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

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Mon, Mar 24, 2025 at 08:50:36AM -0700, Ian Rogers wrote:
> On Mon, Mar 24, 2025 at 8:38 AM Ian Rogers <irogers@google.com> wrote:
> >
> > On Mon, Mar 24, 2025 at 8:28 AM Thomas Richter <tmricht@linux.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.
> 
> Ah, I guess it is a mismatch with the declaration in
> tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c:
> https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c?h=perf-tools-next#n31
> ```
> /* bpf-output associated map */
> struct __augmented_syscalls__ {
>        __uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
>        __type(key, int);
>        __type(value, __u32);
>        __uint(max_entries, MAX_CPUS);
> } __augmented_syscalls__ SEC(".maps");
> ```
> but this looks wrong. The values are file descriptors, so should be
> ints. I think it should be:
> ```
> struct __augmented_syscalls__ {
>        __uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
>        __type(key, int16_t);
>        __type(value, int);
>        __uint(max_entries, MAX_CPUS);
> } __augmented_syscalls__ SEC(".maps");
> ```
> I'm not sure if max_entries can be a uint16_t too. I suspect this may
> well interfere with BPF_MAP_TYPE_PERF_EVENT_ARRAY and its use by
> bpf_perf_event_output. Probably best to keep things in the BPF code as
> they are and do your builtin-trace.c fix. Perhaps someone else can
> clean this up.

I don't think max_entries can be uint16_t.  Also I'm skeptical to change
the key type to int16_t as you said.  I'll pick Thomas's patch up.

Thanks,
Namhyung

> 
> > > 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@linux.ibm.com>
> > > Cc: Ian Rogers <irogers@google.com>
> > > Cc: James Clark <james.clark@linaro.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@google.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
> > >