Re: [PATCH bpf-next 3/6] libbpf: Add feature for kernel extended vlen/kind support

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

On 4/14/26 8:50 PM, 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        | 39 +++++++++++++++++++++++++++++++++
>   tools/lib/bpf/libbpf.c          | 17 +++++++++++++-
>   tools/lib/bpf/libbpf_internal.h |  2 ++
>   3 files changed, 57 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
> index 4f19a0d79b0c..4a75d6717bbb 100644
> --- a/tools/lib/bpf/features.c
> +++ b/tools/lib/bpf/features.c
> @@ -615,6 +615,42 @@ static int probe_kern_btf_layout(int token_fd)
>   						 (char *)layout, token_fd));
>   }
>   
> +static int probe_kern_btf_vlen_kind_extended(int token_fd)
> +{
> +	struct btf *btf;
> +	int ret = 0;
> +	__s32 id;
> +	int err;
> +
> +	btf = btf__load_vmlinux_btf();
> +	err = libbpf_get_error(btf);
> +	if (err)
> +		return err;
> +	id = btf__find_by_name_kind(btf, "btf_max", BTF_KIND_ENUM);

Looks like if id > 0 then extended vlen and kinds are available, so 
there is no point to continue processing the enum and checking value of 
the BTF_MAX_VLEN. Or is there a scenario, where BTF_MAX_VLEN goes back 
to 0xffff?

> +	if (id > 0) {
> +		const struct btf_type *t;
> +		const struct btf_enum *e;
> +		const char *name;
> +		__u32 i, vlen;
> +
> +		t = btf__type_by_id(btf, id);
> +		if (t) {
> +			vlen = btf_vlen(t);
> +
> +			for (i = 0, e = btf_enum(t); i < vlen; i++, e++) {
> +				name = btf__name_by_offset(btf, e->name_off);
> +				if (!name || strcmp(name, "BTF_MAX_VLEN") != 0)
> +					continue;
> +				if (e->val > 0xffff)
> +					ret = 1;
> +			}
> +		}
> +	}
> +	btf__free(btf);
> +
> +	return ret;
> +}
> +
>   typedef int (*feature_probe_fn)(int /* token_fd */);
>   
>   static struct kern_feature_cache feature_cache;
> @@ -699,6 +735,9 @@ 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);
> +			}
> +		}
> +
>   		if ((!has_datasec && btf_is_var(t)) || (!has_decl_tag && btf_is_decl_tag(t))) {
>   			/* replace VAR/DECL_TAG with INT */
>   			t->info = BTF_INFO_ENC(BTF_KIND_INT, 0, 0);
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 3781c45b46d3..8ef1fc96b73a 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -398,6 +398,8 @@ enum kern_feature_id {
>   	FEAT_UPROBE_SYSCALL,
>   	/* Kernel supports BTF layout information */
>   	FEAT_BTF_LAYOUT,
> +	/* Kernel supports BTF vlen > 65535 */
> +	FEAT_BTF_VLEN_KIND_EXTENDED,
>   	__FEAT_CNT,
>   };
>