Re: hit a KASan bug related to Perf during stress test

[Oleg Nesterov <oleg@redhat.com>]

On 10/24, Peter Zijlstra wrote:
>
> On Mon, Oct 24, 2016 at 02:29:42PM +0200, Oleg Nesterov wrote:
> > On 10/24, Peter Zijlstra wrote:
> > >
> > > Still, I wonder if returning 0 is the right thing. 0 is a 'valid' PID
> > > for the init/idle task.
> >
> > Yes, now I think that -1 would make more sense. Unfortunately we can't
> > just change __task_pid_nr_ns(), it already has the users which assume
> > it returns zero... attach_to_pi_state() for example.
>
> Indeed. And I have a patch that assumes task_pid_vnr(&init_task) == 0,
> is that true because of this !alive case or true in general?

This is true in general. Idle threads are always alive but they use the
the special init_struct_pid with .nr == 0.

> No worries though, we can revert to your earlier explicit test and
> return -1 while adding a comment to explain details?

...

> Ah, ok. So whould we change that to match pid and return (explicit) -1
> there too?

Well, if we add that PIDTYPE_TGID hack, I think we can do something
like below...

Or do you think we should add a perf_alive() check into perf_event_pid()
for a quick fix?

Either way it's a pity we can't report at least the valid tid, perhaps
perf_event_tid() could use task_pid_nr() if event->ns == init_pid_ns,
I dunno.

Oleg.

--- x/kernel/events/core.c
+++ x/kernel/events/core.c
@@ -1249,26 +1249,30 @@ unclone_ctx(struct perf_event_context *c
 	return parent_ctx;
 }
 
-static u32 perf_event_pid(struct perf_event *event, struct task_struct *p)
+static u32 perf_event_xxx(struct perf_event *event, struct task_struct *p,
+			 	enum pid_type type)
 {
+	pid_t nr;
 	/*
 	 * only top level events have the pid namespace they were created in
 	 */
 	if (event->parent)
 		event = event->parent;
 
-	return task_tgid_nr_ns(p, event->ns);
+	nr = __task_pid_nr_ns(p, type, event->ns);
+	if (!nr && !is_idle_task(p))
+		nr = -1;
+	return nr;
 }
 
-static u32 perf_event_tid(struct perf_event *event, struct task_struct *p)
+static u32 perf_event_pid(struct perf_event *event, struct task_struct *p)
 {
-	/*
-	 * only top level events have the pid namespace they were created in
-	 */
-	if (event->parent)
-		event = event->parent;
+	return perf_event_xxx(p, event, PIDTYPE_TGID);
+}
 
-	return task_pid_nr_ns(p, event->ns);
+static u32 perf_event_tid(struct perf_event *event, struct task_struct *p)
+{
+	return perf_event_xxx(p, event, PIDTYPE_PID);
 }
 
 /*