On Mon, Jul 30, 2018 at 12:48:06PM -0700, Nick Desaulniers wrote:

On Mon, Jul 30, 2018 at 11:39 AM Nathan Chancellor
[off-list ref] wrote:

quoted

On Mon, Jul 30, 2018 at 10:06:20AM -0700, Nick Desaulniers wrote:

quoted

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: stable@vger.kernel.org # 4.17, 4.14, 4.9, 4.4
Signed-off-by: Nick Desaulniers <ndesaulniers@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

Ah, good catch.  I thought I had tested locally with gcc-8, but it
seems that it was likely only godbolt.  I do see the errors locally
when building with gcc-8.

quoted

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.

I'll fix this up, triple check with gcc-8, and attribute you in the
Suggested-by tag.  Thank you for verifying.
-- 
Thanks,
~Nick Desaulniers

I forgot to mention there is a similar macro that will need this same
fix in arch/arm64/include/asm/processor.h (current_text_addr), if you
want to tackle it in v2. Do CC me on the next series so that I can test,
thank you for doing this!

Cheers,
Nathan