Re: [PATCH for 5.2] rseq/selftests: Fix Thumb mode build failure on arm32

[shuah <shuah@kernel.org>]

On 7/8/19 11:58 AM, Mathieu Desnoyers wrote:
> ----- On Jun 30, 2019, at 9:56 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:
> 
>> Using ".arm .inst" for the arm signature introduces build issues for
>> programs compiled in Thumb mode because the assembler stays in the
>> arm mode for the rest of the inline assembly. Revert to using a ".word"
>> to express the signature as data instead.
>>
>> The choice of signature is a valid trap instruction on arm32 little
>> endian, where both code and data are little endian.
>>
>> ARMv6+ big endian (BE8) generates mixed endianness code vs data:
>> little-endian code and big-endian data. The data value of the signature
>> needs to have its byte order reversed to generate the trap instruction.
>>
>> Prior to ARMv6, -mbig-endian generates big-endian code and data
>> (which match), so the endianness of the data representation of the
>> signature should not be reversed. However, the choice between BE32
>> and BE8 is done by the linker, so we cannot know whether code and
>> data endianness will be mixed before the linker is invoked. So rather
>> than try to play tricks with the linker, the rseq signature is simply
>> data (not a trap instruction) prior to ARMv6 on big endian. This is
>> why the signature is expressed as data (.word) rather than as
>> instruction (.inst) in assembler.
>>
>> Because a ".word" is used to emit the signature, it will be interpreted
>> as a literal pool by a disassembler, not as an actual instruction.
>> Considering that the signature is not meant to be executed except in
>> scenarios where the program execution is completely bogus, this should
>> not be an issue.
> 
> Now that 5.2 is out before this patch has been merged, can we please
> integrate this patch through the kernel selftests or ARM tree so it
> can be merged into the stable 5.2 branch ?
> 

I will apply it to selftests and send it for 5.3-rc1 and mark it for
stable.

thanks,
-- Shuah