[PATCH 2/4] Makefile: Mute warning for __builtin_return_address(>0) for tracing only

[Steven Rostedt]

From: Steven Rostedt <rostedt@xxxxxxxxxxx>

With the latest gcc compilers, they give a warning if
__builtin_return_address() parameter is greater than 0. That is because if
it is used by a function called by a top level function (or in the case of
the kernel, by assembly), it can try to access stack frames outside the
stack and crash the system.

The tracing system uses __builtin_return_address() of up to 2! But it is
well aware of the dangers that it may have, and has even added precautions
to protect against it (see the thunk code in arch/x86/entry/thunk*.S)

Linus originally added KBUILD_CFLAGS that would suppress the warning for the
entire kernel, as simply adding KBUILD_CFLAGS to the tracing directory
wouldn't work. The tracing directory plays a bit with the CFLAGS and
requires a little more logic.

This adds that special logic to only suppress the warning for the tracing
directory. If it is used anywhere else outside of tracing, the warning will
still be triggered.

Link: http://lkml.kernel.org/r/20160728223043.51996267@xxxxxxxxxxxxxxxx

Tested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 Makefile              | 1 -
 kernel/trace/Makefile | 4 ++++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 393b6159ae92..d384848478b9 100644
--- a/Makefile
+++ b/Makefile
@@ -620,7 +620,6 @@ include arch/$(SRCARCH)/Makefile
 
 KBUILD_CFLAGS	+= $(call cc-option,-fno-delete-null-pointer-checks,)
 KBUILD_CFLAGS	+= $(call cc-disable-warning,maybe-uninitialized,)
-KBUILD_CFLAGS	+= $(call cc-disable-warning,frame-address,)
 
 ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS	+= -Os
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 979e7bfbde7a..d0a1617b52b4 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -1,4 +1,8 @@
 
+# We are fully aware of the dangers of __builtin_return_address()
+FRAME_CFLAGS := $(call cc-disable-warning,frame-address)
+KBUILD_CFLAGS += $(FRAME_CFLAGS)
+
 # Do not instrument the tracer itself:
 
 ifdef CONFIG_FUNCTION_TRACER
-- 
2.8.1