Re: [linux-next:master 1430/3917] drivers/platform/surface/surface_aggregator_registry.c:398:25: warning: implicit conversion from 'int' to '__u8' (aka 'unsigned char') changes value from 65535 to 255

[Maximilian Luz <luzmaximilian@gmail.com>]

On 3/11/21 7:51 PM, Nathan Chancellor wrote:
> On Thu, Mar 11, 2021 at 02:39:03PM +0100, Maximilian Luz wrote:
>> On 3/11/21 2:06 PM, kernel test robot wrote:
>>> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
>>> head:   98546348153dee5f8ced572fd6c4690461d20f51
>>> commit: 797e78564634275ed4fe6b3f586c4b96eb1d86bc [1430/3917] platform/surface: aggregator_registry: Add base device hub
>>> config: arm64-randconfig-r026-20210311 (attached as .config)
>>> compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project 574a9dabc63ba1e7a04c08d4bde2eacd61b44ce1)
>>> reproduce (this is a W=1 build):
>>>           wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>>>           chmod +x ~/bin/make.cross
>>>           # install arm64 cross compiling tool for clang build
>>>           # apt-get install binutils-aarch64-linux-gnu
>>>           # https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=797e78564634275ed4fe6b3f586c4b96eb1d86bc
>>>           git remote add linux-next https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
>>>           git fetch --no-tags linux-next master
>>>           git checkout 797e78564634275ed4fe6b3f586c4b96eb1d86bc
>>>           # save the attached .config to linux build tree
>>>           COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=arm64
>>>
>>> If you fix the issue, kindly add following tag as appropriate
>>> Reported-by: kernel test robot <lkp@intel.com>
>>>
>>> All warnings (new ones prefixed by >>):
>>>
>>>      In file included from drivers/platform/surface/surface_aggregator_registry.c:12:
>>>      In file included from include/linux/acpi.h:35:
>>>      In file included from include/acpi/acpi_io.h:7:
>>>      In file included from arch/arm64/include/asm/acpi.h:12:
>>>      include/linux/efi.h:1093:34: warning: passing 1-byte aligned argument to 4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer access [-Walign-mismatch]
>>>              status = get_var(L"SecureBoot", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size,
>>>                                              ^
>>>      include/linux/efi.h:1101:24: warning: passing 1-byte aligned argument to 4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer access [-Walign-mismatch]
>>>              get_var(L"SetupMode", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size, &setupmode);
>>>                                    ^
>>>>> drivers/platform/surface/surface_aggregator_registry.c:398:25: warning: implicit conversion from 'int' to '__u8' (aka 'unsigned char') changes value from 65535 to 255 [-Wconstant-conversion]
>>>              { SSAM_VDEV(HUB, 0x02, SSAM_ANY_IID, 0x00) },
>>>              ~                      ^~~~~~~~~~~~
>>
>> This is a false positive:
>>
>>>      include/linux/surface_aggregator/device.h:71:23: note: expanded from macro 'SSAM_ANY_IID'
>>>      #define SSAM_ANY_IID            0xffff
>>>                                      ^~~~~~
>>
>> Here, clang complains that the input is SSAM_ANY_ID. That is a special
>> value which has special considerations below. The SSAM_DEVICE() and
>> thus SSAM_VDEV() macros are intended to only allow either __u8 or
>> SSAM_ANY_ID as input in this place.
>>
>>>      include/linux/surface_aggregator/device.h:126:63: note: expanded from macro 'SSAM_VDEV'
>>>              SSAM_DEVICE(SSAM_DOMAIN_VIRTUAL, SSAM_VIRTUAL_TC_##cat, tid, iid, fun)
>>>                                                                           ^~~
>>>      include/linux/surface_aggregator/device.h:102:41: note: expanded from macro 'SSAM_DEVICE'
>>>              .instance = ((iid) != SSAM_ANY_IID) ? (iid) : 0,                        \
>>>                                                     ^~~
>>
>> Here is the special handling for SSAM_ANY_ID. So there is an implicit
>> conversion from iid, which may be __u8, to int (i.e. the type of
>> SSAM_ANY_ID), but there is at no point any implicit conversion of
>> SSAM_ANY_ID to __u8, as clang alleges.
> 
> Looks like we are getting bit by https://llvm.org/pr38789 here (also
> tracked at https://github.com/ClangBuiltLinux/linux/issues/92).

Thanks for the links! Looks like this is the same issue.

>> Is there any way to silence this warning in particular without
>> suppressing it (e.g. by explicit casting) when users of this macro
>> _actually_ input any non-u8 and non-SSAM_ANY_ID value as iid,
>> leading to an _actual_ implicit value-changing cast?
> 
> This has been worked around once before, commit b27aca2e555f ("soc:
> rockchip: work around clang warning"). I am not sure of another way to
> silence the warning in specific instances like you described aside from
> just fixing clang (which is obviously the correct solution) so that we
> will get real warnings.
> 
> Unfortunately, the patch that is linked in the LLVM PR above does not
> appear to fix this instance.

So what's the way forward here? Ignore this warning for now and wait for
the clang team to fix it or find a workaround?

The best workaround I can come up with is setting the match_flags
parameter directly as macro argument, which would be a bit more verbose
and feel slightly less clean, but would do the trick.

Also note that any changes to those macros would have to be synchronized
with a couple of patches which are still out to non-pdx86 trees, so if
we're working around this it's best to do this as soon as possible.
Specifically talking to Hans here, as those patches all have a
dependency on stuff that's only in the pdx86 tree right now. I.e.
getting the workaround in before setting up a tag for those dependencies
would simplify things.

Regards,
Max

> Cheers,
> Nathan
> 
>> I believe GCC does get this right and only emits a warning if a
>> non-u8 _and_ non-SSAM_ANY_ID value is input.
>>
>> Regards,
>> Max
>>
>>>      3 warnings generated.
>>>
>>>
>>> vim +398 drivers/platform/surface/surface_aggregator_registry.c
>>>
>>>      396	
>>>      397	static const struct ssam_device_id ssam_base_hub_match[] = {
>>>    > 398		{ SSAM_VDEV(HUB, 0x02, SSAM_ANY_IID, 0x00) },
>>>      399		{ },
>>>      400	};
>>>      401	
>>>
>>> ---
>>> 0-DAY CI Kernel Test Service, Intel Corporation
>>> https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org
>>>
>>