linux-kernel - [PATCH] lockdep: Quiet gcc about dangerous __builtin_return

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Date:   Thu, 8 Sep 2016 12:34:33 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH] lockdep: Quiet gcc about dangerous
 __builtin_return_address() operations

[
  Boris, does this quiet gcc for you?
  I haven't fully tested this yet, as I still don't have a compiler
  that does the warning.
]

Gcc's new warnings about __builtin_return_address(n) operations with
n > 0 is popping up around the kernel. The operation is dangerous, and
the warning is "good to know". But there's instances that we use
__builtin_return_address(n) with n > 0 and are aware of the issues,
and work around them. And its used mostly for tracing and debugging. In
these cases, the warning becomes a distraction and is not helpful.

To get better lock issue traces, a function like get_lock_parent_ip()
uses __builtin_return_address() to find the caller of the lock, and
skip over the internal callers of the lock itself. Currently it is only
used in the kernel/ directory and only if certain configs are enabled.

Create a new config called CONFIG_USING_GET_LOCK_PARENT_IP that gets
selected when another config relies on get_lock_parent_ip(), and this
will now enable the function get_lock_parent_ip(), otherwise it wont be
defined. It will also disable the frame-address warnings from gcc in
the kernel directory.

Reported-by: Borislav Petkov <bp@...en8.de>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 1e2b316d6693..7862c59ac725 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -714,6 +714,7 @@ static inline void __ftrace_enabled_restore(int enabled)
 #define CALLER_ADDR5 ((unsigned long)ftrace_return_address(5))
 #define CALLER_ADDR6 ((unsigned long)ftrace_return_address(6))
 
+#ifdef CONFIG_USING_GET_LOCK_PARENT_IP
 static inline unsigned long get_lock_parent_ip(void)
 {
 	unsigned long addr = CALLER_ADDR0;
@@ -725,6 +726,7 @@ static inline unsigned long get_lock_parent_ip(void)
 		return addr;
 	return CALLER_ADDR2;
 }
+#endif
 
 #ifdef CONFIG_IRQSOFF_TRACER
   extern void time_hardirqs_on(unsigned long a0, unsigned long a1);
diff --git a/kernel/Makefile b/kernel/Makefile
index e2ec54e2b952..bff8214bf5f6 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -11,6 +11,13 @@ obj-y     = fork.o exec_domain.o panic.o \
 	    notifier.o ksysfs.o cred.o reboot.o \
 	    async.o range.o smpboot.o
 
+# Tracing may do some dangerous __builtin_return_address() operations
+# We know they are dangerous, we don't need gcc telling us that.
+ifdef CONFIG_USING_GET_LOCK_PARENT_IP
+FRAME_CFLAGS := $(call cc-disable-warning,frame-address)
+KBUILD_CFLAGS += $(FRAME_CFLAGS)
+endif
+
 obj-$(CONFIG_MULTIUSER) += groups.o
 
 ifdef CONFIG_FUNCTION_TRACER
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 72c07c2ffd79..f02a02fdc3ed 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -197,6 +197,7 @@ config PREEMPT_TRACER
 	select RING_BUFFER_ALLOW_SWAP
 	select TRACER_SNAPSHOT
 	select TRACER_SNAPSHOT_PER_CPU_SWAP
+	select USING_GET_LOCK_PARENT_IP
 	help
 	  This option measures the time spent in preemption-off critical
 	  sections, with microsecond accuracy.
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 2307d7c89dac..9a2bd7af64df 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -977,6 +977,7 @@ config TIMER_STATS
 config DEBUG_PREEMPT
 	bool "Debug preemptible kernel"
 	depends on DEBUG_KERNEL && PREEMPT && TRACE_IRQFLAGS_SUPPORT
+	select USING_GET_LOCK_PARENT_IP
 	default y
 	help
 	  If you say Y here then the kernel will use a debug variant of the
@@ -1159,8 +1160,17 @@ config LOCK_TORTURE_TEST
 
 endmenu # lock debugging
 
+config USING_GET_LOCK_PARENT_IP
+        bool
+	help
+	  Enables the use of the function get_lock_parent_ip() that
+	  will use __builtin_return_address(n) with n > 0 causing
+	  some gcc warnings. When this is selected, those warnings
+	  will be suppressed.
+
 config TRACE_IRQFLAGS
 	bool
+	select USING_GET_LOCK_PARENT_IP
 	help
 	  Enables hooks to interrupt enabling and disabling for
 	  either tracing or lock debugging.