Subject: Re: [PATCH v2] powerpc/uaccess: Use flexible addressing with __put_user()/__get_user()

From: Segher Boessenkool <[email-masked]>
Date: Tue, 30 Jun 2020 11:33:24 -0500

On Tue, Jun 30, 2020 at 04:55:05PM +0200, Christophe Leroy wrote:
> Le 30/06/2020 à 03:19, Michael Ellerman a écrit :
> >Michael Ellerman <[email-masked]> writes:
> >>Because it uses the "m<>" constraint which didn't work on GCC 4.6.
> >>
> >>https://github.com/linuxppc/issues/issues/297
> >>
> >>So we should be able to pick it up for v5.9 hopefully.
> >
> >It seems to break the build with the kernel.org 4.9.4 compiler and
> >corenet64_smp_defconfig:
> 
> Looks like 4.9.4 doesn't accept "m<>" constraint either.

The evidence contradicts this assertion.

> Changing it to "m" make it build.

But that just means something else is wrong.

> >+ make -s CC=powerpc64-linux-gnu-gcc -j 160
> >In file included from /linux/include/linux/uaccess.h:11:0,
> >                  from /linux/include/linux/sched/task.h:11,
> >                  from /linux/include/linux/sched/signal.h:9,
> >                  from /linux/include/linux/rcuwait.h:6,
> >                  from /linux/include/linux/percpu-rwsem.h:7,
> >                  from /linux/include/linux/fs.h:33,
> >                  from /linux/include/linux/huge_mm.h:8,
> >                  from /linux/include/linux/mm.h:675,
> >                  from /linux/arch/powerpc/kernel/signal_32.c:17:
> >/linux/arch/powerpc/kernel/signal_32.c: In function 
> >'save_user_regs.isra.14.constprop':
> >/linux/arch/powerpc/include/asm/uaccess.h:161:2: error: 'asm' operand has 
> >impossible constraints
> >   __asm__ __volatile__(     \
> >   ^
> >/linux/arch/powerpc/include/asm/uaccess.h:197:12: note: in expansion of 
> >macro '__put_user_asm'
> >     case 4: __put_user_asm(x, ptr, retval, "stw"); break; \
> >             ^
> >/linux/arch/powerpc/include/asm/uaccess.h:206:2: note: in expansion of 
> >macro '__put_user_size_allowed'
> >   __put_user_size_allowed(x, ptr, size, retval);  \
> >   ^
> >/linux/arch/powerpc/include/asm/uaccess.h:220:2: note: in expansion of 
> >macro '__put_user_size'
> >   __put_user_size(__pu_val, __pu_addr, __pu_size, __pu_err); \
> >   ^
> >/linux/arch/powerpc/include/asm/uaccess.h:96:2: note: in expansion of 
> >macro '__put_user_nocheck'
> >   __put_user_nocheck((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)))
> >   ^
> >/linux/arch/powerpc/kernel/signal_32.c:120:7: note: in expansion of macro 
> >'__put_user'
> >    if (__put_user((unsigned int)gregs[i], &frame->mc_gregs[i]))
> >        ^

Can we see what that was after the macro jungle?  Like, the actual
preprocessed code?

Also, what GCC version *does* work on this?

Segher