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

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Thu Apr 16, 2026 at 7:38 AM PDT, Alan Maguire wrote:
> Add feature check for kernel extended vlen/kind support, and reject
> BTF that uses extended vlens/kinds if the kernel does not support
> it.  There is no reasonable path to generally sanitize such BTF.
>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---
>  tools/lib/bpf/features.c        | 43 +++++++++++++++++++++++++++++++++
>  tools/lib/bpf/libbpf.c          | 17 ++++++++++++-
>  tools/lib/bpf/libbpf_internal.h |  2 ++
>  3 files changed, 61 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
> index 4f19a0d79b0c..f835931f4afe 100644
> --- a/tools/lib/bpf/features.c
> +++ b/tools/lib/bpf/features.c
> @@ -615,6 +615,46 @@ static int probe_kern_btf_layout(int token_fd)
>  						 (char *)layout, token_fd));
>  }
>  
> +#define EXTEND_TYPE_LEN	((3 * sizeof(struct btf_type) + sizeof(__u32))/sizeof(__u32))
> +#define EXTEND_VLEN		0x10000
> +#define EXTEND_SECINFO_LEN	(EXTEND_VLEN * sizeof(struct btf_var_secinfo)/sizeof(__u32))
> +
> +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),
> +		/* datasec */
> +		BTF_TYPE_ENC(9 /* "bar" */, BTF_INFO_ENC(BTF_KIND_DATASEC, EXTEND_VLEN, 0), 0),
> +	};
> +	struct btf_var_secinfo *s;
> +	__u32 *types_data;
> +	__u32 i;
> +	int ret;
> +
> +	types_data = calloc(EXTEND_TYPE_LEN + EXTEND_SECINFO_LEN, sizeof(__u32));
> +	if (!types_data)
> +		return -ENOMEM;
> +	memcpy(types_data, types, sizeof(types));
> +
> +	for (i = 0, s = (struct btf_var_secinfo *)&types_data[EXTEND_TYPE_LEN];
> +	     i < EXTEND_VLEN; i++, s++) {
> +		s->type = 2;
> +		s->offset = 4 * i;
> +		s->size = 4;
> +	}
> +
> +	ret = probe_fd(libbpf__load_raw_btf((char *)types_data,
> +					    (EXTEND_TYPE_LEN + EXTEND_SECINFO_LEN) * sizeof(__u32),
> +					    strs, sizeof(strs), token_fd));
> +	free(types_data);
> +
> +	return ret;
> +}
> +
>  typedef int (*feature_probe_fn)(int /* token_fd */);
>  
>  static struct kern_feature_cache feature_cache;
> @@ -699,6 +739,9 @@ static struct kern_feature_desc {
>  	[FEAT_BTF_LAYOUT] = {
>  		"kernel supports BTF layout", probe_kern_btf_layout,
>  	},
> +	[FEAT_BTF_VLEN_KIND_EXTENDED] = {
> +		"kernel supports extended BTF vlen/kind", probe_kern_btf_vlen_kind_extended,
> +	},
>  };
>  
>  bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 8b0c3246097f..5f19d8ac17a9 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -3139,10 +3139,11 @@ static bool btf_needs_sanitization(struct bpf_object *obj)
>  	bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);
>  	bool has_qmark_datasec = kernel_supports(obj, FEAT_BTF_QMARK_DATASEC);
>  	bool has_layout = kernel_supports(obj, FEAT_BTF_LAYOUT);
> +	bool has_vlen_kind_extended = kernel_supports(obj, FEAT_BTF_VLEN_KIND_EXTENDED);
>  
>  	return !has_func || !has_datasec || !has_func_global || !has_float ||
>  	       !has_decl_tag || !has_type_tag || !has_enum64 || !has_qmark_datasec ||
> -	       !has_layout;
> +	       !has_layout || !has_vlen_kind_extended;
>  }
>  
>  struct btf *bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *orig_btf)
> @@ -3156,6 +3157,7 @@ struct btf *bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *orig_bt
>  	bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);
>  	bool has_qmark_datasec = kernel_supports(obj, FEAT_BTF_QMARK_DATASEC);
>  	bool has_layout = kernel_supports(obj, FEAT_BTF_LAYOUT);
> +	bool has_vlen_kind_extended = kernel_supports(obj, FEAT_BTF_VLEN_KIND_EXTENDED);
>  	int enum64_placeholder_id = 0;
>  	const struct btf_header *hdr;
>  	struct btf *btf = NULL;
> @@ -3217,6 +3219,19 @@ struct btf *bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *orig_bt
>  	for (i = 1; i < btf__type_cnt(btf); i++) {
>  		t = (struct btf_type *)btf__type_by_id(btf, i);
>  
> +		/*
> +		 * If BTF uses extended vlen/kind and kernel does not support
> +		 * it, there is nothing we can do.
> +		 */
> +		if (!has_vlen_kind_extended) {
> +			if (btf_vlen(t) > 0xffff || btf_kind(t) > 0x1f) {
> +				pr_debug("Unsupported %s for id %u\n",
> +					 btf_kind(t) > 0x1f ? "BTF kind" : "BTF vlen", i);
> +				btf__free(btf);
> +				return ERR_PTR(-EINVAL);
> +			}
> +		}

What's the point of the patch?
Just to make libbpf error early?
It seems it can be dropped. The kernel will error if it's old.