Re: [PATCH v3 1/2] libbpf: Add setter for initial value for internal maps

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

On Sat, Mar 28, 2020 at 11:29 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> For internal maps (most notably the maps backing global variables), libbpf
> uses an internal mmaped area to store the data after opening the object.
> This data is subsequently copied into the kernel map when the object is
> loaded.
>
> This adds a function to set a new value for that data, which can be used to
> before it is loaded into the kernel. This is especially relevant for RODATA
> maps, since those are frozen on load.
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
> v3:
>   - Add a setter for the initial value instead of a getter for the pointer to it
>   - Add selftest
> v2:
>   - Add per-map getter for data area instead of a global rodata getter for bpf_obj
>
>  tools/lib/bpf/libbpf.c   | 11 +++++++++++
>  tools/lib/bpf/libbpf.h   |  2 ++
>  tools/lib/bpf/libbpf.map |  1 +
>  3 files changed, 14 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 085e41f9b68e..f9953a8ffcfa 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -6756,6 +6756,17 @@ void *bpf_map__priv(const struct bpf_map *map)
>         return map ? map->priv : ERR_PTR(-EINVAL);
>  }
>
> +int bpf_map__set_initial_value(struct bpf_map *map,
> +                              void *data, size_t size)

nit: const void *


> +{
> +       if (!map->mmaped || map->libbpf_type == LIBBPF_MAP_KCONFIG ||
> +           size != map->def.value_size)
> +               return -EINVAL;
> +

How about also checking that bpf_map wasn't yet created? Checking
map->fd >= 0 should be enough.

> +       memcpy(map->mmaped, data, size);
> +       return 0;
> +}
> +
>  bool bpf_map__is_offload_neutral(const struct bpf_map *map)
>  {
>         return map->def.type == BPF_MAP_TYPE_PERF_EVENT_ARRAY;
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index d38d7a629417..ee30ed487221 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -407,6 +407,8 @@ typedef void (*bpf_map_clear_priv_t)(struct bpf_map *, void *);
>  LIBBPF_API int bpf_map__set_priv(struct bpf_map *map, void *priv,
>                                  bpf_map_clear_priv_t clear_priv);
>  LIBBPF_API void *bpf_map__priv(const struct bpf_map *map);
> +LIBBPF_API int bpf_map__set_initial_value(struct bpf_map *map,
> +                                         void *data, size_t size);
>  LIBBPF_API int bpf_map__reuse_fd(struct bpf_map *map, int fd);
>  LIBBPF_API int bpf_map__resize(struct bpf_map *map, __u32 max_entries);
>  LIBBPF_API bool bpf_map__is_offload_neutral(const struct bpf_map *map);
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 5129283c0284..f46873b9fe5e 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -243,5 +243,6 @@ LIBBPF_0.0.8 {
>                 bpf_link__pin;
>                 bpf_link__pin_path;
>                 bpf_link__unpin;
> +               bpf_map__set_initial_value;
>                 bpf_program__set_attach_target;
>  } LIBBPF_0.0.7;
> --
> 2.26.0
>