Re: [PATCH v4 34/39] perf: Skip user unwind if !current->mm

Josh Poimboeuf
2025-01-22T23:08:41Z

On Wed, Jan 22, 2025 at 03:29:10PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 21, 2025 at 06:31:26PM -0800, Josh Poimboeuf wrote:
> > If the task doesn't have any memory, there's no stack to unwind.
> >
> > Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> > ---
> > kernel/events/core.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 99f0f28feeb5..a886bb83f4d0 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -7792,7 +7792,7 @@ struct perf_callchain_entry *
> > perf_callchain(struct perf_event *event, struct pt_regs *regs)
> > {
> > bool kernel = !event->attr.exclude_callchain_kernel;
> > - bool user = !event->attr.exclude_callchain_user;
> > + bool user = !event->attr.exclude_callchain_user && current->mm;
>
> What about things like io_uring helpers, don't they keep ->mm but are
> never in userspace?

Hm, that's news to me. At least this patch doesn't make things worse in
that regard.

IIRC, user_regs(task_pt_regs(current)) returns false for kthreads
because the regs area is cleared. We could add in a user_regs() check?

--
Josh