libbpf: Introduce bpf_map__initial_value().

Introduce bpf_map__initial_value() to read initial contents
of mmaped data/rodata/bss maps.
Note that bpf_map__set_initial_value() doesn't allow modifying
kconfig map while bpf_map__initial_value() allows reading
its values.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210514003623.28033-17-alexei.starovoitov@gmail.com

src/libbpf.c

@@ -9758,6 +9758,14 @@ int bpf_map__set_initial_value(struct bpf_map *map,
 	return 0;
 }
 
+const void *bpf_map__initial_value(struct bpf_map *map, size_t *psize)
+{
+	if (!map->mmaped)
+		return NULL;
+	*psize = map->def.value_size;
+	return map->mmaped;
+}
+
 bool bpf_map__is_offload_neutral(const struct bpf_map *map)
 {
 	return map->def.type == BPF_MAP_TYPE_PERF_EVENT_ARRAY;

src/libbpf.h

@@ -471,6 +471,7 @@ LIBBPF_API int bpf_map__set_priv(struct bpf_map *map, void *priv,
 LIBBPF_API void *bpf_map__priv(const struct bpf_map *map);
 LIBBPF_API int bpf_map__set_initial_value(struct bpf_map *map,
 					  const void *data, size_t size);
+LIBBPF_API const void *bpf_map__initial_value(struct bpf_map *map, size_t *psize);
 LIBBPF_API bool bpf_map__is_offload_neutral(const struct bpf_map *map);
 LIBBPF_API bool bpf_map__is_internal(const struct bpf_map *map);
 LIBBPF_API int bpf_map__set_pin_path(struct bpf_map *map, const char *path);

src/libbpf.map

@@ -359,6 +359,7 @@ LIBBPF_0.4.0 {
 		bpf_linker__finalize;
 		bpf_linker__free;
 		bpf_linker__new;
+		bpf_map__initial_value;
 		bpf_map__inner_map;
 		bpf_object__gen_loader;
 		bpf_object__set_kversion;