Nios II Classic Software Developer’s Handbook

ID 683282
Date 5/14/2015
Public
Document Table of Contents

2.1.7.3.2. Stack Overflow

To enable Stack Checking, go to the BSP Editor and click on the Settings tab, click on Advanced, hal, and then click enable_runtime_stack_checking. When the Stack Checking is enabled, extra code is added at the start of each function call to:
  • Check the current value of the stack pointer
  • Compare this to the max stack size, which is stored in the Exception Temp (ET) Register
If the stack pointed to is outside of the valid range, the software branches and calls a “break 3” instruction. This is seen by the Debug Control module.
Note: With stack checking on, malloc() and new() can detect heap exhaustion, as well.

Example of function with stack checking code

    ___vfprintf_internal_r:
000002ec:   addi sp,sp,-1308
000002f0:   bgeu sp,et,0x2f8 <___vfprintf_internal_r+12>
000002f4:   break 3

The bgeu and break 3 lines are what is added for the stack overflow checking. If the stack pointer has grown beyond its limits the break is called.

Recognizing and Debugging a Stack Overflow

When a stack overflow occurs, having registered an instruction-related exception handler helps you identify it by its behavior.

Default Instruction-Related Exception Handler

The default value for an instruction-related exception handler is when it is not registered.

If you don’t register an instruction-related exception handler, the “break 3” instruction is picked up by the software trap logic and a break is passed to the debugger. You must roll back through the history in the debugger to find the memory operation that triggered the stack checking break.
Note: With stack checking on, malloc() and new() can detect heap exhaustion.

How to Isolate the Cause of a Sigtrap

How to isolate the cause of a sigtrap seen in the debugger with no instruction-related exception handle?

The Debugger breaks with sigtrap:

  1. Use the thread view in the debug window and select the last state.

    This is the highest number. The last thread will be the actual call than overflowed.

  2. Switch to instruction stepping mode in the debugger by pressing the i-> button in the debug window, which opens the memory disassembly view.
    If there has been a stack overflow the disassembly view should show execution pointing to a break3 after the stack check:
            
              ___vfprintf_internal_r:
    000002ec:   addi sp,sp,-1308
    000002f0:   bgeu sp,et,0x2f8 <___vfprintf_internal_r+12>
    000002f4:   break 3
  3. Check the value of sp and et which holds the max stack side in the Nios II register view.
  4. Move to the prior state in the debug window and re-check sp vs et.
    Figure 2. Nios II Debug window

Custom Instruction-Related Exception Handler

For use outside the debugger, you can register your own instruction-related exception handler which is called when the break (or any exception) is seen.

On an exception, including overflow, the HAL calls the instruction-related exception handler, passing in the cause field from the exception register, and the address which caused the exception. At this point, it is up to you to decide what do.

For more information about how to register an instruction-related exception, refer to