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author | John Moon <quic_johmoo@quicinc.com> | 2023-12-12 05:02:58 +0300 |
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committer | Masahiro Yamada <masahiroy@kernel.org> | 2023-12-29 16:25:21 +0300 |
commit | 8c88bc5b489e785c7ead94ce6fc3adb7f76e8715 (patch) | |
tree | b8403616abd4dac4787f6773a3f804c3cd810caa /Documentation/dev-tools/checkuapi.rst | |
parent | 1f7f31bf7202adcab9616307bcb11a65fb565f63 (diff) | |
download | linux-8c88bc5b489e785c7ead94ce6fc3adb7f76e8715.tar.xz |
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>
Diffstat (limited to 'Documentation/dev-tools/checkuapi.rst')
-rw-r--r-- | Documentation/dev-tools/checkuapi.rst | 477 |
1 files changed, 477 insertions, 0 deletions
diff --git a/Documentation/dev-tools/checkuapi.rst b/Documentation/dev-tools/checkuapi.rst new file mode 100644 index 000000000000..9072f21b50b0 --- /dev/null +++ b/Documentation/dev-tools/checkuapi.rst @@ -0,0 +1,477 @@ +.. 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! |