| Message ID | 20230217202234.32260-1-quic_johmoo@quicinc.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Validating UAPI backwards compatibility | expand |
On Sat, Feb 18, 2023 at 5:23 AM John Moon <quic_johmoo@quicinc.com> wrote: > > + linux-arm-kernel and Greg KH > > Hi all, > > 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. > > In the vendor driver community (out-of-tree modules), there's been a > lack of discipline when it comes to maintaining UAPI backwards > compatibility. This has been a maintenance burden and limits our options > for long-term support of older devices. > > Our goal is to add tooling for vendor driver developers because the > upstream model of expert maintainer code review can be difficult to > replicate in-house. Tools may help developers catch simple UAPI > incompatibilities that could be easily overlooked by in-house review. > > 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 enforce this guarantee? We would be interested to > learn about any tools or methods used by kernel developers to make sure > the above statement remains true. > > Could the 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. > > Additionally, I've attached a shell script we've been using internally > to validate changes to our UAPI headers are backwards compatible. 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 CI system > to block changes that fail the check. > > Currently, the script works with gcc. It generates output like this when > a backwards-incompatible change is made to a UAPI header: > > !!! ABI differences detected in include/uapi/linux/acct.h (compared to > file at HEAD^1) !!! > > [C] 'struct acct' changed: > type size changed from 512 to 544 (in bits) > 1 data member insertion: > '__u32 new_val', at offset 512 (in bits) at acct.h:71:1 > > 0/1 UAPI header file changes are backwards compatible > UAPI header ABI check failed > > However, we have not had success with clang. It seems clang is more > aggressive in optimizing dead code away (no matter which options we > pass). Therefore, no ABI differences are found. > > We wanted to share with the community to receive feedback and any advice > when it comes to tooling/policy surrounding this issue. Our hope is that > the script will help all kernel UAPI authors (even those that haven't > upstreamed yet) maintain good discipline and avoid breaking userspace. > > [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 <jorcrous@amazon.com> authored the > original version of the UAPI checker script. Thanks Jordan! If you want to express the authorship of the original, it is possible to add the "Co-developed-by" tag, which is mentioned in Documentation/translations/sp_SP/process/submitting-patches.rst It depends on how much code you rewrote, though. > > John Moon (1): > check-uapi: Introduce check-uapi.sh > > scripts/check-uapi.sh | 245 ++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 245 insertions(+) > create mode 100755 scripts/check-uapi.sh > > > base-commit: 033c40a89f55525139fd5b6342281b09b97d05bf > -- > 2.17.1 > -- Best Regards Masahiro Yamada
+ linux-arm-kernel and Greg KH Hi all, 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. In the vendor driver community (out-of-tree modules), there's been a lack of discipline when it comes to maintaining UAPI backwards compatibility. This has been a maintenance burden and limits our options for long-term support of older devices. Our goal is to add tooling for vendor driver developers because the upstream model of expert maintainer code review can be difficult to replicate in-house. Tools may help developers catch simple UAPI incompatibilities that could be easily overlooked by in-house review. 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 enforce this guarantee? We would be interested to learn about any tools or methods used by kernel developers to make sure the above statement remains true. Could the 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. Additionally, I've attached a shell script we've been using internally to validate changes to our UAPI headers are backwards compatible. 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 CI system to block changes that fail the check. Currently, the script works with gcc. It generates output like this when a backwards-incompatible change is made to a UAPI header: !!! ABI differences detected in include/uapi/linux/acct.h (compared to file at HEAD^1) !!! [C] 'struct acct' changed: type size changed from 512 to 544 (in bits) 1 data member insertion: '__u32 new_val', at offset 512 (in bits) at acct.h:71:1 0/1 UAPI header file changes are backwards compatible UAPI header ABI check failed However, we have not had success with clang. It seems clang is more aggressive in optimizing dead code away (no matter which options we pass). Therefore, no ABI differences are found. We wanted to share with the community to receive feedback and any advice when it comes to tooling/policy surrounding this issue. Our hope is that the script will help all kernel UAPI authors (even those that haven't upstreamed yet) maintain good discipline and avoid breaking userspace. [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 <jorcrous@amazon.com> authored the original version of the UAPI checker script. Thanks Jordan! John Moon (1): check-uapi: Introduce check-uapi.sh scripts/check-uapi.sh | 245 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 245 insertions(+) create mode 100755 scripts/check-uapi.sh base-commit: 033c40a89f55525139fd5b6342281b09b97d05bf -- 2.17.1