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

Andrii Nakryiko
2020-04-29T06:40:43Z

On Tue, Apr 28, 2020 at 11:30 PM Alexei Starovoitov <ast@fb.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@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?)...
> >
> > 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.

We don't allow seeking on seq_file created from bpf_iter_link, so
there should be no lseek'ing?

> What's the point calling bpf prog with NULL then?

To know that iteration has ended, even if there were 0 elements to
iterate. 0, 1 or N doesn't matter, we might still need to do some
final actions (e.g., submit or print summary).