[PATCH 4/7] pid: modify task_tgid_nr to work without task->tgid.

[Richard Guy Briggs]

On 14/02/20, Oleg Nesterov wrote:
> On 01/23, Richard Guy Briggs wrote:
> >
> > task->tgid is an error prone construct and results in duplicate maintenance.
> > Start it's demise by modifying task_tgid_nr to not use it.
> 
> Well, I disagree.
> 
> Yes I agree that ->tgid should probably die. But this change itself doesn't
> help, it only makes task_tgid_nr() slower. We need to convert other users
> first, then consider this change along with ->tgid removal.

I thought I recently saw a statistic that there were only 7 instances of
->tgid, but a quick search comes up with >50 outside of audit!

> Besides, this is not that simple and the patch doesn't look right:
> 
> >  static inline pid_t task_tgid_nr(struct task_struct *tsk)
> >  {
> > -	return tsk->tgid;
> > +	return pid_nr(task_tgid(tsk));
> >  }
> 
> And what protect task_tgid? This is racy.
> 
> The race is very unlikely, pid_nr() will likely hit pid == NULL if tsk
> exits. But still it can use the freed/unmapped/reused memory.

So at the very least, I'd need

static inline pid_t task_tgid_nr(struct task_struct *tsk)
{
	return pid_nr(is_alive(tsk) ? task_tgid(tsk) : NULL);
}

And it sounds like I might even need this since the status of is_alive
could change between the time I call it and the time I call task_tgid:

static inline pid_t task_tgid_nr(struct task_struct *tsk)
{
	pid_t pid;

	task_lock(&tsk);
	pid = pid_nr(is_alive(tsk) ? task_tgid(tsk) : NULL);
	task_unlock(&tsk);
	return pid;
}

Is task_lock() sufficient, or do I need the heavier
read_lock(&tasklist_lock)?

> And even if we add rcu_read_lock() the patch will add the semantics change,
> task_tgid_nr() can return 0 if tsk has already exited. At least this should
> be documented, but you also need to audit the users.

Basically check for an inline error return of 0 signalling a failure
rather than the idle task.

> Oleg.

- RGB

--
Richard Guy Briggs <rbriggs at redhat.com>
Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635, Alt: +1.613.693.0684x3545