Re: [PATCH bpf-next] bpf: add support for other map types to bpf_map_lookup_and_delete_elem

[Luka Oreskovic <luka.oreskovic@sartura.hr>]

On Fri, Sep 18, 2020 at 1:21 AM Song Liu <song@kernel.org> wrote:
>
> On Thu, Sep 17, 2020 at 7:16 AM Luka Oreskovic
> <luka.oreskovic@sartura.hr> wrote:
> >
> [...]
>
> > +++ b/kernel/bpf/syscall.c
> > @@ -1475,6 +1475,9 @@ static int map_lookup_and_delete_elem(union bpf_attr *attr)
> >         if (CHECK_ATTR(BPF_MAP_LOOKUP_AND_DELETE_ELEM))
> >                 return -EINVAL;
> >
> > +       if (attr->flags & ~BPF_F_LOCK)
> > +               return -EINVAL;
> > +
>
> Please explain (in comments for commit log) the use of BPF_F_LOCK in
> the commit log,
> as it is new for BPF_MAP_LOOKUP_AND_DELETE_ELEM.
>
> >         f = fdget(ufd);
> >         map = __bpf_map_get(f);
> >         if (IS_ERR(map))
> > @@ -1485,13 +1488,19 @@ static int map_lookup_and_delete_elem(union bpf_attr *attr)
> >                 goto err_put;
> >         }
> >
> > +       if ((attr->flags & BPF_F_LOCK) &&
> > +           !map_value_has_spin_lock(map)) {
> > +               err = -EINVAL;
> > +               goto err_put;
> > +       }
> > +
> >         key = __bpf_copy_key(ukey, map->key_size);
> >         if (IS_ERR(key)) {
> >                 err = PTR_ERR(key);
> >                 goto err_put;
> >         }
> >
> > -       value_size = map->value_size;
> > +       value_size = bpf_map_value_size(map);
> >
> >         err = -ENOMEM;
> >         value = kmalloc(value_size, GFP_USER | __GFP_NOWARN);
> > @@ -1502,7 +1511,24 @@ static int map_lookup_and_delete_elem(union bpf_attr *attr)
> >             map->map_type == BPF_MAP_TYPE_STACK) {
> >                 err = map->ops->map_pop_elem(map, value);
> >         } else {
> > -               err = -ENOTSUPP;
> > +               err = bpf_map_copy_value(map, key, value, attr->flags);
> > +               if (err)
> > +                       goto free_value;
>
> IIUC, we cannot guarantee the value returned is the same as the value we
> deleted. If this is true, I guess this may confuse the user with some
> concurrency
> bug.
>
> Thanks,
> Song
>
> [...]

Thank you very much for your review. This is my first time contributing
to the linux community, so I am very grateful for any input.

For the first point, you are correct, the commit message should
have been more detailed. As for the second point, I see the problem,
but I'm not sure how to resolve it. Maybe moving the
bpf_disable_instrumentation call could work, but I'm not sure if
that could create different problems. I'll try to find and acceptable
solution and resubmit the patch.

Best wishes,
Luka Oreskovic