add mpu_stack_guard pseudomodule (just Cortex-M for now)

triggers an exception during stack overflow,
but at a cost of 32-63 bytes of RAM per thread.

Makefile.pseudomodules

@@ -34,6 +34,7 @@ PSEUDOMODULES += lwip_stats
 PSEUDOMODULES += lwip_tcp
 PSEUDOMODULES += lwip_udp
 PSEUDOMODULES += lwip_udplite
+PSEUDOMODULES += mpu_stack_guard
 PSEUDOMODULES += netdev_default
 PSEUDOMODULES += netif
 PSEUDOMODULES += netstats

cpu/cortexm_common/vectors_cortexm.c

@@ -28,9 +28,14 @@
 #include "cpu.h"
 #include "kernel_init.h"
 #include "board.h"
+#include "mpu.h"
 #include "panic.h"
 #include "vectors_cortexm.h"
 
+#ifndef SRAM_BASE
+#define SRAM_BASE 0
+#endif
+
 /**
  * @brief   Memory markers, defined in the linker script
  * @{
@@ -48,6 +53,8 @@ extern uint32_t _sram;
 extern uint32_t _eram;
 /** @} */
 
+static const uintptr_t stack_base = (uintptr_t)&_sstack;
+
 /**
  * @brief   Allocation of the interrupt stack
  */
@@ -94,6 +101,18 @@ void reset_handler_default(void)
         *(dst++) = 0;
     }
 
+#ifdef MODULE_MPU_STACK_GUARD
+    if (stack_base != SRAM_BASE) {
+        mpu_configure(
+            0,                                              /* MPU region 0 */
+            stack_base + 31,                                /* Base Address (rounded up) */
+            MPU_ATTR(1, AP_RO_RO, 0, 1, 0, 1, MPU_SIZE_32B) /* Attributes and Size */
+        );
+
+        mpu_enable();
+    }
+#endif
+
     post_startup();
 
     /* initialize the board (which also initiates CPU initialization) */