[PATCH bpf-next v3 4/5] libbpf: fix error when st-prefix_ops and ops from differ btf

D. Wythe
2024-12-18T02:44:34Z

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 | 25 +++++++++++++++++--------
1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 66173ddb5a2d..56bf74894110 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/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;
}

--
2.45.0