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

[Siddharth Nayyar <sidnayyar@google.com>]

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`.
- 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@google.com>
---
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@google.com

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@google.com

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@google.com

---
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@google.com>