libbpf: fix error when st-prefix_ops and ops from differ btf

When a struct_ops named xxx_ops was registered by a module, and
it will be used in both built-in modules and the module itself,
so that the btf_type of xxx_ops will be present in btf_vmlinux
instead of in btf_mod, which means that the btf_type of
bpf_struct_ops_xxx_ops and xxx_ops will not be in the same btf.

Here are four possible case:

+--------+-------------+-------------+---------------------------------+
|        | st_opx_xxx  | xxx         |                                 |
+--------+-------------+-------------+---------------------------------+
| case 0 | btf_vmlinux | bft_vmlinux | be used and reg only in vmlinux |
+--------+-------------+-------------+---------------------------------+
| case 1 | btf_vmlinux | bpf_mod     | INVALID                         |
+--------+-------------+-------------+---------------------------------+
| case 2 | btf_mod     | btf_vmlinux | reg in mod but be used both in  |
|        |             |             | vmlinux and mod.                |
+--------+-------------+-------------+---------------------------------+
| case 3 | btf_mod     | btf_mod     | be used and reg only in mod     |
+--------+-------------+-------------+---------------------------------+

At present, cases 0, 1, and 3 can be correctly identified, because
st_ops_xxx is searched from the same btf with xxx. In order to
handle case 2 correctly without affecting other cases, we cannot simply
change the search method for st_ops_xxx from find_btf_by_prefix_kind()
to find_ksym_btf_id(), because in this way, case 1 will not be
recognized anymore.

To address this issue, if st_ops_xxx cannot be found in the btf with xxx
and mod_btf does not exist, do find_ksym_btf_id() again to
avoid such issue.

Fixes: 590a008 ("bpf: libbpf: Add STRUCT_OPS support")
Signed-off-by: D. Wythe <[email protected]>

tools/lib/bpf/libbpf.c

@@ -1005,7 +1005,8 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
 	const struct btf_member *kern_data_member;
 	struct btf *btf = NULL;
 	__s32 kern_vtype_id, kern_type_id;
-	char tname[256];
+	char tname[256], stname[256];
+	int ret;
 	__u32 i;
 
 	snprintf(tname, sizeof(tname), "%.*s",
@@ -1020,17 +1021,25 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
 	}
 	kern_type = btf__type_by_id(btf, kern_type_id);
 
+	ret = snprintf(stname, sizeof(stname), "%s%s", STRUCT_OPS_VALUE_PREFIX, tname);
+	if (ret < 0 || ret >= sizeof(stname))
+		return -ENAMETOOLONG;
+
 	/* Find the corresponding "map_value" type that will be used
 	 * in map_update(BPF_MAP_TYPE_STRUCT_OPS).  For example,
 	 * find "struct bpf_struct_ops_tcp_congestion_ops" from the
 	 * btf_vmlinux.
 	 */
-	kern_vtype_id = find_btf_by_prefix_kind(btf, STRUCT_OPS_VALUE_PREFIX,
-						tname, BTF_KIND_STRUCT);
+	kern_vtype_id = btf__find_by_name_kind(btf, stname, BTF_KIND_STRUCT);
 	if (kern_vtype_id < 0) {
-		pr_warn("struct_ops init_kern: struct %s%s is not found in kernel BTF\n",
-			STRUCT_OPS_VALUE_PREFIX, tname);
-		return kern_vtype_id;
+		if (kern_vtype_id == -ENOENT && !*mod_btf)
+			kern_vtype_id = find_ksym_btf_id(obj, stname, BTF_KIND_STRUCT,
+							 &btf, mod_btf);
+		if (kern_vtype_id < 0) {
+			pr_warn("struct_ops init_kern: struct %s is not found in kernel BTF\n",
+				stname);
+			return kern_vtype_id;
+		}
 	}
 	kern_vtype = btf__type_by_id(btf, kern_vtype_id);
 
@@ -1046,8 +1055,8 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
 			break;
 	}
 	if (i == btf_vlen(kern_vtype)) {
-		pr_warn("struct_ops init_kern: struct %s data is not found in struct %s%s\n",
-			tname, STRUCT_OPS_VALUE_PREFIX, tname);
+		pr_warn("struct_ops init_kern: struct %s data is not found in struct %s\n",
+			tname, stname);
 		return -EINVAL;
 	}