Re: [PATCH v2 bpf-next 3/8] libbpf: Add feature for kernel extended vlen/kind support

[sashiko-bot@kernel.org]

Sashiko AI review found 1 potential issue(s):
- [High] Feature probe `probe_kern_btf_vlen_kind_extended` is malformed and always fails, breaking extended BTF support.
--

commit 45c7cfd438137b97d67612513eb9d60104376b8f
Author: Alan Maguire <alan.maguire@oracle.com>

libbpf: Add feature for kernel extended vlen/kind support

This commit adds a feature check for kernel extended vlen/kind support, and
rejects BTF that uses extended vlens/kinds if the kernel does not support it.

> diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
> index 4f19a0d79b0cf..f835931f4afe7 100644
> --- a/tools/lib/bpf/features.c
> +++ b/tools/lib/bpf/features.c

[ ... ]

> +static int probe_kern_btf_vlen_kind_extended(int token_fd)
> +{
> +	static const char strs[] = "\0int\0foo\0bar";
> +	__u32 types[EXTEND_TYPE_LEN] = {
> +		/* int */
> +		BTF_TYPE_INT_ENC(1, BTF_INT_SIGNED, 0, 32, 4),
> +		/* var */
> +		BTF_TYPE_ENC(5 /* "foo" */, BTF_INFO_ENC(BTF_KIND_VAR, 0, 0), 1),

Is this BTF_KIND_VAR type missing its required trailing struct btf_var
payload? Without it, the kernel's BTF parser might become misaligned
and reject the entire raw BTF payload.

> +		/* datasec */
> +		BTF_TYPE_ENC(9 /* "bar" */, BTF_INFO_ENC(BTF_KIND_DATASEC, EXTEND_VLEN, 0), 0),

Looking at BTF_INFO_ENC(kind, kflag, vlen), it appears EXTEND_VLEN is being
passed as the kflag argument rather than vlen. This leaves vlen as 0 and
sets the kflag bit. Could the kernel's btf_datasec_check_meta() reject this?

Additionally, the size argument to BTF_TYPE_ENC here is 0. Will the kernel
reject this DATASEC type since a zero size usually returns -EINVAL?

Finally, if vlen evaluates to 0, will the appended EXTEND_VLEN
btf_var_secinfo structures below be parsed by the kernel as independent
trailing BTF types? If the first struct evaluates to BTF_KIND_UNKN, the
entire probe might fail unconditionally.

> +	};
> +	struct btf_var_secinfo *s;
> +	__u32 *types_data;

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260416143904.1220662-1-alan.maguire@oracle.com?part=3