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

Masahiro Yamada
2017-04-30T14:00:03Z

Hi Matthias,

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(); \
} \

I'd like to see as much effort as possible
before we decide to hide this kind of warning.

Is it OK with you ?

--
Best Regards
Masahiro Yamada