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_ops_xxx_ops| xxx_ops     |                                 |
+--------+---------------+-------------+---------------------------------+
| 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_ops is searched from the same btf with xxx_ops. In order to
handle case 2 correctly without affecting other cases, we cannot simply
change the search method for st_ops_xxx_ops from find_btf_by_prefix_kind()
to find_ksym_btf_id(), because in this way, case 1 will not be
recognized anymore.

To address the issue, we always look for st_ops_xxx_ops first,
figure out the btf, and then look for xxx_ops with the very btf to avoid
such issue.

Fixes: 590a008 ("bpf: libbpf: Add STRUCT_OPS support")
Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>

Author: D. Wythe

tools/lib/bpf/libbpf.c

@@ -1005,35 +1005,40 @@ 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",
 		 (int)bpf_core_essential_name_len(tname_raw), tname_raw);
 
-	kern_type_id = find_ksym_btf_id(obj, tname, BTF_KIND_STRUCT,
+	ret = snprintf(stname, sizeof(stname), "%s%s", STRUCT_OPS_VALUE_PREFIX,
+		       tname);
+	if (ret < 0 || ret >= sizeof(stname))
+		return -ENAMETOOLONG;
+
+	/* Look for the corresponding "map_value" type that will be used
+	 * in map_update(BPF_MAP_TYPE_STRUCT_OPS) first, figure out the btf
+	 * and the mod_btf.
+	 * For example, find "struct bpf_struct_ops_tcp_congestion_ops".
+	 */
+	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);
+
+	kern_type_id = btf__find_by_name_kind(btf, tname, BTF_KIND_STRUCT);
 	if (kern_type_id < 0) {
 		pr_warn("struct_ops init_kern: struct %s is not found in kernel BTF\n",
 			tname);
 		return kern_type_id;
 	}
 	kern_type = btf__type_by_id(btf, kern_type_id);
 
-	/* 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);
-	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;
-	}
-	kern_vtype = btf__type_by_id(btf, kern_vtype_id);
-
 	/* Find "struct tcp_congestion_ops" from
 	 * struct bpf_struct_ops_tcp_congestion_ops {
 	 *	[ ... ]
@@ -1046,8 +1051,8 @@ 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;
 	}