bpf: Extend BTF UAPI vlen, kinds to use unused bits

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>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260417143023.1551481-2-alan.maguire@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 14 insertions, 12 deletions

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 {