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SubjectRe: [PATCH bpf-next 2/3] bpf: Add bpf_perf_event_read_sample() helper
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On 11/3/22 1:55 PM, Song Liu wrote:
>
>
>> On Nov 3, 2022, at 12:45 PM, Yonghong Song <yhs@meta.com> wrote:
>>
>>
>>
>> On 11/1/22 3:02 AM, Jiri Olsa 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
>>
>> Martin and I have discussed an idea to introduce a generic helper like
>> bpf_get_kern_ctx(void *ctx)
>> Given a context, the helper will return a PTR_TO_BTF_ID representing the
>> corresponding kernel ctx. So in the above example, user could call
>>
>> struct bpf_perf_event_data_kern *kctx = bpf_get_kern_ctx(ctx);
>> ...
>
> This is an interesting idea!
>
>> To implement bpf_get_kern_ctx helper, the verifier can find the type
>> of the context and provide a hidden btf_id as the second parameter of
>> the actual kernel helper function like
>> bpf_get_kern_ctx(ctx) {
>> return ctx;
>> }
>> /* based on ctx_btf_id, find kctx_btf_id and return it to verifier */
>
> I think we will need a map of ctx_btf_id => kctx_btf_id. Shall we somehow
> expose this to the user?

Yes, inside the kernel we need ctx_btf_id -> kctx_btf_id mapping.
Good question. We might not want to this mapping as a stable API.
So using kfunc might be more appropriate.

>
> Thanks,
> Song
>
>
>> The bpf_get_kern_ctx helper can be inlined as well.
>
>

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