Re: [PATCH 1/2] creds: kill __task_cred()->task_is_dead() check

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Sep 20, 2011 at 04:39:42PM +0200, Oleg Nesterov wrote:
> >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.

>From a first quick scan...

							Thanx, Paul

> Signed-off-by: Oleg Nesterov <oleg@redhat.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);		\

The "0" above will make lockdep-RCU complain unconditionally.  My guess
is that you want rcu_dereference_raw().

>  	})
> 
>  /**
> --- 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))
>