libbpf: Skip modules BTF loading when CAP_SYS_ADMIN is missing

If during CO-RE relocations libbpf is not able to find the target type
in the running kernel BTF, it searches for it in modules' BTF.
The downside of this approach is that loading modules' BTF requires
CAP_SYS_ADMIN and this prevents BPF applications from running with more
granular capabilities (e.g. CAP_BPF) when they don't need to search
types into modules' BTF.

This patch skips by default modules' BTF loading phase when
CAP_SYS_ADMIN is missing.
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Co-developed-by: Federico Di Pierro <nierro92@gmail.com>
Signed-off-by: Federico Di Pierro <nierro92@gmail.com>
Signed-off-by: Andrea Terzolo <andreaterzolo3@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/CAGQdkDvYU_e=_NX+6DRkL_-TeH3p+QtsdZwHkmH0w3Fuzw0C4w@mail.gmail.com
Link: https://lore.kernel.org/bpf/20230626093614.21270-1-andreaterzolo3@gmail.com

tools/lib/bpf/libbpf.c

@@ -5471,6 +5471,10 @@ static int load_module_btfs(struct bpf_object *obj)
 		err = bpf_btf_get_next_id(id, &id);
 		if (err && errno == ENOENT)
 			return 0;
+		if (err && errno == EPERM) {
+			pr_debug("skipping module BTFs loading, missing privileges\n");
+			return 0;
+		}
 		if (err) {
 			err = -errno;
 			pr_warn("failed to iterate BTF objects: %d\n", err);