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

From: Bill Wendling
Date: Tue Nov 30 2021 - 13:39:11 EST

On Tue, Nov 30, 2021 at 10:17 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> On Tue, Nov 30, 2021 at 10:25:43PM +1100, Michael Ellerman wrote:
> > Christophe Leroy <christophe.leroy@xxxxxxxxxx> 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@xxxxxxxxx/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@xxxxxxxxx>
> > >>
> > >> 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