Re: [PATCH 1/2] kbuild: clang: Disable 'address-of-packed-member' warning

[Masahiro Yamada]

Hi Matthias,

Sorry for my late reply.

2017-05-09 8:18 GMT+09:00 Matthias Kaehlcke <mka@xxxxxxxxxxxx>:
> Hi Masahiro,
>
> El Sun, May 07, 2017 at 01:52:25AM +0900 Masahiro Yamada ha dit:
>
>> 2017-05-02 10:23 GMT+09:00 Matthias Kaehlcke <mka@xxxxxxxxxxxx>:
>> > Hi Masahiro,
>> >
>> > El Sun, Apr 30, 2017 at 10:59:52PM +0900 Masahiro Yamada ha dit:
>> >
>> >> 2017-04-22 6:39 GMT+09:00 Matthias Kaehlcke <mka@xxxxxxxxxxxx>:
>> >> > clang generates plenty of these warnings in different parts of the code,
>> >> > to an extent that the warnings are little more than noise. Disable the
>> >> > 'address-of-packed-member' warning.
>> >> >
>> >> > Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
>> >>
>> >>
>> >> As far as I compiled arch/x86/configs/x86_64_defconfig,
>> >> all address-of-packed-member warnings came from the single point:
>> >>
>> >> ./arch/x86/include/asm/processor.h:534:30: warning: taking address of
>> >> packed member 'sp0' of class or structure 'x86_hw_tss' may result in
>> >> an unaligned pointer value [-Waddress-of-packed-member]
>> >>         return this_cpu_read_stable(cpu_tss.x86_tss.sp0);
>> >>                                     ^~~~~~~~~~~~~~~~~~~
>> >> ./arch/x86/include/asm/percpu.h:391:59: note: expanded from macro
>> >> 'this_cpu_read_stable'
>> >> #define this_cpu_read_stable(var)       percpu_stable_op("mov", var)
>> >>                                                                 ^~~
>> >> ./arch/x86/include/asm/percpu.h:228:16: note: expanded from macro
>> >> 'percpu_stable_op'
>> >>                     : "p" (&(var)));                    \
>> >>                              ^~~
>> >>
>> >>
>> >>
>> >> For this case, I was able to fix it with the following patch:
>> >>
>> >>
>> >> diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
>> >> index 9fa0360..de25d1c 100644
>> >> --- a/arch/x86/include/asm/percpu.h
>> >> +++ b/arch/x86/include/asm/percpu.h
>> >> @@ -211,26 +211,27 @@ do {
>> >>         \
>> >>  #define percpu_stable_op(op, var)                      \
>> >>  ({                                                     \
>> >>         typeof(var) pfo_ret__;                          \
>> >> +       void *__p = &(var);                             \
>> >>         switch (sizeof(var)) {                          \
>> >>         case 1:                                         \
>> >>                 asm(op "b "__percpu_arg(P1)",%0"        \
>> >>                     : "=q" (pfo_ret__)                  \
>> >> -                   : "p" (&(var)));                    \
>> >> +                   : "p" (__p));                       \
>> >>                 break;                                  \
>> >>         case 2:                                         \
>> >>                 asm(op "w "__percpu_arg(P1)",%0"        \
>> >>                     : "=r" (pfo_ret__)                  \
>> >> -                   : "p" (&(var)));                    \
>> >> +                   : "p" (__p));                       \
>> >>                 break;                                  \
>> >>         case 4:                                         \
>> >>                 asm(op "l "__percpu_arg(P1)",%0"        \
>> >>                     : "=r" (pfo_ret__)                  \
>> >> -                   : "p" (&(var)));                    \
>> >> +                   : "p" (__p));                       \
>> >>                 break;                                  \
>> >>         case 8:                                         \
>> >>                 asm(op "q "__percpu_arg(P1)",%0"        \
>> >>                     : "=r" (pfo_ret__)                  \
>> >> -                   : "p" (&(var)));                    \
>> >> +                   : "p" (__p));                       \
>> >>                 break;                                  \
>> >>         default: __bad_percpu_size();                   \
>> >>         }                                               \
>> >
>> > Thanks for having a look!
>> >
>> > It is odd though that you only see warnings from that origin, I
>> > encounter plenty of others with x86_64_defconfig, mostly stemming
>> > from uaccess macros:
>> >
>> > kernel/power/user.c:439:35: warning: taking address of packed member
>> > 'dev' of class or structure 'compat_resume_swap_area' may result in an
>> > unaligned pointer value [-Waddress-of-packed-member]
>> >                 err |= get_user(swap_area.dev, &u_swap_area->dev);
>> >                                                 ^~~~~~~~~~~~~~~~
>> > ./arch/x86/include/asm/uaccess.h:168:23: note: expanded from macro 'get_user'
>> >         register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX);            \
>> >                              ^~~
>> > ./arch/x86/include/asm/uaccess.h:132:41: note: expanded from macro '__inttype'
>> > __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
>> >                                         ^
>> >
>> > I looked into fixing different cases, but didn't see a clear path
>> > forward since we can't just cast the type away as in your patch above.
>>
>>
>> Curious.
>> I tested clang 3.0 thru 4.0, but I could not reproduce this.
>>
>> This part just calculates sizeof(*(ptr)).
>> I think it is a false positive warning bug if clang reports this.
>
> The instance above is indeed somewhat doubtful, in any case there are
> plenty of others, most of them from fs/compat.c using __get/put_user_xyz():
>
> fs/compat.c:366:33: warning: taking address of packed member 'l_whence' of class or structure 'compat_flock64' may result in an unaligned pointer value [-Waddress-of-packed-member]
>             __get_user(kfl->l_whence, &ufl->l_whence) ||
>                                        ^~~~~~~~~~~~~
> arch/x86/include/asm/uaccess.h:505:27: note: expanded from macro '__get_user'
>         __get_user_nocheck((x), (ptr), sizeof(*(ptr)))
>                                  ^~~
> arch/x86/include/asm/uaccess.h:436:29: note: expanded from macro '__get_user_nocheck'
>         __get_user_size(__gu_val, (ptr), (size), __gu_err, -EFAULT);    \
>                                    ^~~
> arch/x86/include/asm/uaccess.h:361:21: note: expanded from macro '__get_user_size'
>                 __get_user_asm(x, ptr, retval, "w", "w", "=r", errret); \
>                                   ^~~
> arch/x86/include/asm/uaccess.h:385:19: note: expanded from macro '__get_user_asm'
>                      : "m" (__m(addr)), "i" (errret), "0" (err))
>                                 ^~~~
> arch/x86/include/asm/uaccess.h:444:51: note: expanded from macro '__m'
> #define __m(x) (*(struct __large_struct __user *)(x))
>                                                   ^
>
> The clang version I use is fairly recent since it includes some
> fixes needed to build a working kernel (mostly for ARM64).
>
> clang --version
> Chromium OS 5.0_pre300080-r1 clang version 5.0.0
>
> Cheers
>
> Matthias


OK.  I understood it is difficult to eliminate these warnings.
I consider this series for v4.13 (unless somebody comes up with a
better solution).


-- 
Best Regards
Masahiro Yamada