linux-kernel - Re: [PATCH v5 5/5] powerpc/inst: Optimise copy_inst_from_kernel

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Message-ID: <87a6hlq408.fsf@mpe.ellerman.id.au>
Date:   Tue, 30 Nov 2021 22:25:43 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        kernel test robot <lkp@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     llvm@...ts.linux.dev, kbuild-all@...ts.01.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 5/5] powerpc/inst: Optimise
 copy_inst_from_kernel_nofault()

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> Le 29/11/2021 à 23:55, kernel test robot a écrit :
>> Hi Christophe,
>> 
>> I love your patch! Perhaps something to improve:
>> 
>> [auto build test WARNING on powerpc/next]
>> [also build test WARNING on v5.16-rc3 next-20211129]
>> [If your patch is applied to the wrong git tree, kindly drop us a note.
>> And when submitting patch, we suggest to use '--base' as documented in
>> https://git-scm.com/docs/git-format-patch]
>> 
>> url:    https://github.com/0day-ci/linux/commits/Christophe-Leroy/powerpc-inst-Refactor-___get_user_instr/20211130-015346
>> base:   https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git next
>> config: powerpc-randconfig-r023-20211129 (https://download.01.org/0day-ci/archive/20211130/202111300652.0yDBNvyJ-lkp@intel.com/config)
>> compiler: clang version 14.0.0 (https://github.com/llvm/llvm-project df08b2fe8b35cb63dfb3b49738a3494b9b4e6f8e)
>> reproduce (this is a W=1 build):
>>          wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>>          chmod +x ~/bin/make.cross
>>          # install powerpc cross compiling tool for clang build
>>          # apt-get install binutils-powerpc-linux-gnu
>>          # https://github.com/0day-ci/linux/commit/fb7bff30cc0efc7e4df1b48bb69de1f325eee826
>>          git remote add linux-review https://github.com/0day-ci/linux
>>          git fetch --no-tags linux-review Christophe-Leroy/powerpc-inst-Refactor-___get_user_instr/20211130-015346
>>          git checkout fb7bff30cc0efc7e4df1b48bb69de1f325eee826
>>          # save the config file to linux build tree
>>          mkdir build_dir
>>          COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=powerpc prepare
>> 
>> If you fix the issue, kindly add following tag as appropriate
>> Reported-by: kernel test robot <lkp@...el.com>
>> 
>> All warnings (new ones prefixed by >>):
>> 
>>     In file included from arch/powerpc/kernel/asm-offsets.c:71:
>>     In file included from arch/powerpc/kernel/../xmon/xmon_bpts.h:7:
>>>> arch/powerpc/include/asm/inst.h:165:20: warning: variable 'val' is uninitialized when used here [-Wuninitialized]
>>                     *inst = ppc_inst(val);
>>                                      ^~~
>>     arch/powerpc/include/asm/inst.h:53:22: note: expanded from macro 'ppc_inst'
>>     #define ppc_inst(x) (x)
>>                          ^
>>     arch/powerpc/include/asm/inst.h:155:18: note: initialize the variable 'val' to silence this warning
>>             unsigned int val, suffix;
>>                             ^
>>                              = 0
>
> I can't understand what's wrong here.
>
> We have
>
> 	__get_kernel_nofault(&val, src, u32, Efault);
> 	if (IS_ENABLED(CONFIG_PPC64) && get_op(val) == OP_PREFIX) {
> 		__get_kernel_nofault(&suffix, src + 1, u32, Efault);
> 		*inst = ppc_inst_prefix(val, suffix);
> 	} else {
> 		*inst = ppc_inst(val);
> 	}
>
> With
>
> #define __get_kernel_nofault(dst, src, type, err_label)			\
> 	__get_user_size_goto(*((type *)(dst)),				\
> 		(__force type __user *)(src), sizeof(type), err_label)
>
>
> And
>
> #define __get_user_size_goto(x, ptr, size, label)				\
> do {										\
> 	BUILD_BUG_ON(size > sizeof(x));						\
> 	switch (size) {								\
> 	case 1: __get_user_asm_goto(x, (u8 __user *)ptr, label, "lbz"); break;	\
> 	case 2: __get_user_asm_goto(x, (u16 __user *)ptr, label, "lhz"); break;	\
> 	case 4: __get_user_asm_goto(x, (u32 __user *)ptr, label, "lwz"); break;	\
> 	case 8: __get_user_asm2_goto(x, (u64 __user *)ptr, label);  break;	\
> 	default: x = 0; BUILD_BUG();						\
> 	}									\
> } while (0)
>
> And
>
> #define __get_user_asm_goto(x, addr, label, op)			\
> 	asm_volatile_goto(					\
> 		"1:	"op"%U1%X1 %0, %1	# get_user\n"	\
> 		EX_TABLE(1b, %l2)				\
> 		: "=r" (x)					\
> 		: "m<>" (*addr)				\
> 		:						\
> 		: label)
>
>
> I see no possibility, no alternative path where val wouldn't be set. The 
> asm clearly has *addr as an output param so it is always set.

I guess clang can't convince itself of that?

>>     1 warning generated.
>>     <stdin>:1559:2: warning: syscall futex_waitv not implemented [-W#warnings]
>>     #warning syscall futex_waitv not implemented
>>      ^
>>     1 warning generated.
>>     arch/powerpc/kernel/vdso32/gettimeofday.S:72:8: error: unsupported directive '.stabs'
>>     .stabs "_restgpr_31_x:F-1",36,0,0,_restgpr_31_x; .globl _restgpr_31_x; _restgpr_31_x:
>>            ^
>>     arch/powerpc/kernel/vdso32/gettimeofday.S:73:8: error: unsupported directive '.stabs'
>>     .stabs "_rest32gpr_31_x:F-1",36,0,0,_rest32gpr_31_x; .globl _rest32gpr_31_x; _rest32gpr_31_x:
>
> How should we fix that ? Will clang support .stabs in the future ?

I think we should drop any stabs annotations / support. AFAICT none of
the toolchains we support produce it anymore.

cheers