Subject: Re: [PATCH V3 2/2] bpf: control all the perf events stored in PERF_EVENT_ARRAY maps

From: Alexei Starovoitov <[email-masked]>
Date: Fri, 16 Oct 2015 15:12:20 -0700

On 10/16/15 12:42 AM, Kaixu Xia wrote:
> This patch implements the function that controlling all the perf
> events stored in PERF_EVENT_ARRAY maps by setting the parameter
> 'index' to maps max_entries.
>
> Signed-off-by: Kaixu Xia <[email-masked]>
> ---
>   kernel/trace/bpf_trace.c | 20 ++++++++++++++++++--
>   1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 3175600..4b385863 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -229,13 +229,30 @@ static u64 bpf_perf_event_dump_control(u64 r1, u64 index, u64 flag, u64 r4, u64
>   	struct bpf_map *map = (struct bpf_map *) (unsigned long) r1;
>   	struct bpf_array *array = container_of(map, struct bpf_array, map);
>   	struct perf_event *event;
> +	int i;
>
> -	if (unlikely(index >= array->map.max_entries))
> +	if (unlikely(index > array->map.max_entries))
>   		return -E2BIG;
>
>   	if (flag & BIT_FLAG_CHECK)
>   		return -EINVAL;
>
> +	if (index == array->map.max_entries) {

I don't like in-band signaling like this, since it's easy to make
mistake on the bpf program side.
Please use 2nd bit of 'flags' for that instead.
Also squash this patch into 1st.