Re: [PATCH v4 32/39] perf: Remove get_perf_callchain() 'crosstask' argument

On Fri, Jan 24, 2025 at 10:13:23AM -0800, Andrii Nakryiko wrote:
> On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <[email protected]> wrote:
> > @@ -430,10 +429,8 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
> >         if (task && user && !user_mode(regs))
> >                 goto err_fault;
> >
> > -       /* get_perf_callchain does not support crosstask user stack walking
> > -        * but returns an empty stack instead of NULL.
> > -        */
> > -       if (crosstask && user) {
> > +       /* get_perf_callchain() does not support crosstask stack walking */
> > +       if (crosstask) {
> 
> crosstask stack trace is supported for kernel stack traces (see
> get_callchain_entry_for_task() call), so this is breaking that case

Oh I see, thanks.

BTW, that seems dubious, does it do anything to ensure the task isn't
running?   Otherwise the unwind is going to be a wild ride.

-- 
Josh