Re: [PATCH v5 5/5] powerpc/inst: Optimise copy_inst_from_kernel_nofault()

Michael Ellerman
2021-11-30T11:32:42Z

Christophe Leroy <christophe.leroy@csgroup.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@intel.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