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

Andrii Nakryiko
2020-04-29T06:08:39Z

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?)...

> 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.