[PATCH v2 0/9] Kernel API Specification Framework

Sasha Levin
2026-03-22T12:10:33Z

This proposal introduces machinery for documenting kernel APIs, addressing the
long-standing challenge of maintaining stable interfaces between the kernel and
user-space programs. Despite the kernel's commitment to never breaking user
space, the lack of machine-readable API specifications has led to breakages and
across system calls and IOCTLs.

Specifications can document parameter types, valid ranges, constraints, and
alignment requirements. They capture return value semantics including success
conditions and error codes with their meaning. Execution context requirements,
capabilities, locking constraints, signal handling behavior, and side effects
can all be formally specified.

These specifications live alongside the code they document and are both
human-readable and machine-parseable. They can be validated at runtime when
CONFIG_KAPI_RUNTIME_CHECKS is enabled, exported via debugfs for userspace
tools, and extracted from either vmlinux or source code.

This enables static analysis tools to verify userspace API usage at compile
time, test generation based on formal specifications, consistent error handling
validation, automated documentation generation, and formal verification of
kernel interfaces.

The implementation includes a core framework with ELF section storage,
kerneldoc integration for inline specification, a debugfs interface for runtime
querying, and a Rust-based extraction tool (tools/kapi) supporting JSON, RST,
and plain text output formats. Example specifications are provided for the four
fundamental file syscalls (sys_open, sys_close, sys_read, sys_write). The
series also includes a KUnit test suite with 38 tests and a runtime
verification selftest with 29+ TAP tests.

The series with runtime testing enabled (CONFIG_KAPI_RUNTIME_CHECKS=y)
currently survives LTP tests in a KVM VM.

Changes since v1:

- Removed DEFINE_KERNEL_API_SPEC macro from user-facing documentation and
examples. The macros are now internal plumbing only; kerneldoc annotations
are the sole authoring interface. (Jonathan Corbet, Mauro Carvalho Chehab)

- Removed IOCTL specification section from documentation, as no IOCTL specs
are included in this series. (Jonathan Corbet)

- Removed since-version field entirely from the framework: struct
kernel_api_spec, debugfs output, JSON export, kdoc parser, and all
extractors/formatters in the kapi tool. (Jonathan Corbet)

- Removed stale :Date: field from documentation. (Jonathan Corbet)

- Fixed kmalloc documentation example to include both parameters (size and
flags) and a side-effect entry, matching the output example. (Jonathan Corbet)

- Simplified DSL references in documentation: converted Common Patterns
section from raw macros to kerneldoc annotation format, removed macro
references from Implementation Details and Troubleshooting. (Jonathan Corbet, Mauro Carvalho Chehab)

- Reworded sys_close spec to describe behavior without referencing kernel
internal callbacks: "flush callback" replaced with "close-time flush
operation", "release callback" references removed entirely. (Greg Kroah-Hartman)

- Removed HP-UX reference and "implementation-defined behavior" phrasing
from sys_close spec. (Greg Kroah-Hartman)

- Added copyright lines to all new files: C headers, kernel modules, Python
scripts, Rust source files, and selftests. (Greg Kroah-Hartman)

- Fixed rebasing artifact where patch 3 removed content from patch 1's
documentation additions. (Greg Kroah-Hartman)

- Removed unnecessary pr_info() from debugfs init and braces from
single-line for loop. (Greg Kroah-Hartman)

- Added commit message changelogs to all patches. (Greg Kroah-Hartman)

References:

v1: https://lore.kernel.org/all/20260313150928.2637368-1-sashal@kernel.org/
RFC v5: https://lore.kernel.org/lkml/20251218204239.4159453-1-sashal@kernel.org/
RFC v4: https://lore.kernel.org/lkml/20250825181434.3340805-1-sashal@kernel.org/
RFC v3: https://lore.kernel.org/lkml/20250711114248.2288591-1-sashal@kernel.org/
RFC v2: https://lore.kernel.org/lkml/20250624180742.5795-1-sashal@kernel.org/
RFC v1: https://lore.kernel.org/lkml/20250614134858.790460-1-sashal@kernel.org/

Sasha Levin (9):
kernel/api: introduce kernel API specification framework
kernel/api: enable kerneldoc-based API specifications
kernel/api: add debugfs interface for kernel API specifications
tools/kapi: Add kernel API specification extraction tool
kernel/api: add API specification for sys_open
kernel/api: add API specification for sys_close
kernel/api: add API specification for sys_read
kernel/api: add API specification for sys_write
kernel/api: add runtime verification selftest

.gitignore | 1 +
Documentation/dev-tools/index.rst | 1 +
Documentation/dev-tools/kernel-api-spec.rst | 620 +++++++
MAINTAINERS | 12 +
arch/x86/include/asm/syscall_wrapper.h | 40 +
fs/open.c | 565 +++++-
fs/read_write.c | 683 +++++++
include/asm-generic/vmlinux.lds.h | 28 +
include/linux/kernel_api_spec.h | 1580 +++++++++++++++++
include/linux/syscall_api_spec.h | 186 ++
include/linux/syscalls.h | 39 +
init/Kconfig | 2 +
kernel/Makefile | 3 +
kernel/api/.gitignore | 2 +
kernel/api/Kconfig | 70 +
kernel/api/Makefile | 14 +
kernel/api/kapi_debugfs.c | 499 ++++++
kernel/api/kapi_kunit.c | 538 ++++++
kernel/api/kernel_api_spec.c | 1277 +++++++++++++
scripts/Makefile.build | 31 +
scripts/Makefile.clean | 3 +
tools/docs/kernel-doc | 5 +
tools/kapi/.gitignore | 4 +
tools/kapi/Cargo.toml | 19 +
tools/kapi/src/extractor/debugfs.rs | 578 ++++++
tools/kapi/src/extractor/kerneldoc_parser.rs | 1555 ++++++++++++++++
tools/kapi/src/extractor/mod.rs | 461 +++++
tools/kapi/src/extractor/source_parser.rs | 408 +++++
.../src/extractor/vmlinux/binary_utils.rs | 508 ++++++
.../src/extractor/vmlinux/magic_finder.rs | 115 ++
tools/kapi/src/extractor/vmlinux/mod.rs | 844 +++++++++
tools/kapi/src/formatter/json.rs | 720 ++++++++
tools/kapi/src/formatter/mod.rs | 142 ++
tools/kapi/src/formatter/plain.rs | 707 ++++++++
tools/kapi/src/formatter/rst.rs | 850 +++++++++
tools/kapi/src/main.rs | 122 ++
tools/lib/python/kdoc/kdoc_apispec.py | 888 +++++++++
tools/lib/python/kdoc/kdoc_output.py | 9 +-
tools/lib/python/kdoc/kdoc_parser.py | 85 +-
tools/testing/selftests/kapi/Makefile | 7 +
tools/testing/selftests/kapi/kapi_test_util.h | 33 +
tools/testing/selftests/kapi/test_kapi.c | 1023 +++++++++++
42 files changed, 15268 insertions(+), 9 deletions(-)
create mode 100644 Documentation/dev-tools/kernel-api-spec.rst
create mode 100644 include/linux/kernel_api_spec.h
create mode 100644 include/linux/syscall_api_spec.h
create mode 100644 kernel/api/.gitignore
create mode 100644 kernel/api/Kconfig
create mode 100644 kernel/api/Makefile
create mode 100644 kernel/api/kapi_debugfs.c
create mode 100644 kernel/api/kapi_kunit.c
create mode 100644 kernel/api/kernel_api_spec.c
create mode 100644 tools/kapi/.gitignore
create mode 100644 tools/kapi/Cargo.toml
create mode 100644 tools/kapi/src/extractor/debugfs.rs
create mode 100644 tools/kapi/src/extractor/kerneldoc_parser.rs
create mode 100644 tools/kapi/src/extractor/mod.rs
create mode 100644 tools/kapi/src/extractor/source_parser.rs
create mode 100644 tools/kapi/src/extractor/vmlinux/binary_utils.rs
create mode 100644 tools/kapi/src/extractor/vmlinux/magic_finder.rs
create mode 100644 tools/kapi/src/extractor/vmlinux/mod.rs
create mode 100644 tools/kapi/src/formatter/json.rs
create mode 100644 tools/kapi/src/formatter/mod.rs
create mode 100644 tools/kapi/src/formatter/plain.rs
create mode 100644 tools/kapi/src/formatter/rst.rs
create mode 100644 tools/kapi/src/main.rs
create mode 100644 tools/lib/python/kdoc/kdoc_apispec.py
create mode 100644 tools/testing/selftests/kapi/Makefile
create mode 100644 tools/testing/selftests/kapi/kapi_test_util.h
create mode 100644 tools/testing/selftests/kapi/test_kapi.c

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2.51.0