bpf: Return hashes of maps in BPF_OBJ_GET_INFO_BY_FD

Currently only array maps are supported, but the implementation can be
extended for other maps and objects. The hash is memoized only for
exclusive and frozen maps as their content is stable until the exclusive
program modifies the map.

This is required for BPF signing, enabling a trusted loader program to
verify a map's integrity. The loader retrieves
the map's runtime hash from the kernel and compares it against an
expected hash computed at build time.

Signed-off-by: KP Singh <kpsingh@kernel.org>
Link: https://lore.kernel.org/r/20250914215141.15144-7-kpsingh@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/include/uapi/linux/bpf.h

@@ -6672,6 +6672,8 @@ struct bpf_map_info {
 	__u32 btf_value_type_id;
 	__u32 btf_vmlinux_id;
 	__u64 map_extra;
+	__aligned_u64 hash;
+	__u32 hash_size;
 } __attribute__((aligned(8)));
 
 struct bpf_btf_info {