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

[Song Liu <songliubraving@fb.com>]

> On Aug 25, 2022, at 4:03 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Thu, Aug 25, 2022 at 3:08 PM Song Liu <songliubraving@fb.com> wrote:
>> 
>> 
>> 
>>> On Aug 25, 2022, at 2:33 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>>> 
>>> On Tue, Aug 23, 2022 at 2:04 PM Namhyung Kim <namhyung@kernel.org> 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@kernel.org>
>>>> ---
>>>> 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

Given the name discussion (the other email), I now like one API better.

Actually, since we are on this, can we make it more generic, and handle
all possible PERF_SAMPLE_* (in enum perf_event_sample_format)? Something
like:

long bpf_perf_event_read_sample(void *ctx, void *buf, u64 size, u64 flags);

WDYT Namhyung?

Another idea is to add another parameter, so that we can pick which 
PERF_SAMPLE_* to output via bpf_perf_event_read_sample().

I think this will cover all cases with sample perf_event. Thoughts?

Thanks,
Song



> 
>> 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),            \
>>>>       /* */
>>>> 
>>> 
>>> [...]
>>