[PATCH bpf-next 0/6] btf: Extend vlen, kind in struct btf_type

Alan Maguire
2026-04-14T19:50:45Z

Currently BTF types can have a maximum of 65535 vlen-specified
objects. While this limit has not yet been surpassed for existing
cases (struct/union fields, enum values, function arguments and
datasec), upcoming BTF location information - specifically inline
sites - will hit that limit. Utilize unused BTF info bits in
struct btf_type to increase limit to 24-bits (over 16 million).
This is more than an order of magnitude greater than inline
site counts for the kernel (~400,000) so should be enough for
the near future at least.

Similarly, struct btf_type uses 5 bits for BTF kind values;
currently we use 20, but BTF location information will consume
another 3, and conceivably providing better support for Rust
types in BTF (now that modules can be built using Rust) could
push us close to the max of 31. Use 2 unused bits to provide
a max possible kind of 127.

Patch 1 handles UAPI and kernel-related changes.

Patch 2 updates libbpf to have btf_vlen() return a __u32 instead
of __u16 and updates consumers in libbpf accordingly.

Patch 3 adds a BTF feature check for extended vlen/kind support;
loading BPF program BTF that supports extended vlen/kinds should
fail if the kernel does not support them.

Patch 4 cleans up __u16 vlen usage in bpftool.

Patch 5 tests the BTF extended vlen/kind feature check.

Patch 6 fixes up a test that relies on BTF info overflowing
maximum kind value; fix up the expected error to be an
invalid kind rather than (now impossible) invalid btf_info.

Alan Maguire (6):
bpf: Extend BTF UAPI vlen, kinds to use unused bits
libbpf: Adjust btf_vlen() to return a __u32
libbpf: Add feature for kernel extended vlen/kind support
bpftool: Support 24-bit vlen
selftests/bpf: Test BTF sanitization rejection for invalid vlen
selftests/bpf: Fix up btf/invalid test for extended kind

include/linux/btf.h | 4 +-
include/uapi/linux/btf.h | 26 ++++----
kernel/bpf/btf.c | 25 +++-----
tools/bpf/bpftool/btf.c | 17 ++---
tools/bpf/bpftool/gen.c | 16 +++--
tools/include/uapi/linux/btf.h | 26 ++++----
tools/lib/bpf/btf.c | 26 ++++----
tools/lib/bpf/btf.h | 2 +-
tools/lib/bpf/btf_dump.c | 24 +++----
tools/lib/bpf/features.c | 39 +++++++++++
tools/lib/bpf/libbpf.c | 17 ++++-
tools/lib/bpf/libbpf_internal.h | 2 +
tools/lib/bpf/relo_core.c | 16 ++---
tools/testing/selftests/bpf/prog_tests/btf.c | 4 +-
.../selftests/bpf/prog_tests/btf_sanitize.c | 64 ++++++++++++++++++-
15 files changed, 210 insertions(+), 98 deletions(-)

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2.39.3