2.11.8. __user_stack_cleanup_space()
If defined, this function enables a SIGSTAK handler to run after a stack overflow. It returns the size of the extra stack your system requires.
__value_in_regs struct __stack_cleanup_space _user_stack_cleanup_space(unsigned ignore);
There is no default implementation of this function.
If you define this function, it must return to r0 the amount (measured in bytes) of extra stack that must be available after stack overflow in order to support recovery from overflow.