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Cortex M3 - How detect stack overflow?

Andre Pereira
Note: This was originally posted on 12th January 2011 at http://forums.arm.com

Hi everybody,

I've started developing whit ARM few weeks a go and I have this doubt:
The Cortex-M3 have some hardware detection of stack overflow?

Let me explain:
I've used PIC24 for my last projects, and using as example, the PIC24 have the register "SPLIM" (Stack Pointer Limit) where you write the address of the last byte of your software stack and if your software try to write in more than the address wrote in the SPLIM register, the processor causes a "trap" (a kind of interruption with an dedicated vector).
Using this "trap" you can know if you have any stack overflow.

The ARM Cortex-M3 have something like that?

I've searched in the ARM v7 and the Cortex-M documentation and I just found something about BusFault in stacking but I don't understand very well.

Sorry for my poor english (I'm from Brazil) and thanks in advance.
André Rairan.
  • Note: This was originally posted on 12th January 2011 at http://forums.arm.com

    Thanks for reply Joseph,

    So, can I set the start stack addres to (initial ram addres + stack size) them when the stack try to decrement less then the (initial ram addres) trigger a bus fault?
  • Note: This was originally posted on 13th January 2011 at http://forums.arm.com

    Great considerations, thanks again.

    I would like to know in runtime, then I could create a kind of LOG to know the problem and solve increasing the stack size or changing the program.
    Just to protect my software from this "bug"

    How could I use the MPU for this?

    Regards.
  • Arm Employee Badge
    Note: This was originally posted on 12th January 2011 at http://forums.arm.com

    In debug environment, you can use the data watchpoint feature to detect access to the last location in stack and trigger a halt of the core when it happen.

    Bus fault happen when the bus slave return an error response (e.g. access to invalid address location). In stack overflow detection, the stack push might still be in a valid location in SRAM, but exceed the expected stack size. In this case it won't trigger the bus fault. Therefore bus fault is not suitable for this.

    Note that in ARM processors, the stack pointer decrement in stack push, and increment in stack pop.
    regards,
    Joseph
  • Arm Employee Badge
    Note: This was originally posted on 13th January 2011 at http://forums.arm.com

    Hi,

    Technically possible, but the trouble is that usually compiler suite place data in the beginning of SRAM. So in this case you will have to place the data above the initial stack pointer value, which can be messy to setup (depends on tool chain).

    Secondly, once you get the bus fault, the stack pointer will be in invalid memory location. So you can't program your bus fault handler or hard fault handler in C as it might try to use stack.

    Thirdly, the C compiler might insert stack pointer initialization code in C startup code (before enter main).
    So the actual value of stack pointer at the start of the main could be quite different from the initial stack pointer in vector table,

    Another thing you need to consider is the amount of stack space the interrupt handlers uses.  If the interrupt can be nestes, you need to calculate the worst case stack size required when a number of interrupt nesting occurred.

    Do you want the stack overflow detection to be used in debug time or runtime?
    For run time maybe you can consider using the MPU (Memory Protection Unit) for this.

    regards,
    Joseph
  • Arm Employee Badge
    Note: This was originally posted on 14th January 2011 at http://forums.arm.com

    First of all, in case you don't know, many compiler suites generate information about how much stack your program required.
    For example, in Keil MDK, after you compile a project, you will find a html file in the project directory. Inside you can find something like this
    main (Thumb, 36 bytes, Stack size 8  bytes, hello.o(.text))

    [Stack]

    • Max Depth = 128
    • Call Chain = main ⇒ __2printf ⇒ _printf_char_common ⇒ __printf
    (it is a long file, but  you should be able to locate this information easily).

    From the same file, you should be able to locate the stack size required for your interrupt handlers.
    For each level of interrupt, you need to reserved 9 words. (Depending on the stack pointer value when interrupt happen, a word of padding might be need so that the stack frame is double word aligned).

    Now you can work out the maximum stack size you need by consider
       the stack size required from main( )
    + the stack size required for interrupt handlers when nested
    + 9 words x number of interrupt level.

    However, this will not check the situations when the stack pointer is modified manually inside the code.

    If using MPU, you can define two regions (the MPU support 8 regions):
    Region 0 cover the whole 4GB address space to allow the program to access program, data and I/O as usual.
    Region 1 define a small memory block (minimum MPU region is 32 bytes) to be blocked from any accesses.
    The rest of the regions (region 2 to region 7) remain disabled.
    Then enable the MPU (MPU_CTRL set to 1).
    When the block memory is accessed, a hardfault (or memoery management fault if you enable this exception) would take place.

    By setting the blocked memory region to the limited of the stack memory, the hardfault will be triggered when your program use more stack then you expected.

    Note:
    1. A MPU region starting address must be aligned to the size of the MPU region.
    2. By using this method, the values of registers pushed to the stack before hardfault is entered could be lost.

    If you want to allow the program to be resumed, it is easier to use data watchpoint comparator in DWT to trigger debug monitor exception.