The kernel community has rigorously enforced a policy of backwards
compatibility in its UAPI headers for a long time. This has allowed user
applications to enjoy stability across kernel upgrades without
recompiling. Our goal is to add tooling and documentation to help kernel
developers maintain this stability.
We see in the kernel documentation:
"Kernel headers are backwards compatible, but not forwards compatible.
This means that a program built against a C library using older kernel
headers should run on a newer kernel (although it may not have access
to new features), but a program built against newer kernel headers may
not work on an older kernel."[1]
How does the kernel community enforce this guarantee? As we understand it,
it's enforced with thorough code review and testing. Is there any tooling
outside of this being used to help the process?
Also, could documentation on UAPI maintenance (from a developer's point
of view) be expanded? Internally, we have a set of guidelines for our
kernel developers regarding UAPI compatibility techniques. If there's
interest in supplying a document on this topic with the kernel, we'd be
happy to submit a draft detailing what we have so far as a jumping off
point.
In terms of tooling, I've attached a shell script we've been using
internally to validate backwards compatibility of our UAPI headers. The
script uses libabigail's[2] tool abidiff[3] to compare a modified
header's ABI before and after a patch is applied. If an existing UAPI is
modified, the script exits non-zero. We use this script in our
continuous integration system to block changes that fail the check.
It generates output like this when a backwards-incompatible change is made
to a UAPI header:
!!! ABI differences detected in include/uapi/linux/bpf.h from HEAD~1 -> HEAD !!!
[C] 'struct bpf_insn' changed:
type size hasn't changed
1 data member change:
type of '__s32 imm' changed:
typedef name changed from __s32 to __u32 at int-ll64.h:27:1
underlying type 'int' changed:
type name changed from 'int' to 'unsigned int'
type size hasn't changed
We wanted to share this script with the community and hopefully also
receive general feedback when it comes to tooling/policy surrounding this
issue. Our hope is that the script will help kernel UAPI authors maintain
good discipline and avoid breaking userspace.
In v4, we've updated the script to operate exclusively on the trees
generated by "make headers_install" at the two git references. This
catches several classes of false negatives brought up in earlier
revisions.
Thanks for the helpful reviews of previous revs! We're looking forward
to any additional feedback you may have on v4.
[1] Documentation/kbuild/headers_install.rst
[2] https://sourceware.org/libabigail/manual/libabigail-overview.html
[3] https://sourceware.org/libabigail/manual/abidiff.html
P.S. While at Qualcomm, Jordan Crouse <[email protected]> authored the
original version of the UAPI checker script. Thanks Jordan!<Paste>
John Moon (2):
check-uapi: Introduce check-uapi.sh
docs: dev-tools: Add UAPI checker documentation
Documentation/dev-tools/checkuapi.rst | 479 +++++++++++++++++++++++++
Documentation/dev-tools/index.rst | 1 +
scripts/check-uapi.sh | 488 ++++++++++++++++++++++++++
3 files changed, 968 insertions(+)
create mode 100644 Documentation/dev-tools/checkuapi.rst
create mode 100755 scripts/check-uapi.sh
base-commit: f2afccfefe7be1f7346564fe619277110d341f9b
--
2.17.1
While the kernel community has been good at maintaining backwards
compatibility with kernel UAPIs, it would be helpful to have a tool
to check if a commit introduces changes that break backwards
compatibility.
To that end, introduce check-uapi.sh: a simple shell script that
checks for changes to UAPI headers using libabigail.
libabigail is "a framework which aims at helping developers and
software distributors to spot some ABI-related issues like interface
incompatibility in ELF shared libraries by performing a static
analysis of the ELF binaries at hand."
The script uses one of libabigail's tools, "abidiff", to compile the
changed header before and after the commit to detect any changes.
abidiff "compares the ABI of two shared libraries in ELF format. It
emits a meaningful report describing the differences between the two
ABIs."
The script also includes the ability to check the compatibility of
all UAPI headers across commits. This allows developers to inspect
the stability of the UAPIs over time.
Signed-off-by: John Moon <[email protected]>
---
- Refactored to exclusively check headers installed by make
headers_install. This simplified the code dramatically and removed
the need to perform complex git diffs.
- Removed the "-m" flag. Since we're checking all installed headers
every time, a flag to check only modified files didn't make sense.
- Added info message when usr/include/Makefile is not present that
it's likely because that file was only introduced in v5.3.
- Changed default behavior of log file. Now, the script will not
create a log file unless you pass "-l <file>".
- Simplified exit handler.
- Added -j $MAX_THREADS to make headers_install to improve speed.
- Cleaned up variable references.
scripts/check-uapi.sh | 488 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 488 insertions(+)
create mode 100755 scripts/check-uapi.sh
diff --git a/scripts/check-uapi.sh b/scripts/check-uapi.sh
new file mode 100755
index 000000000000..1dc50f67ac3e
--- /dev/null
+++ b/scripts/check-uapi.sh
@@ -0,0 +1,488 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0-only
+# Script to check commits for UAPI backwards compatibility
+
+set -o errexit
+set -o pipefail
+
+print_usage() {
+ name=$(basename "$0")
+ cat << EOF
+$name - check for UAPI header stability across Git commits
+
+By default, the script will check to make sure the latest commit (or current
+dirty changes) did not introduce ABI changes when compared to HEAD^1. You can
+check against additional commit ranges with the -b and -p options.
+
+The script will not check UAPI headers for architectures other than the one
+defined in ARCH.
+
+Usage: $name [-b BASE_REF] [-p PAST_REF] [-j N] [-l ERROR_LOG] [-q] [-v]
+
+Options:
+ -b BASE_REF Base git reference to use for comparison. If unspecified or empty,
+ will use any dirty changes in tree to UAPI files. If there are no
+ dirty changes, HEAD will be used.
+ -p PAST_REF Compare BASE_REF to PAST_REF (e.g. -p v6.1). If unspecified or empty,
+ will use BASE_REF^1. Must be an ancestor of BASE_REF. Only headers
+ that exist on PAST_REF will be checked for compatibility.
+ -j JOBS Number of checks to run in parallel (default: number of CPU cores).
+ -l ERROR_LOG Write error log to file (default: no error log is generated).
+ -q Quiet operation (suppress all stdout, still print stderr).
+ -v Verbose operation (print more information about each header being checked).
+
+Environmental args:
+ ABIDIFF Custom path to abidiff binary
+ CC C compiler (default is "gcc")
+ ARCH Target architecture of C compiler (default is host arch)
+
+Exit codes:
+ $SUCCESS) Success
+ $FAIL_ABI) ABI difference detected
+ $FAIL_PREREQ) Prerequisite not met
+ $FAIL_COMPILE) Compilation error
+EOF
+}
+
+readonly SUCCESS=0
+readonly FAIL_ABI=1
+readonly FAIL_PREREQ=2
+readonly FAIL_COMPILE=3
+
+# Print to stderr
+eprintf() {
+ # shellcheck disable=SC2059
+ printf "$@" >&2
+}
+
+# Check if git tree is dirty
+tree_is_dirty() {
+ if git diff --quiet; then
+ return 1
+ else
+ return 0
+ fi
+}
+
+# Get list of files installed in $ref
+get_file_list() {
+ local -r ref="$1"
+ local -r tree="$(get_header_tree "$ref")"
+
+ # Print all installed headers, filtering out ones that can't be compiled
+ find "$tree" -type f -name '*.h' -printf '%P\n' | grep -v -f "$INCOMPAT_LIST"
+}
+
+# Add to the list of incompatible headers
+add_to_incompat_list() {
+ local -r ref="$1"
+
+ # Start with the usr/include/Makefile to get a list of the headers
+ # that don't compile using this method.
+ if [ ! -f usr/include/Makefile ]; then
+ eprintf "error - no usr/include/Makefile present at %s\n" "$ref"
+ eprintf "Note: usr/include/Makefile was added in the v5.3 kernel release\n"
+ exit "$FAIL_PREREQ"
+ fi
+ {
+ # shellcheck disable=SC2016
+ printf 'all: ; @echo $(no-header-test)\n'
+ cat usr/include/Makefile
+ } | SRCARCH="$ARCH" make -f - | tr " " "\n" | grep -v "asm-generic" >> "$INCOMPAT_LIST"
+
+ # The makefile also skips all asm-generic files, but prints "asm-generic/%"
+ # which won't work for our grep match. Instead, print something grep will match.
+ printf "asm-generic/.*\.h\n" >> "$INCOMPAT_LIST"
+
+ sort -u -o "$INCOMPAT_LIST" "$INCOMPAT_LIST"
+}
+
+# Compile the simple test app
+do_compile() {
+ local -r inc_dir="$1"
+ local -r header="$2"
+ local -r out="$3"
+ printf "int main(void) { return 0; }\n" | \
+ "$CC" -c \
+ -o "$out" \
+ -x c \
+ -O0 \
+ -std=c90 \
+ -fno-eliminate-unused-debug-types \
+ -g \
+ "-I${inc_dir}" \
+ -include "$header" \
+ -
+}
+
+# Save the current git tree state, stashing if needed
+save_tree_state() {
+ printf "Saving current tree state... "
+ current_ref="$(git rev-parse HEAD)"
+ readonly current_ref
+ if tree_is_dirty; then
+ unstash="true"
+ git stash push --quiet
+ fi
+ printf "OK\n"
+}
+
+# Restore the git tree state, unstashing if needed
+restore_tree_state() {
+ if [ -z "$current_ref" ]; then
+ return 0
+ fi
+
+ printf "Restoring current tree state... "
+ git checkout --quiet "$current_ref"
+ if [ "$unstash" = "true" ]; then
+ git stash pop --quiet
+ unstash="false"
+ fi
+ printf "OK\n"
+}
+
+# Handle exit cleanup
+exit_handler() {
+ if [ "$DEVIATED_FROM_CURRENT_TREE" = "true" ]; then
+ restore_tree_state
+ fi
+
+ rm -rf "$TMP_DIR"
+}
+
+# Install headers for both git refs
+install_headers() {
+ local -r base_ref="$1"
+ local -r past_ref="$2"
+
+ DEVIATED_FROM_CURRENT_TREE="false"
+ for ref in "$base_ref" "$past_ref"; do
+ if [ -n "$ref" ]; then
+ if [ "$DEVIATED_FROM_CURRENT_TREE" = "false" ]; then
+ save_tree_state
+ DEVIATED_FROM_CURRENT_TREE="true"
+ fi
+ # This script ($0) is already loaded into memory at this point,
+ # so this operation is safe
+ git checkout --quiet "$(git rev-parse "$ref")"
+ fi
+
+ printf "Installing sanitized UAPI headers from %s... " "${ref:-dirty tree}"
+ make -j "$MAX_THREADS" ARCH="$ARCH" INSTALL_HDR_PATH="${TMP_DIR}/${ref}/usr" headers_install > /dev/null 2>&1
+ printf "OK\n"
+
+ # Add to list of incompatible headers while we have $ref checked out
+ add_to_incompat_list "$ref" "$INCOMPAT_LIST"
+ done
+
+ restore_tree_state
+ DEVIATED_FROM_CURRENT_TREE="false"
+}
+
+# Print the path to the headers_install tree for a given ref
+get_header_tree() {
+ local -r ref="$1"
+ printf "%s" "${TMP_DIR}/${ref}/usr"
+}
+
+# Check file list for UAPI compatibility
+check_uapi_files() {
+ local -r base_ref="$1"
+ local -r past_ref="$2"
+
+ local passed=0;
+ local failed=0;
+ local -a threads=()
+
+ printf "Checking changes to UAPI headers between %s and %s\n" "$past_ref" "${base_ref:-dirty tree}"
+ # Loop over all UAPI headers that were installed by $past_ref (if they only exist on $base_ref,
+ # there's no way they're broken and no way to compare anyway)
+ while read -r file; do
+ if [ "${#threads[@]}" -ge "$MAX_THREADS" ]; then
+ if wait "${threads[0]}"; then
+ passed=$((passed + 1))
+ else
+ failed=$((failed + 1))
+ fi
+ threads=("${threads[@]:1}")
+ fi
+
+ check_individual_file "$base_ref" "$past_ref" "$file" &
+ threads+=("$!")
+ done < <(get_file_list "$past_ref")
+
+ for t in "${threads[@]}"; do
+ if wait "$t"; then
+ passed=$((passed + 1))
+ else
+ failed=$((failed + 1))
+ fi
+ done
+
+ total="$((passed + failed))"
+ if [ "$failed" -gt 0 ]; then
+ eprintf "error - %d/%d UAPI headers compatible with %s appear _not_ to be backwards compatible\n" "$failed" "$total" "$ARCH"
+ else
+ printf "All %d UAPI headers compatible with %s appear to be backwards compatible\n" "$total" "$ARCH"
+ fi
+
+ return "$failed"
+}
+
+# Check an individual file for UAPI compatibility
+check_individual_file() {
+ local -r base_ref="$1"
+ local -r past_ref="$2"
+ local -r file="$3"
+
+ local -r base_header="$(get_header_tree "$base_ref")/${file}"
+ local -r past_header="$(get_header_tree "$past_ref")/${file}"
+
+ if [ ! -f "$base_header" ]; then
+ printf "error - UAPI header %s was incorrectly removed\n" "$file" | tee "${base_header}.error" >&2
+ return 1
+ fi
+
+ compare_abi "$file" "$base_header" "$past_header" "$base_ref" "$past_ref"
+}
+
+# Perform the A/B compilation and compare output ABI
+compare_abi() {
+ local -r file="$1"
+ local -r base_header="$2"
+ local -r past_header="$3"
+ local -r base_ref="$4"
+ local -r past_ref="$5"
+ local -r log="${TMP_DIR}/log/${file}.log"
+
+ mkdir -p "$(dirname "$log")"
+
+ if ! do_compile "$(get_header_tree "$base_ref")/include" "$base_header" "${base_header}.bin" 2> "$log"; then
+ eprintf "error - couldn't compile version of UAPI header %s at %s\n" "$file" "$base_ref"
+ cat "$log" >&2
+ exit "$FAIL_COMPILE"
+ fi
+
+ if ! do_compile "$(get_header_tree "$past_ref")/include" "$past_header" "${past_header}.bin" 2> "$log"; then
+ eprintf "error - couldn't compile version of UAPI header %s at %s\n" "$file" "$past_ref"
+ cat "$log" >&2
+ exit "$FAIL_COMPILE"
+ fi
+
+ "$ABIDIFF" --non-reachable-types "${past_header}.bin" "${base_header}.bin" > "$log" && ret="$?" || ret="$?"
+ if [ "$ret" -eq 0 ]; then
+ if [ "$VERBOSE" = "true" ]; then
+ printf "No ABI differences detected in %s from %s -> %s\n" "$file" "$past_ref" "${base_ref:-dirty tree}"
+ fi
+ else
+ # Bits in abidiff's return code can be used to determine the type of error
+ if [ $(("$ret" & 0x1)) -gt 0 ]; then
+ eprintf "error - abidiff did not run properly\n"
+ exit 1
+ fi
+
+ # If the only changes were additions (not modifications to existing APIs), then
+ # there's no problem. Ignore these diffs.
+ if grep "Unreachable types summary" "$log" | grep -q "0 removed" &&
+ grep "Unreachable types summary" "$log" | grep -q "0 changed"; then
+ return 0
+ fi
+ {
+ printf "!!! ABI differences detected in %s from %s -> %s !!!\n\n" "$file" "$past_ref" "${base_ref:-dirty tree}"
+ sed -e '/summary:/d' -e '/changed type/d' -e '/^$/d' -e 's/^/ /g' "$log"
+
+ if ! cmp "$past_header" "$base_header" > /dev/null 2>&1; then
+ printf "\nHeader file diff (after headers_install):\n"
+ diff -Naur "$past_header" "$base_header" \
+ | sed -e "s|${past_header}|${past_ref}/${file}|g" \
+ -e "s|${base_header}|${base_ref:-dirty}/${file}|g"
+ printf "\n"
+ else
+ printf "\n%s did not change between %s and %s...\n" "$file" "$past_ref" "${base_ref:-dirty tree}"
+ printf "It's possible a change to one of the headers it includes caused this error:\n"
+ grep '^#include' "$base_header"
+ printf "\n"
+ fi
+ } | tee "${base_header}.error" >&2
+ return 1
+ fi
+}
+
+min_version_is_satisfied() {
+ local -r min_version="$1"
+ local -r version_installed="$2"
+
+ printf "%s\n%s\n" "$min_version" "$version_installed" | sort -Vc > /dev/null 2>&1
+}
+
+# Make sure we have the tools we need and the arguments make sense
+check_deps() {
+ ABIDIFF="${ABIDIFF:-abidiff}"
+ CC="${CC:-gcc}"
+ ARCH="${ARCH:-$(uname -m)}"
+ if [ "$ARCH" = "x86_64" ]; then
+ ARCH="x86"
+ fi
+
+ local -r abidiff_min_version="1.7"
+ local -r libdw_min_version_if_clang="0.171"
+
+ if ! command -v "$ABIDIFF" > /dev/null 2>&1; then
+ eprintf "error - abidiff not found!\n"
+ eprintf "Please install abigail-tools version %s or greater\n" "$abidiff_min_version"
+ eprintf "See: https://sourceware.org/libabigail/manual/libabigail-overview.html\n"
+ return 1
+ fi
+
+ local -r abidiff_version="$("$ABIDIFF" --version | cut -d ' ' -f 2)"
+ if ! min_version_is_satisfied "$abidiff_min_version" "$abidiff_version"; then
+ eprintf "error - abidiff version too old: %s\n" "$abidiff_version"
+ eprintf "Please install abigail-tools version %s or greater\n" "$abidiff_min_version"
+ eprintf "See: https://sourceware.org/libabigail/manual/libabigail-overview.html\n"
+ return 1
+ fi
+
+ if ! command -v "$CC" > /dev/null 2>&1; then
+ eprintf 'error - %s not found\n' "$CC"
+ return 1
+ fi
+
+ if "$CC" --version | grep -q clang; then
+ local -r libdw_version="$(ldconfig -v 2>/dev/null | grep -v SKIPPED | grep -m 1 -o 'libdw-[0-9]\+.[0-9]\+' | cut -c 7-)"
+ if ! min_version_is_satisfied "$libdw_min_version_if_clang" "$libdw_version"; then
+ eprintf "error - libdw version too old for use with clang: %s\n" "$libdw_version"
+ eprintf "Please install libdw from elfutils version %s or greater\n" "$libdw_min_version_if_clang"
+ eprintf "See: https://sourceware.org/elfutils/\n"
+ return 1
+ fi
+ fi
+
+ if [ ! -d "arch/${ARCH}" ]; then
+ eprintf 'error - ARCH "%s" is not a subdirectory under arch/\n' "$ARCH"
+ eprintf "Please set ARCH to one of:\n%s\n" "$(find arch -maxdepth 1 -mindepth 1 -type d -printf '%f ' | fmt)"
+ return 1
+ fi
+
+ if ! git rev-parse --is-inside-work-tree > /dev/null 2>&1; then
+ eprintf "error - this script requires the kernel tree to be initialized with Git\n"
+ return 1
+ fi
+
+ if ! git rev-parse --verify "$past_ref" > /dev/null 2>&1; then
+ printf 'error - invalid git reference "%s"\n' "$past_ref"
+ return 1
+ fi
+
+ if [ -n "$base_ref" ]; then
+ if ! git merge-base --is-ancestor "$past_ref" "$base_ref" > /dev/null 2>&1; then
+ printf 'error - "%s" is not an ancestor of base ref "%s"\n' "$past_ref" "$base_ref"
+ return 1
+ fi
+ if [ "$(git rev-parse "$base_ref")" = "$(git rev-parse "$past_ref")" ]; then
+ printf 'error - "%s" and "%s" are the same reference\n' "$past_ref" "$base_ref"
+ return 1
+ fi
+ fi
+}
+
+run() {
+ local base_ref="$1"
+ local past_ref="$2"
+ local abi_error_log="$3"
+ shift 3
+
+ if [ -z "$KERNEL_SRC" ]; then
+ KERNEL_SRC="$(realpath "$(dirname "$0")"/..)"
+ fi
+
+ cd "$KERNEL_SRC"
+
+ if [ -z "$base_ref" ] && ! tree_is_dirty; then
+ base_ref=HEAD
+ fi
+
+ if [ -z "$past_ref" ]; then
+ if [ -n "$base_ref" ]; then
+ past_ref="${base_ref}^1"
+ else
+ past_ref=HEAD
+ fi
+ fi
+
+ if ! check_deps; then
+ exit "$FAIL_PREREQ"
+ fi
+
+ TMP_DIR=$(mktemp -d)
+ readonly TMP_DIR
+ trap 'exit_handler' EXIT
+
+ readonly INCOMPAT_LIST="${TMP_DIR}/incompat_list.txt"
+ touch "$INCOMPAT_LIST"
+
+ # Run make install_headers for both refs
+ install_headers "$base_ref" "$past_ref"
+
+ # Check for any differences in the installed header trees
+ if diff -r -q "$(get_header_tree "$base_ref")" "$(get_header_tree "$past_ref")" > /dev/null 2>&1; then
+ printf "No changes to UAPI headers were applied between %s and %s\n" "$past_ref" "${base_ref:-dirty tree}"
+ exit "$SUCCESS"
+ fi
+
+ if ! check_uapi_files "$base_ref" "$past_ref"; then
+ eprintf "error - UAPI header ABI check failed\n"
+ if [ -n "$abi_error_log" ]; then
+ {
+ printf 'Generated by "%s %s" from git ref %s\n\n' "$0" "$*" "$(git rev-parse HEAD)"
+ find "$TMP_DIR" -type f -name '*.error' -exec cat {} +
+ } > "$abi_error_log"
+ eprintf "Failure summary saved to %s\n" "$abi_error_log"
+ fi
+ exit "$FAIL_ABI"
+ fi
+}
+
+main() {
+ MAX_THREADS=$(nproc)
+ VERBOSE="false"
+ local base_ref=""
+ local quiet="false"
+ while getopts "hb:p:mj:l:qv" opt; do
+ case $opt in
+ h)
+ print_usage
+ exit "$SUCCESS"
+ ;;
+ b)
+ base_ref="$OPTARG"
+ ;;
+ p)
+ past_ref="$OPTARG"
+ ;;
+ j)
+ MAX_THREADS="$OPTARG"
+ ;;
+ l)
+ abi_error_log="$OPTARG"
+ ;;
+ q)
+ quiet="true"
+ ;;
+ v)
+ VERBOSE="true"
+ ;;
+ *)
+ exit "$FAIL_PREREQ"
+ esac
+ done
+
+
+ if [ "$quiet" = "true" ]; then
+ run "$base_ref" "$past_ref" "$abi_error_log" "$@" > /dev/null
+ else
+ run "$base_ref" "$past_ref" "$abi_error_log" "$@"
+ fi
+}
+
+main "$@"
--
2.17.1
Add detailed documentation for scripts/check-uapi.sh.
Signed-off-by: John Moon <[email protected]>
Acked-by: Nick Desaulniers <[email protected]>
---
- Removed references to "-m" mode
- Updated script outputs
- Added example of UAPI removal via Kbuild
Documentation/dev-tools/checkuapi.rst | 479 ++++++++++++++++++++++++++
Documentation/dev-tools/index.rst | 1 +
2 files changed, 480 insertions(+)
create mode 100644 Documentation/dev-tools/checkuapi.rst
diff --git a/Documentation/dev-tools/checkuapi.rst b/Documentation/dev-tools/checkuapi.rst
new file mode 100644
index 000000000000..c67598d03cdb
--- /dev/null
+++ b/Documentation/dev-tools/checkuapi.rst
@@ -0,0 +1,479 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+============
+UAPI Checker
+============
+
+The UAPI checker (``scripts/check-uapi.sh``) is a shell script which checks
+UAPI header files for userspace backwards-compatibility across the git tree.
+
+The script can produce false positives in some cases, so developers are
+encouraged to use their best judgement when interpreting the results. Please
+refer to kernel documentation on the topic of IOCTL stability for more
+information (Documentation/process/botching-up-ioctls.rst).
+
+Options
+=======
+
+This section will describe the options ``check-uapi.sh`` can be run with.
+
+Usage::
+
+ check-uapi.sh [-b BASE_REF] [-p PAST_REF] [-j N] [-l ERROR_LOG] [-q] [-v]
+
+Available options::
+
+ -b BASE_REF Base git reference to use for comparison. If unspecified or empty,
+ will use any dirty changes in tree to UAPI files. If there are no
+ dirty changes, HEAD will be used.
+ -p PAST_REF Compare BASE_REF to PAST_REF (e.g. -p v6.1). If unspecified or empty,
+ will use BASE_REF^1. Must be an ancestor of BASE_REF. Only headers
+ that exist on PAST_REF will be checked for compatibility.
+ -j JOBS Number of checks to run in parallel (default: number of CPU cores).
+ -l ERROR_LOG Write error log to file (default: no error log is generated).
+ -q Quiet operation (suppress all stdout, still print stderr).
+ -v Verbose operation (print more information about each header being checked).
+
+Environmental args::
+
+ ABIDIFF Custom path to abidiff binary
+ CC C compiler (default is "gcc")
+ ARCH Target architecture of C compiler (default is host arch)
+
+Exit codes::
+
+ 0) Success
+ 1) ABI difference detected
+ 2) Prerequisite not met
+ 3) Compilation error
+
+Examples
+========
+
+Basic Usage
+-----------
+
+First, let's try making a change to a UAPI header file that obviously won't
+break userspace::
+
+ cat << 'EOF' | patch -l -p1
+ --- a/include/uapi/linux/acct.h
+ +++ b/include/uapi/linux/acct.h
+ @@ -21,7 +21,9 @@
+ #include <asm/param.h>
+ #include <asm/byteorder.h>
+
+ -/*
+ +#define FOO
+ +
+ +/*
+ * comp_t is a 16-bit "floating" point number with a 3-bit base 8
+ * exponent and a 13-bit fraction.
+ * comp2_t is 24-bit with 5-bit base 2 exponent and 20 bit fraction
+ diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
+ EOF
+
+Now, let's use the script to validate::
+
+ % ./scripts/check-uapi.sh
+ Installing sanitized UAPI headers from dirty tree... OK
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from HEAD... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between HEAD and dirty tree
+ All 906 UAPI headers compatible with x86 appear to be backwards compatible
+
+Let's add another change that *would* break userspace::
+
+ cat << 'EOF' | patch -l -p1
+ --- a/include/uapi/linux/bpf.h 2023-02-28 13:32:36.505591077 -0800
+ +++ b/include/uapi/linux/bpf.h 2023-02-28 13:32:57.033494020 -0800
+ @@ -73,7 +73,7 @@
+ __u8 dst_reg:4; /* dest register */
+ __u8 src_reg:4; /* source register */
+ __s16 off; /* signed offset */
+ - __s32 imm; /* signed immediate constant */
+ + __u32 imm; /* unsigned immediate constant */
+ };
+
+ /* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
+ EOF
+
+The script should catch this incompatibility::
+
+ % ./scripts/check-uapi.sh
+ Installing sanitized UAPI headers from dirty tree... OK
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from HEAD... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between HEAD and dirty tree
+ !!! ABI differences detected in include/linux/bpf.h from HEAD -> dirty tree !!!
+
+ [C] 'struct bpf_insn' changed:
+ type size hasn't changed
+ 1 data member change:
+ type of '__s32 imm' changed:
+ typedef name changed from __s32 to __u32 at int-ll64.h:27:1
+ underlying type 'int' changed:
+ type name changed from 'int' to 'unsigned int'
+ type size hasn't changed
+
+ Header file diff (after headers_install):
+ --- HEAD/include/linux/bpf.h 2023-03-24 16:34:58.901204604 -0700
+ +++ dirty/include/linux/bpf.h 2023-03-24 16:34:56.973213331 -0700
+ @@ -73,7 +73,7 @@
+ __u8 dst_reg:4; /* dest register */
+ __u8 src_reg:4; /* source register */
+ __s16 off; /* signed offset */
+ - __s32 imm; /* signed immediate constant */
+ + __u32 imm; /* unsigned immediate constant */
+ };
+
+ /* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
+
+ error - 1/906 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+ error - UAPI header ABI check failed
+
+The script finds the ABI breakage and reports it (along with a diff of the
+offending file).
+
+Let's commit the breaking change, then commit the good change::
+
+ % git commit -m 'Breaking UAPI change' include/uapi/linux/bpf.h
+ [detached HEAD f758e574663a] Breaking UAPI change
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+ % git commit -m 'Innocuous UAPI change' include/uapi/linux/acct.h
+ [detached HEAD 2e87df769081] Innocuous UAPI change
+ 1 file changed, 3 insertions(+), 1 deletion(-)
+
+Now, let's run the script again with no arguments::
+
+ % ./scripts/check-uapi.sh
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from HEAD... OK
+ Installing sanitized UAPI headers from HEAD^1... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between HEAD^1 and HEAD
+
+It doesn't catch any breaking change because, by default, it only compares
+``HEAD`` to ``HEAD^1``. The breaking change was committed on ``HEAD~2``. If we
+wanted the search scope to go back further, we'd have to use the ``-p`` option
+to pass a different past reference to compare to. In this case, let's pass
+``-p HEAD~2`` to the script so it checks UAPI changes between ``HEAD~2`` and
+``HEAD``::
+
+ % ./scripts/check-uapi.sh -p HEAD~2
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from HEAD... OK
+ Installing sanitized UAPI headers from HEAD~2... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between HEAD~2 and HEAD
+ !!! ABI differences detected in include/linux/bpf.h from HEAD~2 -> HEAD !!!
+
+ [C] 'struct bpf_insn' changed:
+ type size hasn't changed
+ 1 data member change:
+ type of '__s32 imm' changed:
+ typedef name changed from __s32 to __u32 at int-ll64.h:27:1
+ underlying type 'int' changed:
+ type name changed from 'int' to 'unsigned int'
+ type size hasn't changed
+
+ Header file diff (after headers_install):
+ --- HEAD~2/include/linux/bpf.h 2023-03-24 16:42:53.999065836 -0700
+ +++ HEAD/include/linux/bpf.h 2023-03-24 16:42:53.307068936 -0700
+ @@ -73,7 +73,7 @@
+ __u8 dst_reg:4; /* dest register */
+ __u8 src_reg:4; /* source register */
+ __s16 off; /* signed offset */
+ - __s32 imm; /* signed immediate constant */
+ + __u32 imm; /* unsigned immediate constant */
+ };
+
+ /* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
+
+ error - 1/906 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+ error - UAPI header ABI check failed
+
+Alternatively, we could have also ran with ``-b HEAD~``. This would set the
+base reference to ``HEAD~`` so then the script would compare it to ``HEAD~^1``.
+
+
+Architecture-specific Headers
+-----------------------------
+
+Consider this change::
+
+ cat << 'EOF' | patch -l -p1
+ --- a/arch/arm64/include/uapi/asm/sigcontext.h
+ +++ b/arch/arm64/include/uapi/asm/sigcontext.h
+ @@ -70,6 +70,7 @@ struct sigcontext {
+ struct _aarch64_ctx {
+ __u32 magic;
+ __u32 size;
+ + __u32 new_var;
+ };
+
+ #define FPSIMD_MAGIC 0x46508001
+ EOF
+
+This is a change to an arm64-specific UAPI header file. In this example, I'm
+running the script from an x86 machine with an x86 compiler, so by default,
+the script only works with x86-compatible UAPI header files::
+
+ % ./scripts/check-uapi.sh
+ Installing sanitized UAPI headers from dirty tree... OK
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from HEAD... OK
+ Restoring current tree state... OK
+ No changes to UAPI headers were applied between HEAD and dirty tree
+
+With an x86 compiler, we can't check header files in ``arch/arm64``, so the
+script doesn't even try.
+
+If we want to check the header file, we'll have to use an arm64 compiler and
+set ``ARCH`` accordingly::
+
+ % CC=aarch64-linux-gnu-gcc ARCH=arm64 ./scripts/check-uapi.sh
+ Installing sanitized UAPI headers from dirty tree... OK
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from HEAD... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between HEAD and dirty tree
+ !!! ABI differences detected in include/asm/sigcontext.h from HEAD -> dirty tree !!!
+
+ [C] 'struct _aarch64_ctx' changed:
+ type size changed from 64 to 96 (in bits)
+ 1 data member insertion:
+ '__u32 new_var', at offset 64 (in bits) at sigcontext.h:73:1
+ --- snip ---
+
+ Header file diff (after headers_install):
+ --- HEAD/include/asm/sigcontext.h 2023-03-24 16:45:48.850281831 -0700
+ +++ dirty/include/asm/sigcontext.h 2023-03-24 16:45:46.922290483 -0700
+ @@ -70,6 +70,7 @@
+ struct _aarch64_ctx {
+ __u32 magic;
+ __u32 size;
+ + __u32 new_var;
+ };
+
+ #define FPSIMD_MAGIC 0x46508001
+
+ error - 1/878 UAPI headers compatible with arm64 appear _not_ to be backwards compatible
+ error - UAPI header ABI check failed
+
+We can see with ``ARCH`` and ``CC`` set properly for the file, the ABI change
+is reported properly. Also notice that the total number of UAPI header files
+checked by the script changes. This is because the number of headers installed
+for arm64 platforms is different than x86.
+
+Cross-Dependency Breakages
+--------------------------
+
+Consider this change::
+
+ cat << 'EOF' | patch -l -p1
+ --- a/include/uapi/linux/types.h
+ +++ b/include/uapi/linux/types.h
+ @@ -52,7 +52,7 @@ typedef __u32 __bitwise __wsum;
+ #define __aligned_be64 __be64 __attribute__((aligned(8)))
+ #define __aligned_le64 __le64 __attribute__((aligned(8)))
+
+ -typedef unsigned __bitwise __poll_t;
+ +typedef unsigned short __bitwise __poll_t;
+
+ #endif /* __ASSEMBLY__ */
+ #endif /* _UAPI_LINUX_TYPES_H */
+ EOF
+
+Here, we're changing a ``typedef`` in ``types.h``. This doesn't break a UAPI in
+``types.h``, but other UAPIs in the tree may break due to this change::
+
+ % ./scripts/check-uapi.sh
+ Installing sanitized UAPI headers from dirty tree... OK
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from HEAD... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between HEAD and dirty tree
+ !!! ABI differences detected in include/linux/eventpoll.h from HEAD -> dirty tree !!!
+
+ [C] 'struct epoll_event' changed:
+ type size changed from 96 to 80 (in bits)
+ 2 data member changes:
+ type of '__poll_t events' changed:
+ underlying type 'unsigned int' changed:
+ type name changed from 'unsigned int' to 'unsigned short int'
+ type size changed from 32 to 16 (in bits)
+ '__u64 data' offset changed from 32 to 16 (in bits) (by -16 bits)
+
+ include/linux/eventpoll.h did not change between HEAD and dirty tree...
+ It's possible a change to one of the headers it includes caused this error:
+ #include <linux/fcntl.h>
+ #include <linux/types.h>
+
+ error - 1/906 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+ error - UAPI header ABI check failed
+
+Note that the script noticed the failing header file did not change, so it
+assumes one of its includes must have caused the breakage. Indeed, we can see
+``linux/types.h`` is used from ``eventpoll.h``.
+
+UAPI Header Removals
+--------------------
+
+Consider this change::
+
+ cat << 'EOF' | patch -l -p1
+ diff --git a/include/uapi/asm-generic/Kbuild b/include/uapi/asm-generic/Kbuild
+ index ebb180aac74e..a9c88b0a8b3b 100644
+ --- a/include/uapi/asm-generic/Kbuild
+ +++ b/include/uapi/asm-generic/Kbuild
+ @@ -31,6 +31,6 @@ mandatory-y += stat.h
+ mandatory-y += statfs.h
+ mandatory-y += swab.h
+ mandatory-y += termbits.h
+ -mandatory-y += termios.h
+ +#mandatory-y += termios.h
+ mandatory-y += types.h
+ mandatory-y += unistd.h
+ EOF
+
+This script removes a UAPI header file from the install list. Let's run the
+script::
+
+ % ./scripts/check-uapi.sh
+ Installing sanitized UAPI headers from dirty tree... OK
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from HEAD... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between HEAD and dirty tree
+ error - UAPI header include/asm/termios.h was incorrectly removed
+ error - 1/906 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+ error - UAPI header ABI check failed
+
+Removing a UAPI header is considered a breaking change and the script will flag
+it as such.
+
+Checking Historic UAPI Compatibility
+------------------------------------
+
+You can use the ``-b`` and ``-p`` options to examine different chunks of your
+git tree. For example, to check all changed UAPI header files between tags
+v6.0 and v6.1, you'd run::
+
+ % ./scripts/check-uapi.sh -b v6.1 -p v6.0
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from v6.1... OK
+ Installing sanitized UAPI headers from v6.0... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between v6.0 and v6.1
+ --- snip ---
+ error - 90/907 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+ error - UAPI header ABI check failed
+
+Note: before v5.3, a header file needed by the script is not present, so the
+script is unable to check changes before then.
+
+You'll notice that the script detected many UAPI changes that are not
+backwards compatible. Knowing that kernel UAPIs are supposed to be stable
+forever, this is an alarming result. This brings us to the next section: false
+positives.
+
+False Positives
+===============
+
+The UAPI checker is very aggressive in detecting ABI changes, so some false
+positives may appear. For example, if you check all UAPI headers between v6.0
+and v6.1, many breakages will be flagged. Run the following::
+
+ ./scripts/check-uapi.sh -b v6.1 -p v6.0 -l abi_error_log.txt
+
+The errors will be logged to ``abi_error_log.txt``. Here, we'll find examples
+of several types of false positives.
+
+Enum Expansion
+--------------
+
+::
+
+ !!! ABI differences detected in include/uapi/linux/openvswitch.h from v6.0 -> v6.1 !!!
+
+ [C] 'enum ovs_datapath_attr' changed:
+ type size hasn't changed
+ 1 enumerator insertion:
+ 'ovs_datapath_attr::OVS_DP_ATTR_IFINDEX' value '9'
+ 1 enumerator change:
+ 'ovs_datapath_attr::__OVS_DP_ATTR_MAX' from value '9' to '10' at openvswitch.h.current:85:1
+
+In this case, an enum was expanded. Consequently, the "MAX" value was
+incremented. This is not considered a breaking change because it's assumed
+userspace programs are using the MAX value in a sane fashion.
+
+Expanding Into Reserved/Padding Fields
+--------------------------------------
+
+::
+
+ !!! ABI differences detected in include/uapi/linux/perf_event.h from v6.0 -> v6.1 !!!
+
+ [C] 'struct perf_branch_entry' changed:
+ type size hasn't changed
+ 3 data member insertions:
+ '__u64 spec', at offset 152 (in bits) at perf_event.h.current:1420:1
+ '__u64 new_type', at offset 154 (in bits) at perf_event.h.current:1421:1
+ '__u64 priv', at offset 158 (in bits) at perf_event.h.current:1422:1
+ 1 data member change:
+ '__u64 reserved' offset changed from 152 to 161 (in bits) (by +9 bits)
+
+In this case, a reserved field was expanded into. Previously, the reserved
+field occupied 40 bits in the struct. After the change, three new members
+were added that took up 9 bits, so the size of the reserved field was
+reduced to 31.
+
+As the size of the struct did not change and none of the fields a userspace
+program could have been using were removed/changed/relocated, this change is
+not considered breaking.
+
+Removals For Refactoring or Deprecation
+---------------------------------------
+
+Sometimes drivers for very old hardware are removed, such as in this example::
+
+ % ./scripts/check-uapi.sh -b ba47652ba655
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from ba47652ba655... OK
+ Installing sanitized UAPI headers from ba47652ba655^1... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between ba47652ba655^1 and ba47652ba655
+ error - UAPI header include/linux/meye.h was incorrectly removed
+ error - 1/910 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+ error - UAPI header ABI check failed
+
+Another example::
+
+ % ./scripts/check-uapi.sh -b f40eb99897af
+ Saving current tree state... OK
+ Installing sanitized UAPI headers from f40eb99897af... OK
+ Installing sanitized UAPI headers from f40eb99897af^1... OK
+ Restoring current tree state... OK
+ Checking changes to UAPI headers between f40eb99897af^1 and f40eb99897af
+ error - UAPI header include/linux/pktcdvd.h was incorrectly removed
+ error - 1/906 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+ error - UAPI header ABI check failed
+
+While technically not false positives, the script will always flag removals.
+
+Other times, refactoring may drive a removal, but have no impact on userspace.
+For example, d759be8953fe. This change is before v5.3, so cannot be checked by
+the script. This is a true false positive as userspace was not impacted but the
+script flagged the change nonetheless.
+
+Summary
+-------
+
+There may be other examples of false positives that are not listed here.
+
+In the future, as tooling improves, we may be able to filter out more of these
+false positives. In any case, use your best judgement (and ideally a unit test
+in userspace) to make sure your UAPI changes are backwards-compatible!
diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 6b0663075dc0..0876f5a2cf55 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -34,6 +34,7 @@ Documentation/dev-tools/testing-overview.rst
kselftest
kunit/index
ktap
+ checkuapi
.. only:: subproject and html
--
2.17.1