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

[Andrii Nakryiko]

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.

> +
>                 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
>