Re: [PATCH bpf-next 1/2] bpf: track contents of read-only maps as scalars

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Oct 8, 2019 at 2:29 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On Tue, Oct 08, 2019 at 12:45:47PM -0700, Andrii Nakryiko wrote:
> > Maps that are read-only both from BPF program side and user space side
> > have their contents constant, so verifier can track referenced values
> > precisely and use that knowledge for dead code elimination, branch
> > pruning, etc. This patch teaches BPF verifier how to do this.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@fb.com>
> > ---
> >  kernel/bpf/verifier.c | 58 +++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 56 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index ffc3e53f5300..1e4e4bd64ca5 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -2739,6 +2739,42 @@ static void coerce_reg_to_size(struct bpf_reg_state *reg, int size)
> >       reg->smax_value = reg->umax_value;
> >  }
> >
> > +static bool bpf_map_is_rdonly(const struct bpf_map *map)
> > +{
> > +     return (map->map_flags & BPF_F_RDONLY_PROG) &&
> > +            ((map->map_flags & BPF_F_RDONLY) || map->frozen);
>
> This is definitely buggy. Testing for 'map->map_flags & BPF_F_RDONLY'
> to assume it's RO from user space side is not correct as it's just
> related to the current fd, but not the map itself. So the second part
> definitely /must/ only be: && map->frozen

Yep, you are right, map->frozen and BPF_F_RDONLY_PROG only.

>
> Thanks,
> Daniel