Re: [PATCH] perf trace: Fix wrong size to bpf_map__update_elem call

From: Ian Rogers
Date: Mon Mar 24 2025 - 11:38:41 EST


On Mon, Mar 24, 2025 at 8:28 AM Thomas Richter <tmricht@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Cc: Ian Rogers <irogers@xxxxxxxxxx>
> Cc: James Clark <james.clark@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>

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
>