[PATCH bpf-next v2 4/5] libbpf: fix error when st-prefix_ops and ops from differ btf

["D. Wythe" <alibuda@linux.alibaba.com>]

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: 590a00888250 ("bpf: libbpf: Add STRUCT_OPS support")
Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
---
 tools/lib/bpf/libbpf.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 66173ddb5a2d..046feab4ec36 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -994,6 +994,27 @@ static int find_ksym_btf_id(struct bpf_object *obj, const char *ksym_name,
 static int find_btf_by_prefix_kind(const struct btf *btf, const char *prefix,
 				   const char *name, __u32 kind);
 
+static int
+find_ksym_btf_id_by_prefix_kind(struct bpf_object *obj, const char *prefix,
+				const char *name,
+				__u16 kind, struct btf **res_btf,
+				struct module_btf **res_mod_btf)
+{
+	char btf_type_name[128];
+	int ret;
+
+	ret = snprintf(btf_type_name, sizeof(btf_type_name),
+		       "%s%s", prefix, name);
+	/* snprintf returns the number of characters written excluding the
+	 * terminating null. So, if >= BTF_MAX_NAME_SIZE are written, it
+	 * indicates truncation.
+	 */
+	if (ret < 0 || ret >= sizeof(btf_type_name))
+		return -ENAMETOOLONG;
+
+	return find_ksym_btf_id(obj, btf_type_name, kind, res_btf, res_mod_btf);
+}
+
 static int
 find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
 			   struct module_btf **mod_btf,
@@ -1028,9 +1049,16 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
 	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;
+		if (kern_vtype_id == -ENOENT && !*mod_btf)
+			kern_vtype_id =
+				find_ksym_btf_id_by_prefix_kind(obj, STRUCT_OPS_VALUE_PREFIX,
+								tname, BTF_KIND_STRUCT, &btf,
+								mod_btf);
+		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);
 
-- 
2.45.0