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

Alexei Starovoitov
2020-04-29T06:30:20Z

On 4/28/20 11:20 PM, Yonghong Song wrote:
>
>
> On 4/28/20 11:08 PM, Andrii Nakryiko wrote:
>> On Tue, Apr 28, 2020 at 10:10 PM Yonghong Song <yhs@fb.com> wrote:
>>>
>>>
>>>
>>> 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,
>>
>> What about the case when there are no elements to iterate to begin
>> with? In that case, we still need to call bpf_prog for (empty)
>> post-aggregation, but we have no valid element... For bpf_map
>> iteration we could have fake empty bpf_map that would be passed, but
>> I'm not sure it's applicable for any time of object (e.g., having a
>> fake task_struct is probably quite a bit more problematic?)...
>
> Oh, yes, thanks for reminding me of this. I put a call to
> bpf_prog in seq_ops->stop() especially to handle no object
> case. In that case, seq_ops->start() will return NULL,
> seq_ops->next() won't be called, and then seq_ops->stop()
> is called. My earlier attempt tries to hook with next()
> and then find it not working in all cases.

wait a sec. seq_ops->stop() is not the end.
With lseek of seq_file it can be called multiple times.
What's the point calling bpf prog with NULL then?