Re: [PATCH v3 1/3] libbpf: load vmlinux BTF in gen_loader mode for struct_ops

[bot+bpf-ci@kernel.org]

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> 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@google.com>

This looks like a bug fix for commit 67234743736a which introduced the
gen_loader functionality. 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))

Does this change provide a complete solution for struct_ops support in
light skeleton mode? In the earlier review thread, Alexei Starovoitov
commented:

"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."

The author responded by expanding this to a 3-patch series in v2/v3.
However, only this first patch is present here. The other patches in
that series were:

- Patch 2/3: Zeroing btf_key_type_id
- Patch 3/3: Plumbing btf_vmlinux_value_type_id in gen_loader

Without those additional changes, will struct_ops maps actually work in
light skeleton generation, or does this incomplete series leave the
feature partially broken?


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