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

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Date:	Wed, 11 Apr 2012 11:41:34 +0100
From:	David Howells <dhowells@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	dhowells@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] creds: kill __task_cred()->task_is_dead() validation

Oleg Nesterov <oleg@...hat.com> wrote:

> commit 8f92054e:
> 
>     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().
> 
> task_is_dead() should die, task->exit_state != 0 means that this task
> has passed exit_notify(), only do_wait-like code paths should use this.
> 
> Unfortunately, we can't kill task_is_dead() right now, it has already
> found the bugy users in drivers/staging/, the fix already exists.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: David Howells <dhowells@...hat.com>
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