Re: [PATCH v2 4/5] bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter

xiakaixu
2015-07-23T02:13:16Z

于 2015/7/23 9:14, Alexei Starovoitov 写道:
> On 7/22/15 1:09 AM, Kaixu Xia wrote:
>> According to the perf_event_map_fd and key, the function
>> bpf_perf_event_read() can convert the corresponding map
>> value to the pointer to struct perf_event and return the
>> Hardware PMU counter value.
>>
>> The key can't be passed to bpf_perf_event_read() directly
>> because the function argument constraint is lacked.
>
> I don't understand above sentence.
>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 69a1f6b..e3bb181 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -250,6 +250,8 @@ enum bpf_func_id {
>> * Return: 0 on success
>> */
>> BPF_FUNC_get_current_comm,
>> +
>> + BPF_FUNC_perf_event_read, /* u64 bpf_perf_event_read(&map, &key) */
>
> no need for extra empty line.
>
>> +
>> +static u64 bpf_perf_event_read(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
>> +{
>> + struct bpf_map *map = (struct bpf_map *) (unsigned long) r1;
>> + void *key = (void *) (unsigned long) r2;
>> + struct perf_event *event;
>> + void *ptr;
>> +
>> + if (map->map_type != BPF_MAP_TYPE_PERF_EVENT_ARRAY)
>> + return -EINVAL;
>
> please check this statically in verifier instead of in run-time.
>
>> +
>> + rcu_read_lock();
>
> unnecessary.
>
>> + ptr = map->ops->map_lookup_elem(map, key);
>> + rcu_read_unlock();
>> + if (!ptr || !(*(unsigned long *)ptr))
>> + return -EBADF;
>
> all these casts can be removed. First cast of 'r1' into
> perf_event_array will be enough.

So you mean like this?

u64 bpf_perf_event_read(u64 r1, u64 index,...)
{
struct bpf_perf_event_array *array = (void *) (long) r1;
struct perf_event *event;
...
event = array->events[index];
...
}
>
>> +const struct bpf_func_proto bpf_perf_event_read_proto = {
>> + .func = bpf_perf_event_read,
>> + .gpl_only = false,
>> + .ret_type = RET_INTEGER,
>> + .arg1_type = ARG_CONST_MAP_PTR,
>> + .arg2_type = ARG_PTR_TO_MAP_KEY,
>
> make it arg2_type = ARG_ANYTHING then you'll just index
> into array the way prog_array does and similar to bpf_tail_call.

ARG_ANYTHING means any (initialized) argument is ok, but we here
really want is map key. So I'm not sure ARG_ANYTHING is suitable.
You know ARG_ANYTHING is not checked enough in verifier.

Thanks.
>
>
> .
>