Re: [PATCH v3 bpf-next 1/6] bpf: Extend BTF UAPI vlen, kinds to use unused bits

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

On 4/17/26 3:30 PM, Alan Maguire wrote:
> BTF maximum vlen is encoded using 16 bits with a maximum vlen
> of 65535.  This has sufficed for structs, function parameters
> and enumerated type values.  However, with upcoming BTF location
> information - in particular information about inline sites -
> this limit is surpassed.  Use bits 16-23 - currently unused in
> BTF info - to extend to 24 bits, giving a max vlen of (2^24 - 1),
> or 16 million.
> 
> Also extend BTF kind encoding from 5 to 7 bits, giving a maximum
> available number of kinds of 128.  Since with the BTF location work
> we use another 3 kinds, we are fast approaching the current limit
> of 32.
> 
> Convert BTF_MAX_* values to enums to allow them to be encoded in
> kernel BTF; this will allow us to detect if the running kernel
> supports a 24-bit vlen or not.  Add one for max _possible_
> (not used) kind.
> 
> Fix up a few places in the kernel where a 16-bit vlen is assumed;
> remove BTF_INFO_MASK as now all bits are used.
> 
> The vlen expansion was suggested by Andrii in [1]; the kind expansion
> is tackled here too as it may be needed also to support new kinds
> in BTF.
> 
> [1] https://lore.kernel.org/bpf/CAEf4BzZx=X6vGqcA8SPU6D+v6k+TR=ZewebXMuXtpmML058piw@mail.gmail.com/
> 
> Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---

It looks like all callsites of vlen have been migrated to u32 (except 
the ones that are guaranteed to work with narrower types: 
btf_func_linkage(), cs->num_params).
Extending kind from 5 to 7 bits does not require modifying users at all.

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>

>   include/linux/btf.h            |  4 ++--
>   include/uapi/linux/btf.h       | 26 ++++++++++++++------------
>   kernel/bpf/btf.c               | 27 ++++++++++-----------------
>   tools/include/uapi/linux/btf.h | 26 ++++++++++++++------------
>   4 files changed, 40 insertions(+), 43 deletions(-)
> 
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index 48108471c5b1..c82d0d689059 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -415,12 +415,12 @@ static inline bool btf_type_is_array(const struct btf_type *t)
>   	return BTF_INFO_KIND(t->info) == BTF_KIND_ARRAY;
>   }
>   
> -static inline u16 btf_type_vlen(const struct btf_type *t)
> +static inline u32 btf_type_vlen(const struct btf_type *t)
>   {
>   	return BTF_INFO_VLEN(t->info);
>   }
>   
> -static inline u16 btf_vlen(const struct btf_type *t)
> +static inline u32 btf_vlen(const struct btf_type *t)
>   {
>   	return btf_type_vlen(t);
>   }
> diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h
> index 638615ebddc2..618167cab4e6 100644
> --- a/include/uapi/linux/btf.h
> +++ b/include/uapi/linux/btf.h
> @@ -33,20 +33,22 @@ struct btf_header {
>   	__u32	layout_len;	/* length of layout section	*/
>   };
>   
> -/* Max # of type identifier */
> -#define BTF_MAX_TYPE	0x000fffff
> -/* Max offset into the string section */
> -#define BTF_MAX_NAME_OFFSET	0x00ffffff
> -/* Max # of struct/union/enum members or func args */
> -#define BTF_MAX_VLEN	0xffff
> +enum btf_max {
> +	/* Max possible kind */
> +	BTF_MAX_KIND =		0x0000007f,
> +	/* Max # of type identifier */
> +	BTF_MAX_TYPE =		0x000fffff,
> +	/* Max offset into the string section */
> +	BTF_MAX_NAME_OFFSET =	0x00ffffff,
> +	/* Max # of struct/union/enum members or func args */
> +	BTF_MAX_VLEN =		0x00ffffff,
> +};
>   
>   struct btf_type {
>   	__u32 name_off;
>   	/* "info" bits arrangement
> -	 * bits  0-15: vlen (e.g. # of struct's members)
> -	 * bits 16-23: unused
> -	 * bits 24-28: kind (e.g. int, ptr, array...etc)
> -	 * bits 29-30: unused
> +	 * bits  0-23: vlen (e.g. # of struct's members)
> +	 * bits 24-30: kind (e.g. int, ptr, array...etc)
>   	 * bit     31: kind_flag, currently used by
>   	 *             struct, union, enum, fwd, enum64,
>   	 *             decl_tag and type_tag
> @@ -65,8 +67,8 @@ struct btf_type {
>   	};
>   };
>   
> -#define BTF_INFO_KIND(info)	(((info) >> 24) & 0x1f)
> -#define BTF_INFO_VLEN(info)	((info) & 0xffff)
> +#define BTF_INFO_KIND(info)	(((info) >> 24) & 0x7f)
> +#define BTF_INFO_VLEN(info)	((info) & 0xffffff)
>   #define BTF_INFO_KFLAG(info)	((info) >> 31)
>   
>   enum {
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index a62d78581207..fedead837b9a 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -182,7 +182,6 @@
>   #define BITS_ROUNDUP_BYTES(bits) \
>   	(BITS_ROUNDDOWN_BYTES(bits) + !!BITS_PER_BYTE_MASKED(bits))
>   
> -#define BTF_INFO_MASK 0x9f00ffff
>   #define BTF_INT_MASK 0x0fffffff
>   #define BTF_TYPE_ID_VALID(type_id) ((type_id) <= BTF_MAX_TYPE)
>   #define BTF_STR_OFFSET_VALID(name_off) ((name_off) <= BTF_MAX_NAME_OFFSET)
> @@ -289,7 +288,7 @@ enum verifier_phase {
>   struct resolve_vertex {
>   	const struct btf_type *t;
>   	u32 type_id;
> -	u16 next_member;
> +	u32 next_member;
>   };
>   
>   enum visit_state {
> @@ -2031,7 +2030,7 @@ static int env_stack_push(struct btf_verifier_env *env,
>   }
>   
>   static void env_stack_set_next_member(struct btf_verifier_env *env,
> -				      u16 next_member)
> +				      u32 next_member)
>   {
>   	env->stack[env->top_stack - 1].next_member = next_member;
>   }
> @@ -3293,7 +3292,7 @@ static s32 btf_struct_check_meta(struct btf_verifier_env *env,
>   	struct btf *btf = env->btf;
>   	u32 struct_size = t->size;
>   	u32 offset;
> -	u16 i;
> +	u32 i;
>   
>   	meta_needed = btf_type_vlen(t) * sizeof(*member);
>   	if (meta_left < meta_needed) {
> @@ -3369,7 +3368,7 @@ static int btf_struct_resolve(struct btf_verifier_env *env,
>   {
>   	const struct btf_member *member;
>   	int err;
> -	u16 i;
> +	u32 i;
>   
>   	/* Before continue resolving the next_member,
>   	 * ensure the last member is indeed resolved to a
> @@ -4447,7 +4446,7 @@ static s32 btf_enum_check_meta(struct btf_verifier_env *env,
>   	const struct btf_enum *enums = btf_type_enum(t);
>   	struct btf *btf = env->btf;
>   	const char *fmt_str;
> -	u16 i, nr_enums;
> +	u32 i, nr_enums;
>   	u32 meta_needed;
>   
>   	nr_enums = btf_type_vlen(t);
> @@ -4555,7 +4554,7 @@ static s32 btf_enum64_check_meta(struct btf_verifier_env *env,
>   	const struct btf_enum64 *enums = btf_type_enum64(t);
>   	struct btf *btf = env->btf;
>   	const char *fmt_str;
> -	u16 i, nr_enums;
> +	u32 i, nr_enums;
>   	u32 meta_needed;
>   
>   	nr_enums = btf_type_vlen(t);
> @@ -4683,7 +4682,7 @@ static void btf_func_proto_log(struct btf_verifier_env *env,
>   			       const struct btf_type *t)
>   {
>   	const struct btf_param *args = (const struct btf_param *)(t + 1);
> -	u16 nr_args = btf_type_vlen(t), i;
> +	u32 nr_args = btf_type_vlen(t), i;
>   
>   	btf_verifier_log(env, "return=%u args=(", t->type);
>   	if (!nr_args) {
> @@ -4929,7 +4928,7 @@ static int btf_datasec_resolve(struct btf_verifier_env *env,
>   {
>   	const struct btf_var_secinfo *vsi;
>   	struct btf *btf = env->btf;
> -	u16 i;
> +	u32 i;
>   
>   	env->resolve_mode = RESOLVE_TBD;
>   	for_each_vsi_from(i, v->next_member, v->t, vsi) {
> @@ -5183,7 +5182,7 @@ static int btf_func_proto_check(struct btf_verifier_env *env,
>   	const struct btf_type *ret_type;
>   	const struct btf_param *args;
>   	const struct btf *btf;
> -	u16 nr_args, i;
> +	u32 nr_args, i;
>   	int err;
>   
>   	btf = env->btf;
> @@ -5278,7 +5277,7 @@ static int btf_func_check(struct btf_verifier_env *env,
>   	const struct btf_type *proto_type;
>   	const struct btf_param *args;
>   	const struct btf *btf;
> -	u16 nr_args, i;
> +	u32 nr_args, i;
>   
>   	btf = env->btf;
>   	proto_type = btf_type_by_id(btf, t->type);
> @@ -5336,12 +5335,6 @@ static s32 btf_check_meta(struct btf_verifier_env *env,
>   	}
>   	meta_left -= sizeof(*t);
>   
> -	if (t->info & ~BTF_INFO_MASK) {
> -		btf_verifier_log(env, "[%u] Invalid btf_info:%x",
> -				 env->log_type_id, t->info);
> -		return -EINVAL;
> -	}
> -
>   	if (BTF_INFO_KIND(t->info) > BTF_KIND_MAX ||
>   	    BTF_INFO_KIND(t->info) == BTF_KIND_UNKN) {
>   		btf_verifier_log(env, "[%u] Invalid kind:%u",
> diff --git a/tools/include/uapi/linux/btf.h b/tools/include/uapi/linux/btf.h
> index 638615ebddc2..618167cab4e6 100644
> --- a/tools/include/uapi/linux/btf.h
> +++ b/tools/include/uapi/linux/btf.h
> @@ -33,20 +33,22 @@ struct btf_header {
>   	__u32	layout_len;	/* length of layout section	*/
>   };
>   
> -/* Max # of type identifier */
> -#define BTF_MAX_TYPE	0x000fffff
> -/* Max offset into the string section */
> -#define BTF_MAX_NAME_OFFSET	0x00ffffff
> -/* Max # of struct/union/enum members or func args */
> -#define BTF_MAX_VLEN	0xffff
> +enum btf_max {
> +	/* Max possible kind */
> +	BTF_MAX_KIND =		0x0000007f,
> +	/* Max # of type identifier */
> +	BTF_MAX_TYPE =		0x000fffff,
> +	/* Max offset into the string section */
> +	BTF_MAX_NAME_OFFSET =	0x00ffffff,
> +	/* Max # of struct/union/enum members or func args */
> +	BTF_MAX_VLEN =		0x00ffffff,
> +};
>   
>   struct btf_type {
>   	__u32 name_off;
>   	/* "info" bits arrangement
> -	 * bits  0-15: vlen (e.g. # of struct's members)
> -	 * bits 16-23: unused
> -	 * bits 24-28: kind (e.g. int, ptr, array...etc)
> -	 * bits 29-30: unused
> +	 * bits  0-23: vlen (e.g. # of struct's members)
> +	 * bits 24-30: kind (e.g. int, ptr, array...etc)
>   	 * bit     31: kind_flag, currently used by
>   	 *             struct, union, enum, fwd, enum64,
>   	 *             decl_tag and type_tag
> @@ -65,8 +67,8 @@ struct btf_type {
>   	};
>   };
>   
> -#define BTF_INFO_KIND(info)	(((info) >> 24) & 0x1f)
> -#define BTF_INFO_VLEN(info)	((info) & 0xffff)
> +#define BTF_INFO_KIND(info)	(((info) >> 24) & 0x7f)
> +#define BTF_INFO_VLEN(info)	((info) & 0xffffff)
>   #define BTF_INFO_KFLAG(info)	((info) >> 31)
>   
>   enum {