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

Yonghong Song
2020-04-29T01:16:21Z

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 do have another potential use case for this ptr_to_btf_id_or_null,
e.g., for tcp6, instead of pointer casting, I could have bpf_prog
like
int prog(..., struct tcp6_sock *tcp_sk,
struct timewait_sock *tw_sk, struct request_sock *req_sk) {
if (tcp_sk) { /* dump tcp_sk ... */ }
if (tw_sk) { /* dump tw_sk ... */ }
if (req_sk) { /* dump req_sk ... */ }
}
The kernel infrastructure will ensure at any time only one
of tcp_sk/tw_sk/req_sk is valid and the other two is NULL.