[PATCH 1/2] powerpc/uaccess: Fix build errors seen with GCC 14

[Michael Ellerman]

Nick Desaulniers <ndesaulniers at google.com> writes:
> On Tue, May 21, 2024 at 5:39 AM Michael Ellerman <mpe at ellerman.id.au> wrote:
>>
>> Building ppc64le_defconfig with GCC 14 fails with assembler errors:
>>
>>     CC      fs/readdir.o
>>   /tmp/ccdQn0mD.s: Assembler messages:
>>   /tmp/ccdQn0mD.s:212: Error: operand out of domain (18 is not a multiple of 4)
>>   /tmp/ccdQn0mD.s:226: Error: operand out of domain (18 is not a multiple of 4)
>>   ... [6 lines]
>>   /tmp/ccdQn0mD.s:1699: Error: operand out of domain (18 is not a multiple of 4)
>>
>> A snippet of the asm shows:
>>
>>   # ../fs/readdir.c:210:         unsafe_copy_dirent_name(dirent->d_name, name, namlen, efault_end);
>>          ld 9,0(29)       # MEM[(u64 *)name_38(D) + _88 * 1], MEM[(u64 *)name_38(D) + _88 * 1]
>>   # 210 "../fs/readdir.c" 1
>>          1:      std 9,18(8)     # put_user       # *__pus_addr_52, MEM[(u64 *)name_38(D) + _88 * 1]
>>
>> The 'std' instruction requires a 4-byte aligned displacement because
>> it is a DS-form instruction, and as the assembler says, 18 is not a
>> multiple of 4.
>>
>> The fix is to change the constraint on the memory operand to put_user(),
>> from "m" which is a general memory reference to "YZ".
>>
>> The "Z" constraint is documented in the GCC manual PowerPC machine
>> constraints, and specifies a "memory operand accessed with indexed or
>> indirect addressing". "Y" is not documented in the manual but specifies
>> a "memory operand for a DS-form instruction". Using both allows the
>> compiler to generate a DS-form "std" or X-form "stdx" as appropriate.
>>
>> The change has to be conditional on CONFIG_PPC_KERNEL_PREFIXED because
>> the "Y" constraint does not guarantee 4-byte alignment when prefixed
>> instructions are enabled.
>>
>> Unfortunately clang doesn't support the "Y" constraint so that has to be
>> behind an ifdef.
>
> Filed: https://github.com/llvm/llvm-project/issues/92939

Thanks. I will file one to have the GCC constraint documented.

cheers