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

Masahiro Yamada
2017-05-06T16:52:37Z

Hi Matthias,

2017-05-02 10:23 GMT+09:00 Matthias Kaehlcke <mka@chromium.org>:
> 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@chromium.org>:
>> > 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@chromium.org>
>>
>>
>> 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