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

From: Nathan Chancellor
Date: Mon Jul 30 2018 - 14:39:50 EST


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'Ãtre: https://gcc.gnu.org/onlinedocs/cpp/Pragmas.html
>
> Cc: stable@xxxxxxxxxxxxxxx # 4.17, 4.14, 4.9, 4.4
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> 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 âlocal_bh_disableâ:
./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 âlocal_bh_enableâ:
./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