Re: [PATCH] kernel.h: Disable -Wreturn-stack-address for _THIS_IP_

[Nathan Chancellor]

On Mon, Jul 30, 2018 at 10:06:20AM -0700, Nick Desaulniers wrote:
> Starting with Clang-7.0, _THIS_IP_ generates -Wreturn-stack-address
> warnings for almost every translation unit. In general, I'd prefer to
> leave this on (returning the address of a stack allocated variable is in
> general a bad idea) and disable it only at whitelisted call sites.
> 
> We can't do something like:
>  #pragma clang diagnostic push
>  #pragma clang diagnostic ignored "-Wreturn-stack-address"
>  <code>
>  #pragma clang diagnostic pop
> 
> in a GNU Statement Expression or macro, hence we use _Pragma, which is
> its raison d'être: https://gcc.gnu.org/onlinedocs/cpp/Pragmas.html
> 
> Cc: sta...@vger.kernel.org # 4.17, 4.14, 4.9, 4.4
> Signed-off-by: Nick Desaulniers <ndesaulni...@google.com>
> ---
>  include/linux/kernel.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 941dc0a5a877..5906f5727f90 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -168,7 +168,15 @@
>  
>  
>  #define _RET_IP_             (unsigned long)__builtin_return_address(0)
> -#define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })
> +#define _THIS_IP_  (                                                 \
> +{                                                                    \
> +     _Pragma("clang diagnostic push")                                \
> +     _Pragma("clang diagnostic ignored \"-Wreturn-stack-address\"")  \
> +     __label__ __here;                                               \
> +__here: (unsigned long)&&__here;                                     \
> +     _Pragma("clang diagnostic pop")                                 \
> +}                                                                    \
> +)
>  
>  #ifdef CONFIG_LBDAF
>  # include <asm/div64.h>
> -- 
> 2.18.0.233.g985f88cf7e-goog
> 

This generates a ton of warnings with GCC:

In file included from ./include/linux/spinlock.h:58,
                 from ./include/linux/mmzone.h:8,
                 from ./include/linux/gfp.h:6,
                 from ./include/linux/slab.h:15,
                 from ./include/linux/crypto.h:24,
                 from arch/x86/kernel/asm-offsets.c:9:
./include/linux/bottom_half.h: In function ‘local_bh_disable’:
./include/linux/bottom_half.h:19: error: ignoring #pragma clang diagnostic 
[-Werror=unknown-pragmas]
  __local_bh_disable_ip(_THIS_IP_, SOFTIRQ_DISABLE_OFFSET);

./include/linux/bottom_half.h:19: error: ignoring #pragma clang diagnostic 
[-Werror=unknown-pragmas]   
./include/linux/bottom_half.h:19: error: ignoring #pragma clang diagnostic 
[-Werror=unknown-pragmas]                             
./include/linux/bottom_half.h: In function ‘local_bh_enable’:
./include/linux/bottom_half.h:32: error: ignoring #pragma clang diagnostic 
[-Werror=unknown-pragmas]
  __local_bh_enable_ip(_THIS_IP_, SOFTIRQ_DISABLE_OFFSET);

./include/linux/bottom_half.h:32: error: ignoring #pragma clang diagnostic 
[-Werror=unknown-pragmas]
./include/linux/bottom_half.h:32: error: ignoring #pragma clang diagnostic 
[-Werror=unknown-pragmas]
cc1: all warnings being treated as errors
make[1]: *** [Kbuild:56: arch/x86/kernel/asm-offsets.s] Error 1
make: *** [Makefile:1081: prepare0] Error 2

A proper solution is probably going to involve what was done for the
-Wattribute-alias warnings from GCC 8 in commits 8793bb7f4a9d ("kbuild:
add macro for controlling warnings to linux/compiler.h") and
bee20031772a ("disable -Wattribute-alias warning for SYSCALL_DEFINEx()")

I'll take a look at it in a bit unless someone beats me to it.

Thanks!
Nathan