On Fri, Mar 27, 2020 at 5:58 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> 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 getter for the pointer to that internal data store. This can be
> used to modify the data before it is loaded into the kernel, which is
> especially relevant for RODATA, which is frozen on load. This same pointer
> is already exposed to the auto-generated skeletons, so access to it is
> already API; this just adds a way to get at it without pulling in the full
> skeleton infrastructure.
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> ---
> v2:
> - Add per-map getter for data area instead of a global rodata getter for bpf_obj
>
> tools/lib/bpf/libbpf.c | 9 +++++++++
> tools/lib/bpf/libbpf.h | 1 +
> tools/lib/bpf/libbpf.map | 1 +
> 3 files changed, 11 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 085e41f9b68e..a0055f8908fd 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -6756,6 +6756,15 @@ void *bpf_map__priv(const struct bpf_map *map)
> return map ? map->priv : ERR_PTR(-EINVAL);
> }
>
> +void *bpf_map__data_area(const struct bpf_map *map, size_t *size)
I'm not entirely thrilled about "data_area" name. This is entirely for
providing initial value for maps, so maybe something like
bpf_map__init_value() or something along those lines?
Actually, how about a different API altogether:
bpf_map__set_init_value(struct bpf_map *map, void *data, size_t size)?
Application will have to prepare data of correct size, which will be
copied to libbpf's internal storage. It also doesn't expose any of
internal pointer. I don't think extra memcopy is a big deal here.
Thoughts?
> +{
> + if (map->mmaped && map->libbpf_type != LIBBPF_MAP_KCONFIG) {
> + *size = map->def.value_size;
> + return map->mmaped;
> + }
> + return NULL;
> +}
> +
> 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..baef0d2f3205 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -407,6 +407,7 @@ 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 void *bpf_map__data_area(const struct bpf_map *map, 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..258528045a85 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__data_area;
> bpf_program__set_attach_target;
> } LIBBPF_0.0.7;
> --
> 2.26.0
>
