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

[Menglong Dong]

On 2026/2/12 01:20, Andrii Nakryiko wrote:
> On Wed, Feb 11, 2026 at 4:57 AM Menglong Dong <menglong8.dong@xxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > ---
> >  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.

Great! I found that there are already similar usages in kernel.
I did some tests, and it works well. Thanks!

BTW, do you think that if we need do some wrapper for bpf_session_{cookie,is_return}
like this in case that someone else don't know the compatible problem?

extern bool bpf_session_is_return___legacy(void) __weak __ksym;
extern __u64 *bpf_session_cookie___legacy(void) __weak __ksym;


#define bpf_session_is_return(ctx)								\
	(bpf_ksym_exists(bpf_session_is_return___legacy) ? bpf_session_is_return___legacy() :	\
							   bpf_session_is_return(ctx))
#define bpf_session_cookie(ctx)								\
	(bpf_ksym_exists(bpf_session_cookie___legacy) ? bpf_session_cookie___legacy() :	\
							bpf_session_cookie(ctx))

Thanks!
Menglong Dong

> 
> 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
> >
> 
>