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

Oleg Nesterov
2016-10-24T13:27:30Z

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);
}

/*