libbpf: support "module: Function" syntax for tracing programs

In some situations, it is useful to explicitly specify a kernel module
to search for a tracing program target (e.g. when a function of the same
name exists in multiple modules or in vmlinux).

This patch enables that by allowing the "module:function" syntax for the
find_kernel_btf_id function. Thanks to this, the syntax can be used both
from a SEC macro (i.e. `SEC(fentry/module:function)`) and via the
bpf_program__set_attach_target API call.
Signed-off-by: Viktor Malik <vmalik@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/9085a8cb9a552de98e554deb22ff7e977d025440.1714469650.git.vmalik@redhat.com

tools/lib/bpf/libbpf.c

@@ -9862,16 +9862,28 @@ static int find_kernel_btf_id(struct bpf_object *obj, const char *attach_name,
 			      enum bpf_attach_type attach_type,
 			      int *btf_obj_fd, int *btf_type_id)
 {
-	int ret, i;
+	int ret, i, mod_len;
+	const char *fn_name, *mod_name = NULL;
 
-	ret = find_attach_btf_id(obj->btf_vmlinux, attach_name, attach_type);
-	if (ret > 0) {
-		*btf_obj_fd = 0; /* vmlinux BTF */
-		*btf_type_id = ret;
-		return 0;
+	fn_name = strchr(attach_name, ':');
+	if (fn_name) {
+		mod_name = attach_name;
+		mod_len = fn_name - mod_name;
+		fn_name++;
+	}
+
+	if (!mod_name || strncmp(mod_name, "vmlinux", mod_len) == 0) {
+		ret = find_attach_btf_id(obj->btf_vmlinux,
+					 mod_name ? fn_name : attach_name,
+					 attach_type);
+		if (ret > 0) {
+			*btf_obj_fd = 0; /* vmlinux BTF */
+			*btf_type_id = ret;
+			return 0;
+		}
+		if (ret != -ENOENT)
+			return ret;
 	}
-	if (ret != -ENOENT)
-		return ret;
 
 	ret = load_module_btfs(obj);
 	if (ret)
@@ -9880,7 +9892,12 @@ static int find_kernel_btf_id(struct bpf_object *obj, const char *attach_name,
 	for (i = 0; i < obj->btf_module_cnt; i++) {
 		const struct module_btf *mod = &obj->btf_modules[i];
 
-		ret = find_attach_btf_id(mod->btf, attach_name, attach_type);
+		if (mod_name && strncmp(mod->name, mod_name, mod_len) != 0)
+			continue;
+
+		ret = find_attach_btf_id(mod->btf,
+					 mod_name ? fn_name : attach_name,
+					 attach_type);
 		if (ret > 0) {
 			*btf_obj_fd = mod->fd;
 			*btf_type_id = ret;