Message-ID: <CAEf4Bzas7E4bSFnxiObJysf4hDv2AJVd4B4Q+me1wmGtdHVVbQ@mail.gmail.com>
Date: Fri, 10 Jan 2025 15:38:19 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: "D. Wythe" <alibuda@...ux.alibaba.com>
Cc: kgraul@...ux.ibm.com, wenjia@...ux.ibm.com, jaka@...ux.ibm.com, 
	ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	pabeni@...hat.com, song@...nel.org, sdf@...gle.com, haoluo@...gle.com, 
	yhs@...com, edumazet@...gle.com, john.fastabend@...il.com, kpsingh@...nel.org, 
	jolsa@...nel.org, guwen@...ux.alibaba.com, kuba@...nel.org, 
	davem@...emloft.net, netdev@...r.kernel.org, linux-s390@...r.kernel.org, 
	linux-rdma@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 4/5] libbpf: fix error when st-prefix_ops and
 ops from differ btf

On Tue, Dec 17, 2024 at 6:44 PM D. Wythe <alibuda@...ux.alibaba.com> 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@...ux.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;
> +               }

purely from the coding perspective, this is unnecessarily nested and
convoluted. Wouldn't this work the same but be less nested:

kern_vtype_id = btf__find_by_name_kind(btf, stname, BTF_KIND_STRUCT);
if (kern_vtype_id == -ENOENT && !*mod_btf)
    kern_vtype_id = find_ksym_btf_id(...);
if (kern_vtype_id < 0) {
    pr_warn(...);
    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
>

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