docs: dev-tools: Add UAPI checker documentation

Add detailed documentation for scripts/check-uapi.sh.
Signed-off-by: John Moon <quic_johmoo@quicinc.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/dev-tools/checkuapi.rst

+.. 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.
+
+Options
+=======
+
+This section will describe the options with which ``check-uapi.sh``
+can be run.
+
+Usage::
+
+    check-uapi.sh [-b BASE_REF] [-p PAST_REF] [-j N] [-l ERROR_LOG] [-i] [-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).
+    -i             Ignore ambiguous changes that may or may not break UAPI compatibility.
+    -q             Quiet operation.
+    -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
+
+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 user-facing UAPI headers from dirty tree... OK
+    Installing user-facing UAPI headers from HEAD... OK
+    Checking changes to UAPI headers between HEAD and dirty tree...
+    All 912 UAPI headers compatible with x86 appear to be backwards compatible
+
+Let's add another change that *might* break userspace::
+
+    cat << 'EOF' | patch -l -p1
+    --- a/include/uapi/linux/bpf.h
+    +++ b/include/uapi/linux/bpf.h
+    @@ -74,7 +74,7 @@ struct bpf_insn {
+            __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 will catch this::
+
+    % ./scripts/check-uapi.sh
+    Installing user-facing UAPI headers from dirty tree... OK
+    Installing user-facing UAPI headers from HEAD... 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
+    ==================================================================================
+
+    error - 1/912 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+
+In this case, the script is reporting the type change because it could
+break a userspace program that passes in a negative number. Now, let's
+say you know that no userspace program could possibly be using a negative
+value in ``imm``, so changing to an unsigned type there shouldn't hurt
+anything. You can pass the ``-i`` flag to the script to ignore changes
+in which the userspace backwards compatibility is ambiguous::
+
+    % ./scripts/check-uapi.sh -i
+    Installing user-facing UAPI headers from dirty tree... OK
+    Installing user-facing UAPI headers from HEAD... OK
+    Checking changes to UAPI headers between HEAD and dirty tree...
+    All 912 UAPI headers compatible with x86 appear to be backwards compatible
+
+Now, let's make a similar change that *will* break userspace::
+
+    cat << 'EOF' | patch -l -p1
+    --- a/include/uapi/linux/bpf.h
+    +++ b/include/uapi/linux/bpf.h
+    @@ -71,8 +71,8 @@ enum {
+
+     struct bpf_insn {
+            __u8    code;           /* opcode */
+    -       __u8    dst_reg:4;      /* dest register */
+            __u8    src_reg:4;      /* source register */
+    +       __u8    dst_reg:4;      /* dest register */
+            __s16   off;            /* signed offset */
+            __s32   imm;            /* signed immediate constant */
+     };
+    EOF
+
+Since we're re-ordering an existing struct member, there's no ambiguity,
+and the script will report the breakage even if you pass ``-i``::
+
+    % ./scripts/check-uapi.sh -i
+    Installing user-facing UAPI headers from dirty tree... OK
+    Installing user-facing UAPI headers from HEAD... 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
+          2 data member changes:
+            '__u8 dst_reg' offset changed from 8 to 12 (in bits) (by +4 bits)
+            '__u8 src_reg' offset changed from 12 to 8 (in bits) (by -4 bits)
+    ==================================================================================
+
+    error - 1/912 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+
+Let's commit the breaking change, then commit the innocuous 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
+    Installing user-facing UAPI headers from HEAD... OK
+    Installing user-facing UAPI headers from HEAD^1... OK
+    Checking changes to UAPI headers between HEAD^1 and HEAD...
+    All 912 UAPI headers compatible with x86 appear to be backwards compatible
+
+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. 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
+    Installing user-facing UAPI headers from HEAD... OK
+    Installing user-facing UAPI headers from HEAD~2... 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
+          2 data member changes:
+            '__u8 dst_reg' offset changed from 8 to 12 (in bits) (by +4 bits)
+            '__u8 src_reg' offset changed from 12 to 8 (in bits) (by -4 bits)
+    ==============================================================================
+
+    error - 1/912 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+
+Alternatively, we could have also run 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 checks x86-compatible UAPI header files::
+
+    % ./scripts/check-uapi.sh
+    Installing user-facing UAPI headers from dirty tree... OK
+    Installing user-facing UAPI headers from HEAD... 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 user-facing UAPI headers from dirty tree... OK
+    Installing user-facing UAPI headers from HEAD... 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 --
+        [C] 'struct zt_context' changed:
+          type size changed from 128 to 160 (in bits)
+          2 data member changes (1 filtered):
+            '__u16 nregs' offset changed from 64 to 96 (in bits) (by +32 bits)
+            '__u16 __reserved[3]' offset changed from 80 to 112 (in bits) (by +32 bits)
+    =======================================================================================
+
+    error - 1/884 UAPI headers compatible with arm64 appear _not_ to be backwards compatible
+
+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 user-facing UAPI headers from dirty tree... OK
+    Installing user-facing UAPI headers from HEAD... 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>
+
+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 user-facing UAPI headers from dirty tree... OK
+    Installing user-facing UAPI headers from HEAD... OK
+    Checking changes to UAPI headers between HEAD and dirty tree...
+    ==== UAPI header include/asm/termios.h was removed between HEAD and dirty tree ====
+
+    error - 1/912 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+
+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
+    Installing user-facing UAPI headers from v6.1... OK
+    Installing user-facing UAPI headers from v6.0... OK
+    Checking changes to UAPI headers between v6.0 and v6.1...
+
+    --- snip ---
+    error - 37/907 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+
+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:
+caveats.
+
+Caveats
+=======
+
+The UAPI checker makes no assumptions about the author's intention, so some
+types of changes may be flagged even though they intentionally break UAPI.
+
+Removals For Refactoring or Deprecation
+---------------------------------------
+
+Sometimes drivers for very old hardware are removed, such as in this example::
+
+    % ./scripts/check-uapi.sh -b ba47652ba655
+    Installing user-facing UAPI headers from ba47652ba655... OK
+    Installing user-facing UAPI headers from ba47652ba655^1... OK
+    Checking changes to UAPI headers between ba47652ba655^1 and ba47652ba655...
+    ==== UAPI header include/linux/meye.h was removed between ba47652ba655^1 and ba47652ba655 ====
+
+    error - 1/910 UAPI headers compatible with x86 appear _not_ to be backwards compatible
+
+The script will always flag removals (even if they're intentional).
+
+Struct Expansions
+-----------------
+
+Depending on how a structure is handled in kernelspace, a change which
+expands a struct could be non-breaking.
+
+If a struct is used as the argument to an ioctl, then the kernel driver
+must be able to handle ioctl commands of any size. Beyond that, you need
+to be careful when copying data from the user. Say, for example, that
+``struct foo`` is changed like this::
+
+    struct foo {
+        __u64 a; /* added in version 1 */
+    +   __u32 b; /* added in version 2 */
+    +   __u32 c; /* added in version 2 */
+    }
+
+By default, the script will flag this kind of change for further review::
+
+    [C] 'struct foo' changed:
+      type size changed from 64 to 128 (in bits)
+      2 data member insertions:
+        '__u32 b', at offset 64 (in bits)
+        '__u32 c', at offset 96 (in bits)
+
+However, it is possible that this change was made safely.
+
+If a userspace program was built with version 1, it will think
+``sizeof(struct foo)`` is 8. That size will be encoded in the
+ioctl value that gets sent to the kernel. If the kernel is built
+with version 2, it will think the ``sizeof(struct foo)`` is 16.
+
+The kernel can use the ``_IOC_SIZE`` macro to get the size encoded
+in the ioctl code that the user passed in and then use
+``copy_struct_from_user()`` to safely copy the value::
+
+    int handle_ioctl(unsigned long cmd, unsigned long arg)
+    {
+        switch _IOC_NR(cmd) {
+        0x01: {
+            struct foo my_cmd;  /* size 16 in the kernel */
+
+            ret = copy_struct_from_user(&my_cmd, arg, sizeof(struct foo), _IOC_SIZE(cmd));
+            ...
+
+``copy_struct_from_user`` will zero the struct in the kernel and then copy
+only the bytes passed in from the user (leaving new members zeroized).
+If the user passed in a larger struct, the extra members are ignored.
+
+If you know this situation is accounted for in the kernel code, you can
+pass ``-i`` to the script, and struct expansions like this will be ignored.
+
+Flex Array Migration
+--------------------
+
+While the script handles expansion into an existing flex array, it does
+still flag initial migration to flex arrays from 1-element fake flex
+arrays. For example::
+
+    struct foo {
+          __u32 x;
+    -     __u32 flex[1]; /* fake flex */
+    +     __u32 flex[];  /* real flex */
+    };
+
+This change would be flagged by the script::
+
+    [C] 'struct foo' changed:
+      type size changed from 64 to 32 (in bits)
+      1 data member change:
+        type of '__u32 flex[1]' changed:
+          type name changed from '__u32[1]' to '__u32[]'
+          array type size changed from 32 to 'unknown'
+          array type subrange 1 changed length from 1 to 'unknown'
+
+At this time, there's no way to filter these types of changes, so be
+aware of this possible false positive.
+
+Summary
+-------
+
+While many types of false positives are filtered out by the script,
+it's possible there are some cases where the script flags a change
+which does not break UAPI. It's also possible a change which *does*
+break userspace would not be flagged by this script. While the script
+has been run on much of the kernel history, there could still be corner
+cases that are not accounted for.
+
+The intention is for this script to be used as a quick check for
+maintainers or automated tooling, not as the end-all authority on
+patch compatibility. It's best to remember: use your best judgment
+(and ideally a unit test in userspace) to make sure your UAPI changes
+are backwards-compatible!

Documentation/dev-tools/index.rst

@@ -34,6 +34,7 @@ Documentation/dev-tools/testing-overview.rst
    kselftest
    kunit/index
    ktap
+   checkuapi
 
 
 .. only::  subproject and html