Re: [PATCH bpf-next 2/8] bpf: inline map creation logic in map_create() function

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

On Wed, Apr 12, 2023 at 10:53 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Tue, Apr 11, 2023 at 09:32:54PM -0700, Andrii Nakryiko wrote:
> > Keep all the relevant generic sanity checks, permission checks, and
> > creation and initialization logic in one linear piece of code. Currently
> > helper function that handles memory allocation and partial
> > initialization is split apart and is about 1000 lines higher in the
> > file, hurting readability.
>
> At first glance, this seems like a step in the wrong direction: having a
> single-purpose function pulled out of a larger one seems like a good
> thing for stuff like unit testing, etc. Unless there's a reason later in
> the series for this inlining (which should be called out in the
> changelog here), I would say if it is only readability, just move the
> function down 1000 lines but leave it a separate function.

Oh, I should probably clarify this in the commit message. This
function is not that single-function, really. It performs some sanity
checking and then allocates and (partially) initializes the BPF map
itself. By "inlining" it, it makes it possible to perform these sanity
checks first, then do capabilities/security checks, and only if both
pass, allocate and initialize the map. Next patch inserts
(centralizes) all the spread out capabilities checks from per-map
custom callbacks into the same function, right before performing map
allocation and initialization, but after validation of parameters.

So yeah, I do take advantage of this in the next patch, because LSM
hook gets validated bpf_uattr. I'll call this out more clearly. It's
definitely not just moving code around for no good reason.


>
> -Kees
>
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > ---
> >  kernel/bpf/syscall.c | 54 ++++++++++++++++++--------------------------
> >  1 file changed, 22 insertions(+), 32 deletions(-)
> >
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index c1d268025985..a090737f98ea 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -108,37 +108,6 @@ const struct bpf_map_ops bpf_map_offload_ops = {
> >       .map_mem_usage = bpf_map_offload_map_mem_usage,
> >  };
> >
> > -static struct bpf_map *find_and_alloc_map(union bpf_attr *attr)
> > -{
> > -     const struct bpf_map_ops *ops;
> > -     u32 type = attr->map_type;
> > -     struct bpf_map *map;
> > -     int err;
> > -
> > -     if (type >= ARRAY_SIZE(bpf_map_types))
> > -             return ERR_PTR(-EINVAL);
> > -     type = array_index_nospec(type, ARRAY_SIZE(bpf_map_types));
> > -     ops = bpf_map_types[type];
> > -     if (!ops)
> > -             return ERR_PTR(-EINVAL);
> > -
> > -     if (ops->map_alloc_check) {
> > -             err = ops->map_alloc_check(attr);
> > -             if (err)
> > -                     return ERR_PTR(err);
> > -     }
> > -     if (attr->map_ifindex)
> > -             ops = &bpf_map_offload_ops;
> > -     if (!ops->map_mem_usage)
> > -             return ERR_PTR(-EINVAL);
> > -     map = ops->map_alloc(attr);
> > -     if (IS_ERR(map))
> > -             return map;
> > -     map->ops = ops;
> > -     map->map_type = type;
> > -     return map;
> > -}
> > -
> >  static void bpf_map_write_active_inc(struct bpf_map *map)
> >  {
> >       atomic64_inc(&map->writecnt);
> > @@ -1124,7 +1093,9 @@ static int map_check_btf(struct bpf_map *map, const struct btf *btf,
> >  /* called via syscall */
> >  static int map_create(union bpf_attr *attr)
> >  {
> > +     const struct bpf_map_ops *ops;
> >       int numa_node = bpf_map_attr_numa_node(attr);
> > +     u32 map_type = attr->map_type;
> >       struct btf_field_offs *foffs;
> >       struct bpf_map *map;
> >       int f_flags;
> > @@ -1167,9 +1138,28 @@ static int map_create(union bpf_attr *attr)
> >               return -EINVAL;
> >
> >       /* find map type and init map: hashtable vs rbtree vs bloom vs ... */
> > -     map = find_and_alloc_map(attr);
> > +     map_type = attr->map_type;
> > +     if (map_type >= ARRAY_SIZE(bpf_map_types))
> > +             return -EINVAL;
> > +     map_type = array_index_nospec(map_type, ARRAY_SIZE(bpf_map_types));
> > +     ops = bpf_map_types[map_type];
> > +     if (!ops)
> > +             return -EINVAL;
> > +
> > +     if (ops->map_alloc_check) {
> > +             err = ops->map_alloc_check(attr);
> > +             if (err)
> > +                     return err;
> > +     }
> > +     if (attr->map_ifindex)
> > +             ops = &bpf_map_offload_ops;
> > +     if (!ops->map_mem_usage)
> > +             return -EINVAL;
> > +     map = ops->map_alloc(attr);
> >       if (IS_ERR(map))
> >               return PTR_ERR(map);
> > +     map->ops = ops;
> > +     map->map_type = map_type;
> >
> >       err = bpf_obj_name_cpy(map->name, attr->map_name,
> >                              sizeof(attr->map_name));
> > --
> > 2.34.1
> >
>
> --
> Kees Cook