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

[John Fastabend <john.fastabend@gmail.com>]

Namhyung Kim 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>
> ---

Acked-by: John Fastabend <john.fastabend@gmail.com>

> 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.

not a pmu expert but also no good ideas on my side.

...

>  
> +BPF_CALL_4(bpf_read_raw_record, struct bpf_perf_event_data_kern *, ctx,
> +	   void *, buf, u32, size, u64, flags)
> +{
> +	struct perf_raw_record *raw = ctx->data->raw;
> +	struct perf_raw_frag *frag;
> +	u32 to_copy;
> +
> +	if (unlikely(flags & ~BPF_F_GET_RAW_RECORD_SIZE))
> +		return -EINVAL;
> +
> +	if (unlikely(!raw))
> +		return -ENOENT;
> +
> +	if (flags & BPF_F_GET_RAW_RECORD_SIZE)
> +		return raw->size;
> +
> +	if (!buf || (size % sizeof(u32) != 0))
> +		return -EINVAL;
> +
> +	frag = &raw->frag;
> +	WARN_ON_ONCE(!perf_raw_frag_last(frag));
> +
> +	to_copy = min_t(u32, frag->size, size);
> +	memcpy(buf, frag->data, to_copy);
> +
> +	return to_copy;
> +}
> +
> +static const struct bpf_func_proto bpf_read_raw_record_proto = {
> +	.func           = bpf_read_raw_record,
> +	.gpl_only       = true,
> +	.ret_type       = RET_INTEGER,
> +	.arg1_type      = ARG_PTR_TO_CTX,
> +	.arg2_type      = ARG_PTR_TO_MEM_OR_NULL,
> +	.arg3_type      = ARG_CONST_SIZE_OR_ZERO,
> +	.arg4_type      = ARG_ANYTHING,
> +};

Patch lgtm but curious why allow the ARG_PTR_TO_MEM_OR_NULL from API
side instead of just ARG_PTR_TO_MEM? Maybe, just to match the
existing perf_event_read()? I acked it as I think matching existing
API is likely good enough reason.

> +
>  static const struct bpf_func_proto *
>  pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  {
> @@ -1548,6 +1587,8 @@ pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  		return &bpf_read_branch_records_proto;
>  	case BPF_FUNC_get_attach_cookie:
>  		return &bpf_get_attach_cookie_proto_pe;
> +	case BPF_FUNC_read_raw_record:
> +		return &bpf_read_raw_record_proto;
>  	default:
>  		return bpf_tracing_func_proto(func_id, prog);
>  	}
> -- 
> 2.37.2.609.g9ff673ca1a-goog
>