Re: [PATCH v4 1/3] libbpf: load vmlinux BTF in gen_loader mode for struct_ops

[bot+bpf-ci]

> commit 29cedaf6d7d80a7e880a2e639f40fbafcbaf2e65
> Author: Siddharth Nayyar <sidnayyar@xxxxxxxxxx>
> Date:   Wed May 27 12:52:57 2026 +0000
>
>     libbpf: load vmlinux BTF in gen_loader mode for struct_ops
>
>     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 contains `struct_ops` maps.
>
>     Signed-off-by: Siddharth Nayyar <sidnayyar@xxxxxxxxxx>

Alexei Starovoitov requested in v3 review that a selftest be added to
prove struct_ops light skeletons work end-to-end:

  "The selftest is mandatory to prove that."
  https://lore.kernel.org/bpf/CAADnVQ+AA_fmRw7uRwdL2TMyMbpWeebOQKb=0wvpKiQR6Ee5xQ@xxxxxxxxxxxxxx/

This requirement has not been addressed in v4. Should a selftest be added
to demonstrate that struct_ops light skeletons work correctly with this
change?

> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index ab2071fdd3e8..420d37f215ea 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -3544,15 +3544,20 @@ static bool prog_needs_vmlinux_btf(struct bpf_program *prog)
>  	return false;
>  }
>
> -static bool map_needs_vmlinux_btf(struct bpf_map *map)
> +static bool obj_maps_need_vmlinux_btf(const struct bpf_object *obj)
>  {
> -	return bpf_map__is_struct_ops(map);
> +	struct bpf_map *map;
> +
> +	bpf_object__for_each_map(map, obj) {
> +		if (bpf_map__is_struct_ops(map))
> +			return true;
> +	}
> +	return false;
>  }
>
>  static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
>  {
>  	struct bpf_program *prog;
> -	struct bpf_map *map;
>  	int i;
>
>  	/* CO-RE relocations need kernel BTF, only when btf_custom_path
> @@ -3577,12 +3582,7 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
>  			return true;
>  	}
>
> -	bpf_object__for_each_map(map, obj) {
> -		if (map_needs_vmlinux_btf(map))
> -			return true;
> -	}
> -
> -	return false;
> +	return obj_maps_need_vmlinux_btf(obj);
>  }
>
>  static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
> @@ -3590,7 +3590,11 @@ 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;
> +
> +	/* only struct_ops maps need btf_vmlinux in gen_loader */
> +	if (obj->gen_loader && !obj_maps_need_vmlinux_btf(obj))
>  		return 0;
>
>  	if (!force && !obj_needs_vmlinux_btf(obj))

This looks like a bug fix for gen_loader mode with struct_ops maps.
Multiple CI reviews (v1 and v3) noted that this should include:

  Fixes: 67234743736a ("libbpf: Generate loader program out of BPF ELF file.")
  https://lore.kernel.org/bpf/b4b4ca51ae8e281a37707cbcd2ba8349dabc6c630e96461929be0872fd8ed0e6@xxxxxxxxxxxxxxx/

Should this tag be added to indicate it fixes a limitation in the original
gen_loader implementation?


---
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