[PATCH 1/2] tracing/arm64: Have max stack tracer handle the case of return address after data

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

Most archs (well at least x86) store the function call return address on the
stack before storing the local variables for the function. The max stack
tracer depends on this in its algorithm to display the stack size of each
function it finds in the back trace.

Some archs (arm64), may store the return address (from its link register)
just before calling a nested function. There's no reason to save the link
register on leaf functions, as it wont be updated. This breaks the algorithm
of the max stack tracer.

Add a new define ARCH_RET_ADDR_AFTER_LOCAL_VARS that an architecture may set
if it stores the return address (link register) after it stores the
function's local variables, and have the stack trace shift the values of the
mapped stack size to the appropriate functions.

Link: 20190802094103.163576-1-jiping.ma2@xxxxxxxxxxxxx

Reported-by: Jiping Ma <jiping.ma2@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 arch/arm64/include/asm/ftrace.h |  1 +
 kernel/trace/trace_stack.c      | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 5ab5200b2bdc..13a4832cfb00 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -13,6 +13,7 @@
 #define HAVE_FUNCTION_GRAPH_FP_TEST
 #define MCOUNT_ADDR		((unsigned long)_mcount)
 #define MCOUNT_INSN_SIZE	AARCH64_INSN_SIZE
+#define ARCH_RET_ADDR_AFTER_LOCAL_VARS 1
 
 #ifndef __ASSEMBLY__
 #include <linux/compat.h>
diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index 5d16f73898db..40e4a88eea8f 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -158,6 +158,20 @@ static void check_stack(unsigned long ip, unsigned long *stack)
 			i++;
 	}
 
+#ifdef ARCH_RET_ADDR_AFTER_LOCAL_VARS
+	/*
+	 * Some archs will store the link register before calling
+	 * nested functions. This means the saved return address
+	 * comes after the local storage, and we need to shift
+	 * for that.
+	 */
+	if (x > 1) {
+		memmove(&stack_trace_index[0], &stack_trace_index[1],
+			sizeof(stack_trace_index[0]) * (x - 1));
+		x--;
+	}
+#endif
+
 	stack_trace_nr_entries = x;
 
 	if (task_stack_end_corrupted(current)) {
-- 
2.20.1