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

[Song Liu]

> On Nov 3, 2022, at 12:45 PM, Yonghong Song <yhs@xxxxxxxx> 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@xxxxxxxxxx>
>>> ---
>>>  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? 

Thanks,
Song


>    The bpf_get_kern_ctx helper can be inlined as well.