Re: [PATCH bpf-next v3 04/11] bpf: add bpf_for_each_map_elem() helper

[Yonghong Song <yhs@fb.com>]

On 2/25/21 6:27 PM, Cong Wang wrote:
> On Wed, Feb 24, 2021 at 11:33 PM Yonghong Song <yhs@fb.com> wrote:
>> diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
>> index a0d9eade9c80..931870f9cf56 100644
>> --- a/kernel/bpf/bpf_iter.c
>> +++ b/kernel/bpf/bpf_iter.c
>> @@ -675,3 +675,19 @@ int bpf_iter_run_prog(struct bpf_prog *prog, void *ctx)
>>           */
>>          return ret == 0 ? 0 : -EAGAIN;
>>   }
>> +
>> +BPF_CALL_4(bpf_for_each_map_elem, struct bpf_map *, map, void *, callback_fn,
>> +          void *, callback_ctx, u64, flags)
>> +{
>> +       return map->ops->map_for_each_callback(map, callback_fn, callback_ctx, flags);
>> +}
> 
> A quick question: is ->map_for_each_callback() now mandatory for
> every map? I do not see you check for NULL here. Or you check map
> types somewhere I miss?

At the call site of bpf_for_each_map_elem(), verifier knows the map and 
ensure check map->ops->map_for_each_callback() is not null. Otherwise,
it will reject the program. So we are fine here.

> 
> At least some maps do not support iteration, for example, queue/stack.
> If you can document supported maps in bpf_for_each_map_elem() doc,
> it would be very nice.

Yes, will add more info in uapi/linux/bpf.h. Andrii suggested the same.

> 
> Thanks for working on this!

You are welcome.