Quick and dirty cortex m stack watermarking

cortex_m_stack_paint.rs

extern "C" {
    static _stack_start: u32;  // End of the stack (top of memory)
    static _ram_length: u32;   // Total RAM length (stack size in bytes)
}

const SAFE_ZONE_BYTES: usize = 256;  // Reserve top and bottom bytes

#[inline(always)]
pub fn check_stack_high_water_mark() -> usize {
    check_stack_high_water_mark_inner::<SAFE_ZONE_BYTES>()
}
#[inline(always)]
pub fn paint_stack() {
    paint_stack_inner::<SAFE_ZONE_BYTES>();
}

pub fn paint_stack_inner<const SAFE_ZONE_BYTES: usize>() {
    unsafe {
        let stack_start = &_stack_start as *const u32 as *mut u8;
        let stack_size = &_ram_length as *const u32 as usize;
        let stack_end: *mut u8 = stack_start.offset(-(stack_size as isize));
        let safe_stack_end = stack_end.offset(SAFE_ZONE_BYTES as isize);

        let bytes_to_write = stack_size - (2 * SAFE_ZONE_BYTES);
        core::ptr::write_bytes(safe_stack_end, 0xAA, bytes_to_write);
    }
}


pub fn check_stack_high_water_mark_inner<const SAFE_ZONE_BYTES: usize>() -> usize {
    unsafe {
        let stack_start = &_stack_start as *const u32 as *mut u8;
        let stack_size = &_ram_length as *const u32 as usize;
        let stack_end = stack_start.offset(-(stack_size as isize));
        let safe_stack_end = stack_end.offset(SAFE_ZONE_BYTES as isize);

        let mut current = safe_stack_end;
        while current < stack_start && *current == 0xAA {
            current = current.offset(1);
        }

        let used_stack = stack_start.offset_from(current) as usize;
        used_stack
    }
}