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Date:   Tue, 28 Apr 2020 23:51:33 -0700
From:   Yonghong Song <yhs@...com>
To:     Martin KaFai Lau <kafai@...com>
CC:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next v1 03/19] bpf: add bpf_map iterator



On 4/28/20 11:34 PM, Martin KaFai Lau wrote:
> On Tue, Apr 28, 2020 at 11:20:30PM -0700, 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@...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.
> I think "show()" is convoluted with "stop()/eof()".  Could "stop()/eof()"
> be its own separate (and optional) bpf_prog which only does "stop()/eof()"?

I thought this before. But user need to write a program instead of
a simple "if" condition in the main program...

> 
>>>> 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.

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