[PATCH 1/2] tools/lib/bpf/libbpf: Prioritize module kfuncs over vmlinux kfuncs

[Song Chen <chensong_2000@126.com>]

Change the kfunc resolution order in find_ksym_btf_id() to search
module BTFs before vmlinux BTF. This allows kernel modules to override
vmlinux kfuncs with the same name, enabling a form of live-patching
for kfuncs.

Previously, vmlinux kfuncs were always preferred, making it impossible
for modules to provide enhanced or fixed versions of existing kfuncs.
With this change, modules can now override kernel kfuncs, while
programs that don't use module BTFs remain unaffected.

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Song Chen <chensong_2000@126.com>
---
 tools/lib/bpf/libbpf.c | 34 ++++++++++++++++++----------------
 1 file changed, 18 insertions(+), 16 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 0be7017800fe..9c308930bddb 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8538,29 +8538,31 @@ static int find_ksym_btf_id(struct bpf_object *obj, const char *ksym_name,
 {
 	struct module_btf *mod_btf;
 	struct btf *btf;
-	int i, id, err;
+	int i, id = 0, err;
 
-	btf = obj->btf_vmlinux;
 	mod_btf = NULL;
-	id = btf__find_by_name_kind(btf, ksym_name, kind);
 
-	if (id == -ENOENT) {
-		err = load_module_btfs(obj);
-		if (err)
-			return err;
+	err = load_module_btfs(obj);
+	if (err)
+		goto search_vmlinux;
 
-		for (i = 0; i < obj->btf_module_cnt; i++) {
-			/* we assume module_btf's BTF FD is always >0 */
-			mod_btf = &obj->btf_modules[i];
-			btf = mod_btf->btf;
-			id = btf__find_by_name_kind_own(btf, ksym_name, kind);
-			if (id != -ENOENT)
-				break;
-		}
+	for (i = 0; i < obj->btf_module_cnt; i++) {
+		/* we assume module_btf's BTF FD is always >0 */
+		mod_btf = &obj->btf_modules[i];
+		btf = mod_btf->btf;
+		id = btf__find_by_name_kind_own(btf, ksym_name, kind);
+		if (id != -ENOENT)
+			goto found;
 	}
-	if (id <= 0)
+
+search_vmlinux:
+	btf = obj->btf_vmlinux;
+	mod_btf = NULL;
+	id = btf__find_by_name_kind(btf, ksym_name, kind);
+	if (id == -ENOENT)
 		return -ESRCH;
 
+found:
 	*res_btf = btf;
 	*res_mod_btf = mod_btf;
 	return id;
-- 
2.43.0