[PATCH v3 3/3] openrisc: Fix issue with get_user for 64-bit values

[Stafford Horne]

On Fri, Sep 11, 2020 at 10:55:26PM +0200, Luc Van Oostenryck wrote:
> On Fri, Sep 11, 2020 at 08:39:40AM +0900, Stafford Horne wrote:
> > A build failure was raised by kbuild with the following error.
> > 
> >     drivers/android/binder.c: Assembler messages:
> >     drivers/android/binder.c:3861: Error: unrecognized keyword/register name `l.lwz ?ap,4(r24)'
> >     drivers/android/binder.c:3866: Error: unrecognized keyword/register name `l.addi ?ap,r0,0'
> > 
> > The issue is with 64-bit get_user() calls on openrisc.  I traced this to
> > a problem where in the internally in the get_user macros there is a cast
> > to long __gu_val this causes GCC to think the get_user call is 32-bit.
> > This binder code is really long and GCC allocates register r30, which
> > triggers the issue. The 64-bit get_user asm tries to get the 64-bit pair
> > register, which for r30 overflows the general register names and returns
> > the dummy register ?ap.
> > 
> > The fix here is to move the temporary variables into the asm macros.  We
> > use a 32-bit __gu_tmp for 32-bit and smaller macro and a 64-bit tmp in
> > the 64-bit macro.  The cast in the 64-bit macro has a trick of casting
> > through __typeof__((x)-(x)) which avoids the below warning.  This was
> > barrowed from riscv.
> > 
> >     arch/openrisc/include/asm/uaccess.h:240:8: warning: cast to pointer from integer of different size
> > 
> > I tested this is a small unit test to check reading between 64-bit and
> > 32-bit pointers to 64-bit and 32-bit values in all combinations.  Also I
> > ran make C=1 to confirm no new sparse warnings came up.  It all looks
> > clean to me.
> 
> It looks correct to me too now at C & assembly level.
> Feel free to add my:
> Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck at gmail.com>

Thanks a lot.

-Stafford