powerpc: Avoid clang uninitialized warning in __get_user_size_allowed

Commit 9975f852ce1b ("powerpc/uaccess: Remove calls to __get_user_bad()
and __put_user_bad()") switch to BUILD_BUG() in the default case, which
leaves x uninitialized. This will not be an issue because the build will
be broken in that case but clang does static analysis before it realizes
the default case will be done so it warns about x being uninitialized
(trimmed for brevity):

 In file included from mm/mprotect.c:13:
 In file included from ./include/linux/hugetlb.h:28:
 In file included from ./include/linux/mempolicy.h:16:
 ./include/linux/pagemap.h:772:16: warning: variable '__gu_val' is used
 uninitialized whenever switch default is taken [-Wsometimes-uninitialized]
                 if (unlikely(__get_user(c, uaddr) != 0))
                              ^~~~~~~~~~~~~~~~~~~~
 ./arch/powerpc/include/asm/uaccess.h:266:2: note: expanded from macro '__get_user'
         __get_user_size_allowed(__gu_val, __gu_addr, __gu_size, __gu_err);      \
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 ./arch/powerpc/include/asm/uaccess.h:235:2: note: expanded from macro
 '__get_user_size_allowed'
        default: BUILD_BUG();                                   \
        ^~~~~~~

Commit 5cd29b1fd3e8 ("powerpc/uaccess: Use asm goto for get_user when
compiler supports it") added an initialization for x because of the same
reason. Do the same thing here so there is no warning across all
versions of clang.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://github.com/ClangBuiltLinux/linux/issues/1359
Link: https://lore.kernel.org/r/20210426203518.981550-1-nathan@kernel.org

diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index a4e791bcd3fe24f0dc1bc2a1d17022b0011fbd1e..a09e4240c5b1670ee184d85c93d82d738251ebe5 100644 (file)
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -232,7 +232,7 @@ do {                                                                \
        case 2: __get_user_asm(x, (u16 __user *)ptr, retval, "lhz"); break;     \
        case 4: __get_user_asm(x, (u32 __user *)ptr, retval, "lwz"); break;     \
        case 8: __get_user_asm2(x, (u64 __user *)ptr, retval);  break;  \
-       default: BUILD_BUG();                                   \
+       default: x = 0; BUILD_BUG();                            \
        }                                                       \
 } while (0)