linux-kernel - Re: [PATCH] perf trace: Fix wrong size to bpf_map__update

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Message-ID: <CAP-5=fXPgoocDSRxzYpkzcLsPwKsp9b80b1KPDWv-obkmiX-fA@mail.gmail.com>
Date: Mon, 24 Mar 2025 08:38:19 -0700
From: Ian Rogers <irogers@...gle.com>
To: Thomas Richter <tmricht@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, acme@...nel.org, namhyung@...nel.org, 
	james.clark@...aro.org, agordeev@...ux.ibm.com, gor@...ux.ibm.com, 
	sumanthk@...ux.ibm.com, hca@...ux.ibm.com
Subject: Re: [PATCH] perf trace: Fix wrong size to bpf_map__update_elem call

On Mon, Mar 24, 2025 at 8:28 AM Thomas Richter <tmricht@...ux.ibm.com> wrote:
>
> In linux-next
> commit c760174401f6 ("perf cpumap: Reduce cpu size from int to int16_t")
> causes the perf tests 100 126 to fail on s390:
>
> Output before:
>  # ./perf test 100
>  100: perf trace BTF general tests         : FAILED!
>  #
>
> The root cause is the change from int to int16_t for the
> cpu maps. The size of the CPU key value pair changes from
> four bytes to two bytes. However a two byte key size is
> not supported for bpf_map__update_elem().

Nice catch!

> Note: validate_map_op() in libbpf.c emits warning
>  libbpf: map '__augmented_syscalls__': \
>          unexpected key size 2 provided, expected 4
> when key size is set to int16_t.

Wow, weird.

> Therefore change to variable size back to 4 bytes for
> invocation of bpf_map__update_elem().
>
> Output after:
>  # ./perf test 100
>  100: perf trace BTF general tests         : Ok
>  #
>
> Fixes: c760174401f6 ("perf cpumap: Reduce cpu size from int to int16_t")
> Signed-off-by: Thomas Richter <tmricht@...ux.ibm.com>
> Cc: Ian Rogers <irogers@...gle.com>
> Cc: James Clark <james.clark@...aro.org>
> ---
>  tools/perf/builtin-trace.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index 092c5f6404ba..464c97a11852 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -4375,10 +4375,12 @@ static int trace__run(struct trace *trace, int argc, const char **argv)
>                  * CPU the bpf-output event's file descriptor.
>                  */
>                 perf_cpu_map__for_each_cpu(cpu, i, trace->syscalls.events.bpf_output->core.cpus) {
> +                       int mycpu = cpu.cpu;
> +
>                         bpf_map__update_elem(trace->skel->maps.__augmented_syscalls__,
> -                                       &cpu.cpu, sizeof(int),
> +                                       &mycpu, sizeof(int),

nit: It is usually preferred to do "sizeof(mycpu)" to avoid the
problems like the one this fix is fixing. And I'm blamed for the bad
code in:
5e6da6be3082 perf trace: Migrate BPF augmentation to use a skeleton

Reviewed-by: Ian Rogers <irogers@...gle.com>

Thanks for the fixes!
Ian

>                                         xyarray__entry(trace->syscalls.events.bpf_output->core.fd,
> -                                                      cpu.cpu, 0),
> +                                                      mycpu, 0),
>                                         sizeof(__u32), BPF_ANY);
>                 }
>         }
> --
> 2.48.1
>