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

[Yonghong Song <yhs@fb.com>]

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.

> 
>> 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.