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

Alan Maguire
2026-04-17T14:30:51Z

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>
---
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 {
--
2.39.3