s390/ftrace: Use unwinder instead of __builtin_return_address()

Using __builtin_return_address(n) might return undefined values
when used with values of n outside of the stack. This was noticed
when __builtin_return_address() was called in ftrace on top level
functions like the interrupt handlers.

As this behaviour cannot be fixed, use the s390 stack unwinder and
remove the ftrace compilation flags for unwind_bc.c and stacktrace.c
to prevent the unwinding function polluting function traces.

Another advantage is that this also works with clang.
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/include/asm/ftrace.h

@@ -8,12 +8,8 @@
 
 #ifndef __ASSEMBLY__
 
-#ifdef CONFIG_CC_IS_CLANG
-/* https://llvm.org/pr41424 */
-#define ftrace_return_address(n) 0UL
-#else
-#define ftrace_return_address(n) __builtin_return_address(n)
-#endif
+unsigned long return_address(unsigned int n);
+#define ftrace_return_address(n) return_address(n)
 
 void ftrace_caller(void);
 

arch/s390/kernel/Makefile

@@ -11,6 +11,8 @@ CFLAGS_REMOVE_ftrace.o		= $(CC_FLAGS_FTRACE)
 # Do not trace early setup code
 CFLAGS_REMOVE_early.o		= $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_rethook.o		= $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_stacktrace.o	= $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_unwind_bc.o	= $(CC_FLAGS_FTRACE)
 
 endif
 

arch/s390/kernel/stacktrace.c

@@ -101,3 +101,22 @@ void arch_stack_walk_user(stack_trace_consume_fn consume_entry, void *cookie,
 	}
 	pagefault_enable();
 }
+
+unsigned long return_address(unsigned int n)
+{
+	struct unwind_state state;
+	unsigned long addr;
+
+	/* Increment to skip current stack entry */
+	n++;
+
+	unwind_for_each_frame(&state, NULL, NULL, 0) {
+		addr = unwind_get_return_address(&state);
+		if (!addr)
+			break;
+		if (!n--)
+			return addr;
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(return_address);