({								\
 		const struct task_struct *__t = (task);			\
-		rcu_dereference_check(__t->real_cred,			\
-				      task_is_dead(__t));		\
+		rcu_dereference_check(__t->real_cred, 0);		\
 	})
 
 /**
--- 3.1/kernel/exit.c~1_kill_task_is_dead	2011-09-20 16:28:22.000000000 +0200
+++ 3.1/kernel/exit.c	2011-09-20 16:28:47.000000000 +0200
@@ -1191,7 +1191,7 @@ static int wait_task_zombie(struct wait_
 	unsigned long state;
 	int retval, status, traced;
 	pid_t pid = task_pid_vnr(p);
-	uid_t uid = __task_cred(p)->uid;
+	uid_t uid = task_uid(p);
 	struct siginfo __user *infop;
 
 	if (!likely(wo->wo_flags & WEXITED))

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