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

Martin KaFai Lau
2020-04-29T06:35:08Z

On Tue, Apr 28, 2020 at 11:20:30PM -0700, 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.
>
> >
> > > otherwise, we won't know whether the one in seq_ops->show()
> > > is the last or not.
I think "show()" is convoluted with "stop()/eof()". Could "stop()/eof()"
be its own separate (and optional) bpf_prog which only does "stop()/eof()"?

> > > 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.