static int check_stack_write_var_off(struct bpf_verifier_env *env,
				     /* func where register points to */
				     struct bpf_func_state *state,
				     int ptr_regno, int off, int size,
				     int value_regno, int insn_idx)
{
	struct bpf_func_state *cur; /* state of the current function */
	int min_off, max_off;
	int i, err;
	struct bpf_reg_state *ptr_reg = NULL, *value_reg = NULL;
	bool writing_zero = false;
	/* set if the fact that we're writing a zero is used to let any
	 * stack slots remain STACK_ZERO
	 */
	bool zero_used = false;

	cur = env->cur_state->frame[env->cur_state->curframe];
	ptr_reg = &cur->regs[ptr_regno];
	min_off = ptr_reg->smin_value + off;
	max_off = ptr_reg->smax_value + off + size;
	if (value_regno >= 0)
		value_reg = &cur->regs[value_regno];
	if (value_reg && register_is_null(value_reg))
		writing_zero = true;

	err = grow_stack_state(state, round_up(-min_off, BPF_REG_SIZE));
	if (err)
		return err;


	/* Variable offset writes destroy any spilled pointers in range. */
	for (i = min_off; i < max_off; i++) {
		u8 new_type, *stype;
		int slot, spi;

		slot = -i - 1;
		spi = slot / BPF_REG_SIZE;
		stype = &state->stack[spi].slot_type[slot % BPF_REG_SIZE];
		mark_stack_slot_scratched(env, spi);

		if (!env->allow_ptr_leaks
				&& *stype != NOT_INIT
				&& *stype != SCALAR_VALUE) {
			/* Reject the write if there's are spilled pointers in
			 * range. If we didn't reject here, the ptr status
			 * would be erased below (even though not all slots are
			 * actually overwritten), possibly opening the door to
			 * leaks.
			 */
			verbose(env, "spilled ptr in range of var-offset stack write; insn %d, ptr off: %d",
				insn_idx, i);
			return -EINVAL;
		}

		/* Erase all spilled pointers. */
		state->stack[spi].spilled_ptr.type = NOT_INIT;

		/* Update the slot type. */
		new_type = STACK_MISC;
		if (writing_zero && *stype == STACK_ZERO) {
			new_type = STACK_ZERO;
			zero_used = true;
		}
		/* If the slot is STACK_INVALID, we check whether it's OK to
		 * pretend that it will be initialized by this write. The slot
		 * might not actually be written to, and so if we mark it as
		 * initialized future reads might leak uninitialized memory.
		 * For privileged programs, we will accept such reads to slots
		 * that may or may not be written because, if we're reject
		 * them, the error would be too confusing.
		 */
		if (*stype == STACK_INVALID && !env->allow_uninit_stack) {
			verbose(env, "uninit stack in range of var-offset write prohibited for !root; insn %d, off: %d",
					insn_idx, i);
			return -EINVAL;
		}
		*stype = new_type;
	}
	if (zero_used) {
		/* backtracking doesn't work for STACK_ZERO yet. */
		err = mark_chain_precision(env, value_regno);
		if (err)
			return err;
	}
	return 0;
}