Re: [PATCH v5.10] powerpc/uaccess: Fix build errors seen with GCC 13/14

[Segher Boessenkool <segher@kernel.crashing.org>]

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