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

[Bill Wendling <morbo@google.com>]

On Tue, Nov 30, 2021 at 10:17 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Tue, Nov 30, 2021 at 10:25:43PM +1100, Michael Ellerman wrote:
> > 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?
>
> A simplified reproducer:
>
> $ cat test.c
> static inline int copy_inst_from_kernel_nofault(unsigned int *inst,
>                                                 unsigned int *src)
> {
>         unsigned int val;
>
>         asm goto("1: lwz %U1%X1 %0, %1   # get_user\n"
>                  ".section __ex_table,\"a\";"
>                  ".balign 4;"
>                  ".long (1b) - . ;"
>                  ".long (%l2) - . ;"
>                  ".previous"
>                  : "=r" (*(unsigned int *)(&val))
>                  : "m<>" (*(unsigned int *)(src))
>                  :
>                  : Efault);
>
>         *inst = val;
>         return 0;
>
> Efault:
>         return -14; /* -EFAULT */
> }
>
> $ clang --target=powerpc-linux-gnu -Wuninitialized -fsyntax-only test.c
> test.c:17:10: warning: variable 'val' is uninitialized when used here [-Wuninitialized]
>         *inst = val;
>                 ^~~
> test.c:4:18: note: initialize the variable 'val' to silence this warning
>         unsigned int val;
>                         ^
>                          = 0
> 1 warning generated.
>
> It certainly looks like there is something wrong with how clang is
> tracking the initialization of the variable because it looks to me like
> val is only used in the fallthrough path, which happens after it is
> initialized via lwz.  Perhaps something is wrong with the logic of
> https://reviews.llvm.org/D71314?  I've added Bill to CC (LLVM issues are
> being migrated from Bugzilla to GitHub Issues right now so I cannot file
> this upstream at the moment).
>
If I remove the casts of "val" the warning doesn't appear. I suspect
that when I wrote that patch I forgot to remove those when checking.
#include "Captain_Picard_facepalm.h"

I'll look into it.

-bw