On Sat, Nov 11, 2023 at 9:20 AM 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>
> ---
>
> Changes in v2:
> * move user and crosstask check before get_callchain_entry
>
LGTM.
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> v1:
> https://lore.kernel.org/linux-perf-users/CAEf4BzaWtOeTBb_+b7Td3NHaKjZU+OohuBJje_nvw9kd6xPA3g@xxxxxxxxxxxxxx/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
>
