Re: [PATCH perf] perf: get_perf_callchain return NULL for crosstask

[Jordan Rome]

On Fri, Nov 10, 2023 at 7:10 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Fri, Nov 10, 2023 at 3:51 PM Jordan Rome <linux@xxxxxxxxxxxxxx> wrote:
> >
> > Return NULL instead of returning 1 incorrect frame, which
> > currently happens when trying to walk the user stack for
> > any task that isn't current. Returning NULL is a better
> > indicator that this behavior is not supported.
> >
> > This issue was found using bpf_get_task_stack inside a BPF
> > iterator ("iter/task"), which iterates over all tasks. The
> > single address/frame in the buffer when getting user stacks
> > for tasks that aren't current could not be symbolized (testing
> > multiple symbolizers).
> >
> > Signed-off-by: Jordan Rome <linux@xxxxxxxxxxxxxx>
> > ---
> >  kernel/events/callchain.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> > index 1273be84392c..430fa544fa80 100644
> > --- a/kernel/events/callchain.c
> > +++ b/kernel/events/callchain.c
> > @@ -201,6 +201,9 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
> >         }
> >
> >         if (user) {
> > +               if (crosstask)
> > +                       return NULL;
>
> I think you need that goto exit_put here.
>

Why is that? Wouldn't that be the same behavior that already exists?
That being said we can probably move this check above get_callchain_entry
and exit earlier.

> > +
> >                 if (!user_mode(regs)) {
> >                         if  (current->mm)
> >                                 regs = task_pt_regs(current);
> > @@ -209,9 +212,6 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
> >                 }
> >
> >                 if (regs) {
> > -                       if (crosstask)
> > -                               goto exit_put;
> > -
> >                         if (add_mark)
> >                                 perf_callchain_store_context(&ctx, PERF_CONTEXT_USER);
> >
> > @@ -219,7 +219,6 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
> >                 }
> >         }
> >
> > -exit_put:
> >         put_callchain_entry(rctx);
> >
> >         return entry;
> > --
> > 2.39.3
> >