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

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Date:   Wed, 29 Apr 2020 08:34:16 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Martin KaFai Lau <kafai@...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 11:44 PM, Yonghong Song wrote:
> 
> 
> On 4/28/20 11:40 PM, Andrii Nakryiko wrote:
>> On Tue, Apr 28, 2020 at 11:30 PM Alexei Starovoitov <ast@...com> wrote:
>>>
>>> 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@...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.
> 
> Yes, I have taken care of this. when the object is NULL,
> bpf program will be called. When the object is NULL again,
> it won't be called. The private data remembers it has
> been called with NULL.

Even without lseek stop() will be called multiple times.
If I read seq_file.c correctly it will be called before
every copy_to_user(). Which means that for a lot of text
(or if read() is done with small buffer) there will be
plenty of start,show,show,stop sequences.