I'm da real mvp

sbrk_syscall.c

int sys_sbrk(__intptr_t change, void **retval) {
    struct addrspace *as = proc_getas();
	if (as == NULL) {
		return EFAULT;
	}

    vaddr_t heapbreak = as->as_heapvbase + as->as_heapsize;

    // we're not checking page alignment of 
    // change (amount) here or anything
    // clever like that here (@273)

    as->as_heapsize += change;

    // address lower than the beginning of the heap
    if (as->as_heapvbase + as->as_heapsize < as->as_heapvbase) {
        *retval = (void *)-1;
        return EINVAL;
    }
    
    // address growing into the stack
    if (as->as_heapvbase + as->as_heapsize >= USERSTACK - PAGE_SIZE * VM_STACKPAGES) {
        *retval = (void *)-1;
        return EINVAL;
    }
    
    *retval = (void *) heapbreak;
    return 0;
}