Re: [PATCH v4 0/3] libbpf: support STRUCT_OPS in light skeletons

[Andrii Nakryiko]

On Wed, May 27, 2026 at 5:53 AM Siddharth Nayyar <sidnayyar@xxxxxxxxxx> wrote:
>
> This series enables support for BPF `STRUCT_OPS` maps (such as
> `sched_ext_ops` used by sched_ext) when using light skeletons (i.e.
> `gen_loader` mode).
>
> Previously, generating light skeletons for objects containing
> `STRUCT_OPS` maps would fail or produce incomplete results because
> `gen_loader` lacked support for several key features required by
> `STRUCT_OPS` maps, specifically:
>
> 1. Loading `vmlinux` BTF to resolve kernel-side type information.
> 2. Correctly plumbing `btf_vmlinux_value_type_id` into map creation
>    attributes.
> 3. Ensuring `btf_key_type_id` is zeroed out to satisfy kernel safety
>    checks.
> 4. Plumbing the userspace BTF FD (`btf_fd`) when
>    `btf_vmlinux_value_type_id` is present but `btf_value_type_id` is
>    zero (which is always the case for `STRUCT_OPS` maps).
>
> This series addresses these limitations by:
>
> - Loading `vmlinux` BTF in `gen_loader` mode when the BPF object
>   contains `struct_ops` maps.
> - Explicitly zeroing out `btf_key_type_id` for `STRUCT_OPS` maps to
>   satisfy kernel validations.
> - Plumbing `btf_vmlinux_value_type_id` during map creation in
>   `gen_loader`.

this type ID search has to happen on target kernel at runtime,
otherwise your lskel will be bound to work only on exact kernel you
used to run bpftool to generate lskel

> - Fixing `btf_fd` copying logic in `gen_loader` to populate it if either
>   `btf_value_type_id` or `btf_vmlinux_value_type_id` is set.
>
> With these changes, it is now possible to generate and use light
> skeletons for BPF programs utilizing `STRUCT_OPS`, such as custom
> sched_ext schedulers.
>
> Signed-off-by: Siddharth Nayyar <sidnayyar@xxxxxxxxxx>
> ---
> Changes in v4:
> - Load btf_vmlinux in gen_loader mode only when BPF object contains
>   STRUCT_OPS map.
> - Link to v3:
>   https://lore.kernel.org/r/20260526-libbpf-load-vmlinux-btf-in-gen_loader-mode-v3-0-5b4fa2a5e1a4@xxxxxxxxxx
>
> Changes in v3:
> - Remove erroneous change contracting map creation attribute size in
>   gen_loader.
> - Link to v2:
>   https://lore.kernel.org/r/20260526-libbpf-load-vmlinux-btf-in-gen_loader-mode-v2-0-6750f5859bc6@xxxxxxxxxx
>
> Changes in v2:
> - Expand the series to 3 patches to fully support STRUCT_OPS in
>   gen_loader.
> - Add a patch to explicitly zero out btf_key_type_id for STRUCT_OPS
>   maps.
> - Add a patch to plumb btf_vmlinux_value_type_id and btf_fd in
>   gen_loader.
> - Link to v1:
>   https://lore.kernel.org/r/20260524-libbpf-load-vmlinux-btf-in-gen_loader-mode-v1-1-6f57f191a7ad@xxxxxxxxxx
>
> ---
> Siddharth Nayyar (3):
>       libbpf: load vmlinux BTF in gen_loader mode for struct_ops
>       libbpf: zero out btf_key_type_id for STRUCT_OPS maps
>       libbpf: plumb btf_vmlinux_value_type_id and btf_fd in gen_loader
>
>  tools/lib/bpf/gen_loader.c |  3 ++-
>  tools/lib/bpf/libbpf.c     | 25 +++++++++++++++----------
>  2 files changed, 17 insertions(+), 11 deletions(-)
> ---
> base-commit: c6e99c10fd9855082568cbd71bb2cc5dc90eda53
> change-id: 20260522-libbpf-load-vmlinux-btf-in-gen_loader-mode-4474834aa467
>
> Best regards,
> --
> Siddharth Nayyar <sidnayyar@xxxxxxxxxx>
>