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

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

On Mon, Sep 28, 2020 at 8:20 PM Hangbin Liu <liuhangbin@gmail.com> wrote:
>
> Besides bpf_map__reuse_fd(), which could let us reuse existing map fd.
> bpf_object__reuse_map() could let us reuse existing pinned maps, which
> is helpful.
>
> This functions could also be used when we add iproute2 libbpf support,
> so we don't need to re-use or re-implement new functions like
> bpf_obj_get()/bpf_map_selfcheck_pinned() in iproute2.
>
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
> ---
>  tools/lib/bpf/libbpf.c | 3 +--
>  tools/lib/bpf/libbpf.h | 1 +
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 32dc444224d8..e835d7a3437f 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4033,8 +4033,7 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
>                 map_info.map_flags == map->def.map_flags);
>  }
>
> -static int
> -bpf_object__reuse_map(struct bpf_map *map)
> +int bpf_object__reuse_map(struct bpf_map *map)
>  {
>         char *cp, errmsg[STRERR_BUFSIZE];
>         int err, pin_fd;
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index a750f67a23f6..4b9e615eb393 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -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,
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?

>  /* get map definition */
>  LIBBPF_API const struct bpf_map_def *bpf_map__def(const struct bpf_map *map);
>  /* get map name */
> --
> 2.25.4
>