[PATCH V2 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>

---
changelog:
v1 -->  v2:
1, introduce namespace to specify which module the kfunc belongs to, like:
modulea__foo
moduleb__foo
foo
As a result, kfunc foo can co-exist in modulea, moduleb and vmlinux, ebpf
code owner can specify which one he wants to call.
---
 tools/lib/bpf/libbpf.c | 60 +++++++++++++++++++++++++++++++-----------
 1 file changed, 45 insertions(+), 15 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 0be7017800fe..9c9e5ff4d754 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8532,35 +8532,65 @@ static int bpf_object__read_kallsyms_file(struct bpf_object *obj)
 	return libbpf_kallsyms_parse(kallsyms_cb, obj);
 }
 
+static void split_module_from_ksym(const char *ksym_name,
+			const char **module_name,
+			size_t *module_name_len,
+			const char **kfunc_name)
+{
+	const char *sep = strstr(ksym_name, "__");
+
+	if (!sep) {
+		*module_name = NULL;
+		*module_name_len = 0;
+		*kfunc_name = ksym_name;
+	} else {
+		*module_name = ksym_name;
+		*module_name_len = sep - ksym_name;
+		*kfunc_name = sep + strlen("__");
+	}
+}
+
 static int find_ksym_btf_id(struct bpf_object *obj, const char *ksym_name,
-			    __u16 kind, struct btf **res_btf,
-			    struct module_btf **res_mod_btf)
+			__u16 kind, struct btf **res_btf,
+			struct module_btf **res_mod_btf)
 {
 	struct module_btf *mod_btf;
 	struct btf *btf;
-	int i, id, err;
+	int i, id = 0, err;
+	const char *module_name;
+	const char *kfunc_name;
+	size_t module_name_len;
+
+	split_module_from_ksym(ksym_name, &module_name, &module_name_len, &kfunc_name);
+	if (module_name_len == 0)
+		goto search_vmlinux;
 
-	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];
+	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];
+		if (strlen(mod_btf->name) == module_name_len &&
+			!strncmp(mod_btf->name, module_name, module_name_len)) {
 			btf = mod_btf->btf;
-			id = btf__find_by_name_kind_own(btf, ksym_name, kind);
+			id = btf__find_by_name_kind_own(btf, kfunc_name, kind);
 			if (id != -ENOENT)
-				break;
+				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