libbpf: Add capability for resizing datasec maps

This patch updates bpf_map__set_value_size() so that if the given map is
memory mapped, it will attempt to resize the mapped region. Initial
contents of the mapped region are preserved. BTF is not required, but
after the mapping is resized an attempt is made to adjust the associated
BTF information if the following criteria is met:
 - BTF info is present
 - the map is a datasec
 - the final variable in the datasec is an array

... the resulting BTF info will be updated so that the final array
variable is associated with a new BTF array type sized to cover the
requested size.

Note that the initial resizing of the memory mapped region can succeed
while the subsequent BTF adjustment can fail. In this case, BTF info is
dropped from the map by clearing the key and value type.

Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20230524004537.18614-2-inwardvessel@gmail.com

1 files changed, 16 insertions, 2 deletions

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 0b7362397ea3..754da73c643b 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -869,8 +869,22 @@ LIBBPF_API int bpf_map__set_numa_node(struct bpf_map *map, __u32 numa_node);
 /* get/set map key size */
 LIBBPF_API __u32 bpf_map__key_size(const struct bpf_map *map);
 LIBBPF_API int bpf_map__set_key_size(struct bpf_map *map, __u32 size);
-/* get/set map value size */
+/* get map value size */
 LIBBPF_API __u32 bpf_map__value_size(const struct bpf_map *map);
+/**
+ * @brief **bpf_map__set_value_size()** sets map value size.
+ * @param map the BPF map instance
+ * @return 0, on success; negative error, otherwise
+ *
+ * There is a special case for maps with associated memory-mapped regions, like
+ * the global data section maps (bss, data, rodata). When this function is used
+ * on such a map, the mapped region is resized. Afterward, an attempt is made to
+ * adjust the corresponding BTF info. This attempt is best-effort and can only
+ * succeed if the last variable of the data section map is an array. The array
+ * BTF type is replaced by a new BTF array type with a different length.
+ * Any previously existing pointers returned from bpf_map__initial_value() or
+ * corresponding data section skeleton pointer must be reinitialized.
+ */
 LIBBPF_API int bpf_map__set_value_size(struct bpf_map *map, __u32 size);
 /* get map key/value BTF type IDs */
 LIBBPF_API __u32 bpf_map__btf_key_type_id(const struct bpf_map *map);
@@ -884,7 +898,7 @@ LIBBPF_API int bpf_map__set_map_extra(struct bpf_map *map, __u64 map_extra);
 
 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 void *bpf_map__initial_value(struct bpf_map *map, size_t *psize);
 
 /**
  * @brief **bpf_map__is_internal()** tells the caller whether or not the