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

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Enabling light skeleton generation for `struct_ops` maps exposes incomplete and fundamentally broken loader logic, which will always fail on the target system.
--

commit 241c33de1404c9ec6cd0b388a0a3bc61171e6646
Author: Siddharth Nayyar <sidnayyar@google.com>

libbpf: load vmlinux BTF in gen_loader mode for struct_ops

This patch modifies the BTF loading logic in libbpf to ensure vmlinux BTF
is loaded even in gen_loader mode if the BPF object contains struct_ops maps.
This provides the necessary type information for generating light skeletons.

> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 3a80a018fc7d5..b159faae7f9c6 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c

[ ... ]

> @@ -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;

[Severity: High]
Does enabling struct_ops for light skeletons result in a generated loader
that works correctly on the target system?

It appears the gen_loader logic statically embeds the build host's
btf_vmlinux_value_type_id into the BPF_MAP_CREATE attributes buffer. Since
BTF IDs vary across kernel versions and configurations, won't passing the
build host's BTF ID to a different target kernel cause the syscall to fail
with -EINVAL or incorrectly associate the map with the wrong kernel structure?
Shouldn't the loader dynamically resolve this ID using bpf_btf_find_by_name_kind
on the target kernel instead?

Additionally, struct_ops maps require their map values to be populated with
the runtime FDs of the loaded BPF programs. Does gen_loader currently have
the necessary logic to emit BPF instructions that read dynamically generated
prog_fds from the loader's stack, patch them into the map value buffer, and
update the map?

Furthermore, does the skel__attach() function generated by
bpftool gen skeleton -L handle struct_ops map attachment? It seems to only
iterate over programs, entirely omitting the struct_ops map attachment that
is normally handled by bpf_map__attach_struct_ops().

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260527-libbpf-load-vmlinux-btf-in-gen_loader-mode-v4-0-27daa15487a9@google.com?part=1