Re: [PATCH bpf-next v1 03/19] bpf: add bpf_map iterator

Yonghong Song
2020-04-29T05:09:32Z

On 4/28/20 7:44 PM, Alexei Starovoitov wrote:
> On 4/28/20 6:15 PM, Yonghong Song wrote:
>>
>>
>> On 4/28/20 5:48 PM, Alexei Starovoitov wrote:
>>> On 4/28/20 5:37 PM, Martin KaFai Lau wrote:
>>>>> +    prog = bpf_iter_get_prog(seq, sizeof(struct
>>>>> bpf_iter_seq_map_info),
>>>>> +                 &meta.session_id, &meta.seq_num,
>>>>> +                 v == (void *)0);
>>>>  From looking at seq_file.c, when will show() be called with "v ==
>>>> NULL"?
>>>>
>>>
>>> that v == NULL here and the whole verifier change just to allow NULL...
>>> may be use seq_num as an indicator of the last elem instead?
>>> Like seq_num with upper bit set to indicate that it's last?
>>
>> We could. But then verifier won't have an easy way to verify that.
>> For example, the above is expected:
>>
>>       int prog(struct bpf_map *map, u64 seq_num) {
>>          if (seq_num >> 63)
>>            return 0;
>>          ... map->id ...
>>          ... map->user_cnt ...
>>       }
>>
>> But if user writes
>>
>>       int prog(struct bpf_map *map, u64 seq_num) {
>>           ... map->id ...
>>           ... map->user_cnt ...
>>       }
>>
>> verifier won't be easy to conclude inproper map pointer tracing
>> here and in the above map->id, map->user_cnt will cause
>> exceptions and they will silently get value 0.
>
> I mean always pass valid object pointer into the prog.
> In above case 'map' will always be valid.
> Consider prog that iterating all map elements.
> It's weird that the prog would always need to do
> if (map == 0)
>   goto out;
> even if it doesn't care about finding last.
> All progs would have to have such extra 'if'.
> If we always pass valid object than there is no need
> for such extra checks inside the prog.
> First and last element can be indicated via seq_num
> or via another flag or via helper call like is_this_last_elem()
> or something.

Okay, I see what you mean now. Basically this means
seq_ops->next() should try to get/maintain next two elements,
otherwise, we won't know whether the one in seq_ops->show()
is the last or not. We could do it in newly implemented
iterator bpf_map/task/task_file. Let me check how I could
make existing seq_ops (ipv6_route/netlink) works with
minimum changes.