RE: [PATCH v2 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches() helper

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

Daniel Xu wrote:
> Branch records are a CPU feature that can be configured to record
> certain branches that are taken during code execution. This data is
> particularly interesting for profile guided optimizations. perf has had
> branch record support for a while but the data collection can be a bit
> coarse grained.
> 
> We (Facebook) have seen in experiments that associating metadata with
> branch records can improve results (after postprocessing). We generally
> use bpf_probe_read_*() to get metadata out of userspace. That's why bpf
> support for branch records is useful.
> 
> Aside from this particular use case, having branch data available to bpf
> progs can be useful to get stack traces out of userspace applications
> that omit frame pointers.
> 
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---
>  include/uapi/linux/bpf.h | 13 ++++++++++++-
>  kernel/trace/bpf_trace.c | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 033d90a2282d..7350c5be6158 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2885,6 +2885,16 @@ union bpf_attr {
>   *		**-EPERM** if no permission to send the *sig*.
>   *
>   *		**-EAGAIN** if bpf program can try again.
> + *
> + * int bpf_perf_prog_read_branches(struct bpf_perf_event_data *ctx, void *buf, u32 buf_size)
> + * 	Description
> + * 		For en eBPF program attached to a perf event, retrieve the
> + * 		branch records (struct perf_branch_entry) associated to *ctx*
> + * 		and store it in	the buffer pointed by *buf* up to size
> + * 		*buf_size* bytes.

It seems extra bytes in buf will be cleared. The number of bytes
copied is returned so I don't see any reason to clear the extra bytes I would
just let the BPF program do this if they care. But it should be noted in
the description at least.

> + * 	Return
> + *		On success, number of bytes written to *buf*. On error, a
> + *		negative value.
>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -3004,7 +3014,8 @@ union bpf_attr {
>  	FN(probe_read_user_str),	\
>  	FN(probe_read_kernel_str),	\
>  	FN(tcp_send_ack),		\
> -	FN(send_signal_thread),
> +	FN(send_signal_thread),		\
> +	FN(perf_prog_read_branches),
>  
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>   * function eBPF program intends to call
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 19e793aa441a..24c51272a1f7 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1028,6 +1028,35 @@ static const struct bpf_func_proto bpf_perf_prog_read_value_proto = {
>           .arg3_type      = ARG_CONST_SIZE,
>  };
>  
> +BPF_CALL_3(bpf_perf_prog_read_branches, struct bpf_perf_event_data_kern *, ctx,
> +	   void *, buf, u32, size)
> +{
> +	struct perf_branch_stack *br_stack = ctx->data->br_stack;
> +	u32 to_copy = 0, to_clear = size;
> +	int err = -EINVAL;
> +
> +	if (unlikely(!br_stack))
> +		goto clear;
> +
> +	to_copy = min_t(u32, br_stack->nr * sizeof(struct perf_branch_entry), size);
> +	to_clear -= to_copy;
> +
> +	memcpy(buf, br_stack->entries, to_copy);
> +	err = to_copy;
> +clear:
> +	memset(buf + to_copy, 0, to_clear);

Here, why do this at all? If the user cares they can clear the bytes
directly from the BPF program. I suspect its probably going to be
wasted work in most cases. If its needed for some reason provide 
a comment with it.

> +	return err;
> +}

[...]