[PATCH v5 2/2] docs: dev-tools: Add UAPI checker documentation

John Moon
2023-04-07T20:36:48Z

Add detailed documentation for scripts/check-uapi.sh.

Signed-off-by: John Moon <quic_johmoo@quicinc.com>
---
- Reworded final false positive example.

Documentation/dev-tools/checkuapi.rst | 480 ++++++++++++++++++++++++++
Documentation/dev-tools/index.rst | 1 +
2 files changed, 481 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..18da0ac6426e
--- /dev/null
+++ b/Documentation/dev-tools/checkuapi.rst
@@ -0,0 +1,480 @@
+.. 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. If it could be checked, the script would flag the change, but this
+would be a true false positive. Similar refactors in the future may be flagged
+by this script.
+
+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