Re: [PATCH bpf-next] libbpf: export bpf_object__reuse_map() to libbpf api

[Hangbin Liu <liuhangbin@gmail.com>]

On Mon, Sep 28, 2020 at 08:30:42PM -0700, Andrii Nakryiko wrote:
> > @@ -431,6 +431,7 @@ bpf_map__prev(const struct bpf_map *map, const struct bpf_object *obj);
> >  /* get/set map FD */
> >  LIBBPF_API int bpf_map__fd(const struct bpf_map *map);
> >  LIBBPF_API int bpf_map__reuse_fd(struct bpf_map *map, int fd);
> > +LIBBPF_API int bpf_object__reuse_map(struct bpf_map *map);
> 
> It's internal function, which doesn't check that map->pin_path is set,

How about add a path check in bpf_object__reuse_map()?

And off course users who use it should call bpf_map__set_pin_path() first.

> for one thing. It shouldn't be exposed. libbpf exposes
> bpf_map__set_pin_path() to set pin_path for any map, and then during
> load time libbpf with "reuse map", if pin_path is not NULL. Doesn't
> that work for you?

Long story...

When trying to add iproute2 libbpf support that I'm working on. We need to
create iproute2 legacy map-in-map manually before libbpf load objects, because
libbpf only support BTF type map-in-map(unless I missed something.).

After creating legacy map-in-map and reuse the fd, if the map has legacy
pining defined, only set the pin path would not enough as libbpf will skip
pinning map if map->fd > 0 in bpf_object__create_maps(). See the following
code bellow.

bpf_map__set_pin_path()
bpf_create_map_in_map()    <- create inner or outer map
bpf_map__reuse_fd(map, inner/outer_fd)
bpf_object__load(obj)
  - bpf_object__load_xattr()
    - bpf_object__create_maps()
      - if (map->fd >= 0)
          continue      <- this will skip pinning map

So when handle legacy map-in-map + pin map, we need to create the map
and pin maps manually at the same time. The code would looks like
(err handler skipped).

map_fd = bpf_obj_get(pathname);
if (map_fd > 0) {
	bpf_map__set_pin_path(map, pathname);
	return bpf_object__reuse_map(map);   <- here we need the reuse_map
}
bpf_create_map_in_map()
bpf_map__reuse_fd(map, map_fd);
bpf_map__pin(map, pathname);

So I think this function is needed, what do you think?

Thanks
Hangbin