Re: [PATCH 06/26] rlimit: Remove unnecessary grab of tasklist_lock

Oleg Nesterov [off-list ref] writes:

Hi Eric,

I'll try very much to read this series tomorrow, can't do this today...

On 06/06, Eric W. Biederman wrote:

quoted

@@ -1380,13 +1380,6 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource,
 			return -EPERM;
 	}
 
-	/* protect tsk->signal and tsk->sighand from disappearing */
-	read_lock(&tasklist_lock);
-	if (!tsk->sighand) {
-		retval = -ESRCH;
-		goto out;
-	}

Yes, the comment is wrong.

However we do need read_lock(tasklist_lock) to access ->group_leader. And the
->sighand != NULL check ensures that ->group_leader is the valid
pointer.

As of 4.12-rc1 The code does not access group_leader anymore.

Also, update_rlimit_cpu() is not safe without tasklist / sighand-check.

We can probably change this code to rely on rcu.

Good point a NULL sighand will cause update_rlimit_cpu to OOPS.

Grr.  There is a point in my tree where this is perfectly safe.  But not
at this point.  Consider this patch dropped for the moment.

Eric