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

From: shuah
Date: Mon Jul 08 2019 - 14:10:40 EST

On 7/8/19 11:58 AM, Mathieu Desnoyers wrote:
----- On Jun 30, 2019, at 9:56 AM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx 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

-- Shuah