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From:   Song Liu <songliubraving@fb.com>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
CC:     Namhyung Kim <namhyung@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>, Martin Lau <kafai@fb.com>,
        Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper
Thread-Topic: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper
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Date:   Thu, 25 Aug 2022 22:08:27 +0000
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> 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.

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