linux-kernel - Re: [PATCH bpf-next] libbpf: skip compat check for session kfuncs

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Message-ID: <CAEf4Bzb6ay+gLD0gqZ8j8-=9onZE396YQmyGX-ZLELqF73iDGA@mail.gmail.com>
Date: Wed, 11 Feb 2026 09:20:07 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: andrii@...nel.org, ast@...nel.org, daniel@...earbox.net, 
	martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org, 
	sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] libbpf: skip compat check for session kfuncs

On Wed, Feb 11, 2026 at 4:57 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> The function prototype of bpf_session_{cookie,is_return} is changed in
> the commit 8fe4dc4f6456 ("bpf: change prototype of
> bpf_session_{cookie,is_return}"), which is not friendly to the old kernel,
> as the libbpf will fail on the compatible checking.
>
> Therefore, let's skip the checking of bpf_session_{cookie,is_return} in
> libbpf, and just let the kernel do the checking.
>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> ---
>  tools/lib/bpf/libbpf.c | 31 ++++++++++++++++++++-----------
>  1 file changed, 20 insertions(+), 11 deletions(-)
>

This is not the way. Use bpf_ksym_exists() check and have custom
legacy bpf_session_cookie___legacy() definition with old prototype.
You can then detect whether
bpf_ksym_exists(bpf_session_cookie___legacy) is true and use the
legacy version, otherwise use the current version.

pw-bot: cr


> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 0c8bf0b5cce4..71bdd1c1ac39 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8557,6 +8557,12 @@ static int find_ksym_btf_id(struct bpf_object *obj, const char *ksym_name,
>         return id;
>  }
>
> +static bool kfunc_skip_compat_check(const char *name)
> +{
> +       return name && (strcmp(name, "bpf_session_cookie") == 0 ||
> +                       strcmp(name, "bpf_session_is_return") == 0);
> +}
> +
>  static int bpf_object__resolve_ksym_var_btf_id(struct bpf_object *obj,
>                                                struct extern_desc *ext)
>  {
> @@ -8617,12 +8623,13 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
>         struct module_btf *mod_btf = NULL;
>         const struct btf_type *kern_func;
>         struct btf *kern_btf = NULL;
> +       const char *kfunc_name;
>         int ret;
>
>         local_func_proto_id = ext->ksym.type_id;
>
> -       kfunc_id = find_ksym_btf_id(obj, ext->essent_name ?: ext->name, BTF_KIND_FUNC, &kern_btf,
> -                                   &mod_btf);
> +       kfunc_name = ext->essent_name ?: ext->name;
> +       kfunc_id = find_ksym_btf_id(obj, kfunc_name, BTF_KIND_FUNC, &kern_btf, &mod_btf);
>         if (kfunc_id < 0) {
>                 if (kfunc_id == -ESRCH && ext->is_weak)
>                         return 0;
> @@ -8634,16 +8641,18 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
>         kern_func = btf__type_by_id(kern_btf, kfunc_id);
>         kfunc_proto_id = kern_func->type;
>
> -       ret = bpf_core_types_are_compat(obj->btf, local_func_proto_id,
> -                                       kern_btf, kfunc_proto_id);
> -       if (ret <= 0) {
> -               if (ext->is_weak)
> -                       return 0;
> +       if (!kfunc_skip_compat_check(kfunc_name)) {
> +               ret = bpf_core_types_are_compat(obj->btf, local_func_proto_id,
> +                                               kern_btf, kfunc_proto_id);
> +               if (ret <= 0) {
> +                       if (ext->is_weak)
> +                               return 0;
>
> -               pr_warn("extern (func ksym) '%s': func_proto [%d] incompatible with %s [%d]\n",
> -                       ext->name, local_func_proto_id,
> -                       mod_btf ? mod_btf->name : "vmlinux", kfunc_proto_id);
> -               return -EINVAL;
> +                       pr_warn("extern (func ksym) '%s': func_proto [%d] incompatible with %s [%d]\n",
> +                               ext->name, local_func_proto_id,
> +                               mod_btf ? mod_btf->name : "vmlinux", kfunc_proto_id);
> +                       return -EINVAL;
> +               }
>         }
>
>         /* set index for module BTF fd in fd_array, if unset */
> --
> 2.53.0
>