Re: [PATCH] libbpf: load vmlinux BTF in gen_loader mode for struct_ops

[Alexei Starovoitov]

On Sun, May 24, 2026 at 1:26 PM Siddharth Nayyar <sidnayyar@xxxxxxxxxx> wrote:
>
> During light skeleton generation (`bpftool gen skeleton -L`), libbpf
> runs in gen_loader mode. Previously, `bpf_object__load_vmlinux_btf()`
> completely bypassed loading the kernel vmlinux BTF (`obj->btf_vmlinux`)
> if `gen_loader` was active.
>
> However, BPF `struct_ops` maps (such as `sched_ext_ops` maps) require
> resolving the kernel-side struct type IDs and member sizes at
> compile/skeleton generation time. Without loading `btf_vmlinux`, libbpf
> cannot query the kernel BTF types, causing light skeleton generation for
> `struct_ops` to fail or omit crucial type information.
>
> Fix this by modifying the check to load `btf_vmlinux` even in
> `gen_loader` mode if the BPF object actually requires kernel vmlinux BTF
> (e.g. contains `struct_ops` maps).
>
> Signed-off-by: Siddharth Nayyar <sidnayyar@xxxxxxxxxx>
> ---
>  tools/lib/bpf/libbpf.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 3a80a018fc7d..cb1b7ea884a7 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -3590,7 +3590,10 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
>         int err;
>
>         /* btf_vmlinux could be loaded earlier */
> -       if (obj->btf_vmlinux || obj->gen_loader)
> +       if (obj->btf_vmlinux)
> +               return 0;
> +
> +       if (obj->gen_loader && !obj_needs_vmlinux_btf(obj))
>                 return 0;

That's a step, but please provide an end-to-end solution
where struct-ops and sched-ext programs are supported
in lksel mode.
This step alone may be more confusing to users than helpful.

pw-bot: cr