linux-kernel - Re: [PATCH v2] powerpc/uaccess: Use flexible addressing with __put_user()/__get

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Message-ID: <20200630211817.GZ3598@gate.crashing.org>
Date:   Tue, 30 Jun 2020 16:18:17 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, npiggin@...il.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] powerpc/uaccess: Use flexible addressing with __put_user()/__get_user()

Hi again,

Thanks for your work so far!

On Tue, Jun 30, 2020 at 06:53:39PM +0000, Christophe Leroy wrote:
> On 06/30/2020 04:33 PM, Segher Boessenkool wrote:
> >>>+ 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?
> 
> Sorry for previous misunderstanding
> 
> Here is the code:
> 
> #define __put_user_asm(x, addr, err, op)			\
> 	__asm__ __volatile__(					\
> 		"1:	" op "%U2%X2 %1,%2	# put_user\n"	\
> 		"2:\n"						\
> 		".section .fixup,\"ax\"\n"			\
> 		"3:	li %0,%3\n"				\
> 		"	b 2b\n"					\
> 		".previous\n"					\
> 		EX_TABLE(1b, 3b)				\
> 		: "=r" (err)					\
> 		: "r" (x), "m<>" (*addr), "i" (-EFAULT), "0" (err))

Yeah I don't see it.  I'll have to look at compiler debug dumps, but I
don't have any working 4.9 around, and I cannot reproduce this with
either older or newer compilers.

It is complainig that constrain_operands just does not work *at all* on
this "m<>" constraint apparently, which doesn't make much sense.

I'll try later when I have more time, sorry :-/


Segher