On Mon, Dec 9, 2024 at 8:04 PM D. Wythe <alibuda@xxxxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxxxx>
> ---
> 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);
> +}
I don't think we need this helper, see below.
> +
> 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);
instead of using find_btf_by_prefix_kind, let's have:
1) snprintf(STRUCT_OPS_VALUE_PREFIX, tname) right here in this
function, so we have expected type constructed and ready to be used
and reused, if necessary
2) call btf__find_by_name_kind() instead of find_btf_by_prefix_kind()
3) if (kern_vtype_id < 0 && !*mod_btf)
kern_vtype_id = find_ksym_btf_id(...)
4) if (kern_vtype_id < 0) /* now emit error and error out */
> 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
>
