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Date:   Tue, 28 Apr 2020 19:44:37 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Yonghong Song <yhs@...com>, Martin KaFai Lau <kafai@...com>
CC:     Andrii Nakryiko <andriin@...com>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net>,
        <kernel-team@...com>
Subject: Re: [PATCH bpf-next v1 03/19] bpf: add bpf_map iterator

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.

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