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

[Masahiro Yamada]

Hi Matthias,


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.




-- 
Best Regards
Masahiro Yamada