linux-kernel - [PATCH 3.12 13/84] Makefile: Mute warning for __builtin_return

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Message-Id: <ea837f4b8359c3b30ccc3ed85ee6a06c33503a67.1476690493.git.jslaby@suse.cz>
Date:   Mon, 17 Oct 2016 09:51:00 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     stable@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 13/84] Makefile: Mute warning for __builtin_return_address(>0) for tracing only

From: Steven Rostedt <rostedt@...dmis.org>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit 377ccbb483738f84400ddf5840c7dd8825716985 upstream.

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@grimm.local.home

Tested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 Makefile              | 1 -
 kernel/trace/Makefile | 4 ++++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 56ad5978a4e7..39bfb82f7106 100644
--- a/Makefile
+++ b/Makefile
@@ -583,7 +583,6 @@ endif # $(dot-config)
 all: vmlinux

 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 d7e2068e4b71..67d201b66794 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
-- 
2.10.1