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

[Yonghong Song <yhs@meta.com>]

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.
> 
>