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

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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>
This commit is contained in:
Alan Maguire
2026-04-17 15:30:18 +01:00
committed by Alexei Starovoitov
parent 0aa6378695
commit f7a6b9eaff
4 changed files with 40 additions and 43 deletions
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26
include/uapi/linux/btf.h
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@@ -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 {