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Date:   Sat, 11 Nov 2023 12:16:11 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jordan Rome <linux@...danrome.com>
Cc:     linux-perf-users@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...nel.org>,
        Kernel Team <kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH v2] perf: get_perf_callchain return NULL for crosstask

On Sat, Nov 11, 2023 at 9:20 AM Jordan Rome <linux@...danrome.com> 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@...danrome.com>
> ---
>
> Changes in v2:
> * move user and crosstask check before get_callchain_entry
>

LGTM.

Acked-by: Andrii Nakryiko <andrii@...nel.org>

> v1:
> https://lore.kernel.org/linux-perf-users/CAEf4BzaWtOeTBb_+b7Td3NHaKjZU+OohuBJje_nvw9kd6xPA3g@mail.gmail.com/T/#t
>
>  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..104ea2975a57 100644
> --- a/kernel/events/callchain.c
> +++ b/kernel/events/callchain.c
> @@ -184,6 +184,9 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
>         struct perf_callchain_entry_ctx ctx;
>         int rctx;
>
> +       if (user && crosstask)
> +               return NULL;
> +
>         entry = get_callchain_entry(&rctx);
>         if (!entry)
>                 return NULL;
> @@ -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
>

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