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

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Message-ID: <f8819fa4-94e3-4bf9-4b60-c57d2804e529@csgroup.eu>
Date:   Tue, 30 Jun 2020 18:53:39 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>
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()



On 06/30/2020 04:33 PM, Segher Boessenkool wrote:
> 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 <mpe@...erman.id.au> 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?
> 

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))

Christophe