Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper

[Andrii Nakryiko]

On Thu, Aug 25, 2022 at 3:08 PM Song Liu <songliubraving@xxxxxx> wrote:
>
>
>
> > On Aug 25, 2022, at 2:33 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Tue, Aug 23, 2022 at 2:04 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >>
> >> The helper is for BPF programs attached to perf_event in order to read
> >> event-specific raw data.  I followed the convention of the
> >> bpf_read_branch_records() helper so that it can tell the size of
> >> record using BPF_F_GET_RAW_RECORD flag.
> >>
> >> The use case is to filter perf event samples based on the HW provided
> >> data which have more detailed information about the sample.
> >>
> >> Note that it only reads the first fragment of the raw record.  But it
> >> seems mostly ok since all the existing PMU raw data have only single
> >> fragment and the multi-fragment records are only for BPF output attached
> >> to sockets.  So unless it's used with such an extreme case, it'd work
> >> for most of tracing use cases.
> >>
> >> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> >> ---
> >> I don't know how to test this.  As the raw data is available on some
> >> hardware PMU only (e.g. AMD IBS).  I tried a tracepoint event but it was
> >> rejected by the verifier.  Actually it needs a bpf_perf_event_data
> >> context so that's not an option IIUC.
> >>
> >> include/uapi/linux/bpf.h | 23 ++++++++++++++++++++++
> >> kernel/trace/bpf_trace.c | 41 ++++++++++++++++++++++++++++++++++++++++
> >> 2 files changed, 64 insertions(+)
> >>
> >> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> >> index 934a2a8beb87..af7f70564819 100644
> >> --- a/include/uapi/linux/bpf.h
> >> +++ b/include/uapi/linux/bpf.h
> >> @@ -5355,6 +5355,23 @@ union bpf_attr {
> >>  *     Return
> >>  *             Current *ktime*.
> >>  *
> >> + * long bpf_read_raw_record(struct bpf_perf_event_data *ctx, void *buf, u32 size, u64 flags)
> >> + *     Description
> >> + *             For an eBPF program attached to a perf event, retrieve the
> >> + *             raw record associated to *ctx* and store it in the buffer
> >> + *             pointed by *buf* up to size *size* bytes.
> >> + *     Return
> >> + *             On success, number of bytes written to *buf*. On error, a
> >> + *             negative value.
> >> + *
> >> + *             The *flags* can be set to **BPF_F_GET_RAW_RECORD_SIZE** to
> >> + *             instead return the number of bytes required to store the raw
> >> + *             record. If this flag is set, *buf* may be NULL.
> >
> > It looks pretty ugly from a usability standpoint to have one helper
> > doing completely different things and returning two different values
> > based on BPF_F_GET_RAW_RECORD_SIZE.
>
> Yeah, I had the same thought when I first looked at it. But that's the
> exact syntax with bpf_read_branch_records(). Well, we still have time
> to fix the new helper..
>
> >
> > I'm not sure what's best, but I have two alternative proposals:
> >
> > 1. Add two helpers: one to get perf record information (and size will
> > be one of them). Something like bpf_perf_record_query(ctx, flags)
> > where you pass perf ctx and what kind of information you want to read
> > (through flags), and u64 return result returns that (see
> > bpf_ringbuf_query() for such approach). And then have separate helper
> > to read data.
> >
> > 2. Keep one helper, but specify that it always returns record size,
> > even if user specified smaller size to read. And then allow passing
> > buf==NULL && size==0. So passing NULL, 0 -- you get record size.
> > Passing non-NULL buf -- you read data.
>
> AFAICT, this is also confusing.
>

this is analogous to snprintf() behavior, so not that new and
surprising when you think about it. But if query + read makes more
sense, then it's fine by me

> Maybe we should use two kfuncs for this?
>
> Thanks,
> Song
>
> >
> >
> > And also, "read_raw_record" is way too generic. We have
> > bpf_perf_prog_read_value(), let's use "bpf_perf_read_raw_record()" as
> > a name. We should have called bpf_read_branch_records() as
> > bpf_perf_read_branch_records(), probably, as well. But it's too late.
> >
> >> + *
> >> + *             **-EINVAL** if arguments invalid or **size** not a multiple
> >> + *             of **sizeof**\ (u64\ ).
> >> + *
> >> + *             **-ENOENT** if the event does not have raw records.
> >>  */
> >> #define __BPF_FUNC_MAPPER(FN)          \
> >>        FN(unspec),                     \
> >> @@ -5566,6 +5583,7 @@ union bpf_attr {
> >>        FN(tcp_raw_check_syncookie_ipv4),       \
> >>        FN(tcp_raw_check_syncookie_ipv6),       \
> >>        FN(ktime_get_tai_ns),           \
> >> +       FN(read_raw_record),            \
> >>        /* */
> >>
> >
> > [...]
>