Stack overflow detection

Use guard value to detect stack overflow

Signed-off-by: Vic Yang <victoryang@google.com>

BUG=chrome-os-partner:8069
TEST=Compile with detection enabled. Cause a task to overflow and see
device halted. Hook gdb and see it stopped at the assertion.

Change-Id: I3417cca8edf4e1291ccb7848bd564b631a9ce463

1 files changed, 14 insertions, 2 deletions

diff --git a/core/cortex-m/task.c b/core/cortex-m/task.c
index 34c12d674c..d11024c8ad 100644
--- a/core/cortex-m/task.c
+++ b/core/cortex-m/task.c
@@ -27,6 +27,7 @@ typedef union {
 	struct {
 		uint32_t sp;       /* saved stack pointer for context switch */
 		uint32_t events;   /* bitmaps of received events */
+		uint32_t guard;    /* Guard value to detect stack overflow */
 		uint8_t stack[0];  /* task stack */
 	};
 	uint32_t context[TASK_SIZE/4];
@@ -72,10 +73,12 @@ static void task_exit_trap(void)
 }
 
 
+#define GUARD_VALUE 0x12345678
 
 /* declare and fill the contexts for all the tasks */
 #define TASK(n, r, d)  {						\
 	.context[0] = (uint32_t)(tasks + TASK_ID_##n + 1) - 64,	        \
+	.context[2] = GUARD_VALUE,					\
 	.context[TASK_SIZE/4 - 8/*r0*/] = (uint32_t)d,                  \
 	.context[TASK_SIZE/4 - 3/*lr*/] = (uint32_t)task_exit_trap,     \
 	.context[TASK_SIZE/4 - 2/*pc*/] = (uint32_t)r,                  \
@@ -87,8 +90,13 @@ static task_ tasks[] __attribute__((section(".data.tasks")))
 	CONFIG_TASK_LIST
 };
 #undef TASK
-/* reserve space to discard context on first context switch */
-uint32_t scratchpad[17] __attribute__((section(".data.tasks")));
+/* Reserve space to discard context on first context switch.
+ * Fill in guard value to prevent overflow detection failure at first
+ * context switch. Modify this if the position of guard value changes.
+ * TODO: come up with a better way than hardcoding the position of guard value.
+ */
+uint32_t scratchpad[17] __attribute__((section(".data.tasks"))) =
+	{0, 0, GUARD_VALUE};
 
 /* context switch at the next exception exit if needed */
 /* TODO: who sets this back to 0 after it's set to 1? */
@@ -181,6 +189,10 @@ void svc_handler(int desched, task_id_t resched)
 	asm volatile("cpsid f\n"
 		     "isb\n");
 	current = __get_task_scheduled();
+#ifdef CONFIG_OVERFLOW_DETECT
+	ASSERT(current->guard == GUARD_VALUE);
+#endif
+
 	if (desched && !current->events) {
 		/* Remove our own ready bit */
 		tasks_ready &= ~(1 << (current-tasks));