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

[bot+bpf-ci]

> commit c1abfce71ecf8c95413f5afe0d153662877e0ad0
> Author: Siddharth Nayyar <sidnayyar@xxxxxxxxxx>
>
>     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 actually requires kernel vmlinux BTF
>     (e.g. contains `struct_ops` maps).
>
>     Signed-off-by: Siddharth Nayyar <sidnayyar@xxxxxxxxxx>

This looks like a bug fix for gen_loader mode with struct_ops maps. Should
this include:

  Fixes: 67234743736a ("libbpf: Generate loader program out of BPF ELF file.")

> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index ab2071fdd3e8..6163e53a2056 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;
>
>  	if (!force && !obj_needs_vmlinux_btf(obj))

Alexei Starovoitov raised a design concern about this approach in v1. He
requested an end-to-end solution where struct-ops and sched-ext programs are
fully supported in light skeleton (lksel) mode, rather than just this
incremental fix. His specific comment was: "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."

This feedback has not been addressed - no v2 was posted and no author response
was provided.

Reference: https://lore.kernel.org/bpf/CAADnVQ+M6pJJJ_YY5QO_EYJyDfd=TNs=A1xFB9_RFLTqaM=w_g@xxxxxxxxxxxxxx/


---
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CI run summary: https://github.com/kernel-patches/bpf/actions/runs/26360291945