On Fri, Jun 14, 2024 at 09:27:14PM +1000, Michael Ellerman wrote:
commit 2d43cc701b96f910f50915ac4c2a0cae5deb734c upstream. 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.
You learn something new every day :-)
A similar error is seen with GCC 13 and CONFIG_UBSAN_SIGNED_WRAP=y.
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.
https://gcc.gnu.org/PR115289 It will be documented soon, thanks for the report!
Although the build error is only seen with GCC 13/14, that appears to just be luck. The constraint has been incorrect since it was first added.
Yes, "m" allows any memory operand, an unaligned one is just fine.
Acked-by: Segher Boessenkool segher@kernel.crashing.org
Segher