Re: [PATCH bpf-next 2/3] bpf: Add bpf_perf_event_read_sample() helper

[Song Liu <song@kernel.org>]

On Tue, Nov 1, 2022 at 11:26 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue, Nov 1, 2022 at 3:03 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > On Mon, Oct 31, 2022 at 10:23:39PM -0700, Namhyung Kim wrote:
> > > The bpf_perf_event_read_sample() helper is to get the specified sample
> > > data (by using PERF_SAMPLE_* flag in the argument) from BPF to make a
> > > decision for filtering on samples.  Currently PERF_SAMPLE_IP and
> > > PERF_SAMPLE_DATA flags are supported only.
> > >
> > > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > > ---
> > >  include/uapi/linux/bpf.h       | 23 ++++++++++++++++
> > >  kernel/trace/bpf_trace.c       | 49 ++++++++++++++++++++++++++++++++++
> > >  tools/include/uapi/linux/bpf.h | 23 ++++++++++++++++
> > >  3 files changed, 95 insertions(+)
> > >
> > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > index 94659f6b3395..cba501de9373 100644
> > > --- a/include/uapi/linux/bpf.h
> > > +++ b/include/uapi/linux/bpf.h
> > > @@ -5481,6 +5481,28 @@ union bpf_attr {
> > >   *           0 on success.
> > >   *
> > >   *           **-ENOENT** if the bpf_local_storage cannot be found.
> > > + *
> > > + * long bpf_perf_event_read_sample(struct bpf_perf_event_data *ctx, void *buf, u32 size, u64 sample_flags)
> > > + *   Description
> > > + *           For an eBPF program attached to a perf event, retrieve the
> > > + *           sample data associated to *ctx* and store it in the buffer
> > > + *           pointed by *buf* up to size *size* bytes.
> > > + *
> > > + *           The *sample_flags* should contain a single value in the
> > > + *           **enum perf_event_sample_format**.
> > > + *   Return
> > > + *           On success, number of bytes written to *buf*. On error, a
> > > + *           negative value.
> > > + *
> > > + *           The *buf* can be set to **NULL** to return the number of bytes
> > > + *           required to store the requested sample data.
> > > + *
> > > + *           **-EINVAL** if *sample_flags* is not a PERF_SAMPLE_* flag.
> > > + *
> > > + *           **-ENOENT** if the associated perf event doesn't have the data.
> > > + *
> > > + *           **-ENOSYS** if system doesn't support the sample data to be
> > > + *           retrieved.
> > >   */
> > >  #define ___BPF_FUNC_MAPPER(FN, ctx...)                       \
> > >       FN(unspec, 0, ##ctx)                            \
> > > @@ -5695,6 +5717,7 @@ union bpf_attr {
> > >       FN(user_ringbuf_drain, 209, ##ctx)              \
> > >       FN(cgrp_storage_get, 210, ##ctx)                \
> > >       FN(cgrp_storage_delete, 211, ##ctx)             \
> > > +     FN(perf_event_read_sample, 212, ##ctx)          \
> > >       /* */
> > >
> > >  /* backwards-compatibility macros for users of __BPF_FUNC_MAPPER that don't
> > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > > index ce0228c72a93..befd937afa3c 100644
> > > --- a/kernel/trace/bpf_trace.c
> > > +++ b/kernel/trace/bpf_trace.c
> > > @@ -28,6 +28,7 @@
> > >
> > >  #include <uapi/linux/bpf.h>
> > >  #include <uapi/linux/btf.h>
> > > +#include <uapi/linux/perf_event.h>
> > >
> > >  #include <asm/tlb.h>
> > >
> > > @@ -1743,6 +1744,52 @@ static const struct bpf_func_proto bpf_read_branch_records_proto = {
> > >       .arg4_type      = ARG_ANYTHING,
> > >  };
> > >
> > > +BPF_CALL_4(bpf_perf_event_read_sample, struct bpf_perf_event_data_kern *, ctx,
> > > +        void *, buf, u32, size, u64, flags)
> > > +{
> >
> > I wonder we could add perf_btf (like we have tp_btf) program type that
> > could access ctx->data directly without helpers
> >
> > > +     struct perf_sample_data *sd = ctx->data;
> > > +     void *data;
> > > +     u32 to_copy = sizeof(u64);
> > > +
> > > +     /* only allow a single sample flag */
> > > +     if (!is_power_of_2(flags))
> > > +             return -EINVAL;
> > > +
> > > +     /* support reading only already populated info */
> > > +     if (flags & ~sd->sample_flags)
> > > +             return -ENOENT;
> > > +
> > > +     switch (flags) {
> > > +     case PERF_SAMPLE_IP:
> > > +             data = &sd->ip;
> > > +             break;
> > > +     case PERF_SAMPLE_ADDR:
> > > +             data = &sd->addr;
> > > +             break;
> >
> > AFAICS from pe_prog_convert_ctx_access you should be able to read addr
> > directly from context right? same as sample_period.. so I think if this
> > will be generic way to read sample data, should we add sample_period
> > as well?
>
> +1
> Let's avoid new stable helpers for this.
> Pls use CORE and read perf_sample_data directly.

We have legacy ways to access sample_period and addr with
struct bpf_perf_event_data and struct bpf_perf_event_data_kern. I
think mixing that
with CORE makes it confusing for the user. And a helper or a kfunc would make it
easier to follow. perf_btf might also be a good approach for this.

Thanks,
Song