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

Matthias Kaehlcke
2017-05-02T01:23:33Z

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.

> I'd like to see as much effort as possible
> before we decide to hide this kind of warning.
>
> Is it OK with you ?

Sounds good, though I will probably leave this one for someone else :)

Cheers

Matthias