Re: [PATCH] kernel/pid.c: check pid whether be NULL in __change_pid()

[Oleg Nesterov <oleg@redhat.com>]

On 10/08, Chen Gang wrote:
>
> On 10/07/2013 08:43 PM, Oleg Nesterov wrote:
> >
> >> but still recommend to check it
> >> in __change_pid() to let itself consistency.
> >
> > Why?
> >
> > Contrary, I think we should not hide the problem. If __change_pid() is
> > called when task->pids[type].pid is already NULL there is something
> > seriously wrong.
> >
>
> Hmm... In my opinion, it means need BUG_ON() for original 'link->pid'.
>
> --------------------------------patch begin-----------------------------
>
> [PATCH] kernel/pid.c: add BUG_ON() for "!pid" in __change_pid()
>
>   Within __change_pid(), 'new' may be NULL if it comes from detach_pid(),

Yes, this is fine,

>   and 'link->pid' also may be NULL ("link->pid = new"),
> ...
>   the original 'link->pid' may be NULL, too.

Too? You mean, it becomes NULL after detach_pid().

>   But in real world, all related extern functions always assume "if
>   'link->pid' is already NULL, there must be something seriously wrong",
>   although __change_pid() can accept parameter 'new' as NULL.

I simply can't understand why you mix "new == NULL" and "link->pid == NULL".

>   So in __change_pid(), need add BUG_ON() for it: "it should not happen,
>   when it really happen, OS must be continuing blindly,

OS will crash a couple of lines below trying to dereference this pointer.

> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -396,6 +396,12 @@ static void __change_pid(struct task_struct *task, enum pid_type type,
>  	link = &task->pids[type];
>  	pid = link->pid;
>
> +	/*
> +	 * If task->pids[type].pid is already NULL, there must be something
> +	 * seriously wrong
> +	 */
> +	BUG_ON(!pid);

Following this logic you should also add

	BUG_ON(!task);
	BUG_ON(!link->node.next);
	BUG_ON(!link->node.prev || link->node.prev == LIST_POISON2);
	...

Seriously, I do not understand the point. Yes, detach_pid() should not
be called twice. And it has a single caller. And this caller will crash
too if it is called twice. So you can also add BUG_ON() into
__unhash_process(). And so on.

Oleg.