static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
{
	struct bpf_verifier_state_list *new_sl;
	struct bpf_verifier_state_list *sl, **pprev;
	struct bpf_verifier_state *cur = env->cur_state, *new;
	int i, j, err, states_cnt = 0;
	bool add_new_state = env->test_state_freq ? true : false;

	cur->last_insn_idx = env->prev_insn_idx;
	if (!env->insn_aux_data[insn_idx].prune_point)
		/* this 'insn_idx' instruction wasn't marked, so we will not
		 * be doing state search here
		 */
		return 0;

	/* bpf progs typically have pruning point every 4 instructions
	 * http://vger.kernel.org/bpfconf2019.html#session-1
	 * Do not add new state for future pruning if the verifier hasn't seen
	 * at least 2 jumps and at least 8 instructions.
	 * This heuristics helps decrease 'total_states' and 'peak_states' metric.
	 * In tests that amounts to up to 50% reduction into total verifier
	 * memory consumption and 20% verifier time speedup.
	 */
	if (env->jmps_processed - env->prev_jmps_processed >= 2 &&
	    env->insn_processed - env->prev_insn_processed >= 8)
		add_new_state = true;

	pprev = explored_state(env, insn_idx);
	sl = *pprev;

	clean_live_states(env, insn_idx, cur);

	while (sl) {
		states_cnt++;
		if (sl->state.insn_idx != insn_idx)
			goto next;
		if (sl->state.branches) {
			if (states_maybe_looping(&sl->state, cur) &&
			    states_equal(env, &sl->state, cur)) {
				verbose_linfo(env, insn_idx, "; ");
				verbose(env, "infinite loop detected at insn %d\n", insn_idx);
				return -EINVAL;
			}
			/* if the verifier is processing a loop, avoid adding new state
			 * too often, since different loop iterations have distinct
			 * states and may not help future pruning.
			 * This threshold shouldn't be too low to make sure that
			 * a loop with large bound will be rejected quickly.
			 * The most abusive loop will be:
			 * r1 += 1
			 * if r1 < 1000000 goto pc-2
			 * 1M insn_procssed limit / 100 == 10k peak states.
			 * This threshold shouldn't be too high either, since states
			 * at the end of the loop are likely to be useful in pruning.
			 */
			if (env->jmps_processed - env->prev_jmps_processed < 20 &&
			    env->insn_processed - env->prev_insn_processed < 100)
				add_new_state = false;
			goto miss;
		}
		if (states_equal(env, &sl->state, cur)) {
			sl->hit_cnt++;
			/* reached equivalent register/stack state,
			 * prune the search.
			 * Registers read by the continuation are read by us.
			 * If we have any write marks in env->cur_state, they
			 * will prevent corresponding reads in the continuation
			 * from reaching our parent (an explored_state).  Our
			 * own state will get the read marks recorded, but
			 * they'll be immediately forgotten as we're pruning
			 * this state and will pop a new one.
			 */
			err = propagate_liveness(env, &sl->state, cur);

			/* if previous state reached the exit with precision and
			 * current state is equivalent to it (except precsion marks)
			 * the precision needs to be propagated back in
			 * the current state.
			 */
			err = err ? : push_jmp_history(env, cur);
			err = err ? : propagate_precision(env, &sl->state);
			if (err)
				return err;
			return 1;
		}
miss:
		/* when new state is not going to be added do not increase miss count.
		 * Otherwise several loop iterations will remove the state
		 * recorded earlier. The goal of these heuristics is to have
		 * states from some iterations of the loop (some in the beginning
		 * and some at the end) to help pruning.
		 */
		if (add_new_state)
			sl->miss_cnt++;
		/* heuristic to determine whether this state is beneficial
		 * to keep checking from state equivalence point of view.
		 * Higher numbers increase max_states_per_insn and verification time,
		 * but do not meaningfully decrease insn_processed.
		 */
		if (sl->miss_cnt > sl->hit_cnt * 3 + 3) {
			/* the state is unlikely to be useful. Remove it to
			 * speed up verification
			 */
			*pprev = sl->next;
			if (sl->state.frame[0]->regs[0].live & REG_LIVE_DONE) {
				u32 br = sl->state.branches;

				WARN_ONCE(br,
					  "BUG live_done but branches_to_explore %d\n",
					  br);
				free_verifier_state(&sl->state, false);
				kfree(sl);
				env->peak_states--;
			} else {
				/* cannot free this state, since parentage chain may
				 * walk it later. Add it for free_list instead to
				 * be freed at the end of verification
				 */
				sl->next = env->free_list;
				env->free_list = sl;
			}
			sl = *pprev;
			continue;
		}
next:
		pprev = &sl->next;
		sl = *pprev;
	}

	if (env->max_states_per_insn < states_cnt)
		env->max_states_per_insn = states_cnt;

	if (!env->bpf_capable && states_cnt > BPF_COMPLEXITY_LIMIT_STATES)
		return push_jmp_history(env, cur);

	if (!add_new_state)
		return push_jmp_history(env, cur);

	/* There were no equivalent states, remember the current one.
	 * Technically the current state is not proven to be safe yet,
	 * but it will either reach outer most bpf_exit (which means it's safe)
	 * or it will be rejected. When there are no loops the verifier won't be
	 * seeing this tuple (frame[0].callsite, frame[1].callsite, .. insn_idx)
	 * again on the way to bpf_exit.
	 * When looping the sl->state.branches will be > 0 and this state
	 * will not be considered for equivalence until branches == 0.
	 */
	new_sl = kzalloc(sizeof(struct bpf_verifier_state_list), GFP_KERNEL);
	if (!new_sl)
		return -ENOMEM;
	env->total_states++;
	env->peak_states++;
	env->prev_jmps_processed = env->jmps_processed;
	env->prev_insn_processed = env->insn_processed;

	/* add new state to the head of linked list */
	new = &new_sl->state;
	err = copy_verifier_state(new, cur);
	if (err) {
		free_verifier_state(new, false);
		kfree(new_sl);
		return err;
	}
	new->insn_idx = insn_idx;
	WARN_ONCE(new->branches != 1,
		  "BUG is_state_visited:branches_to_explore=%d insn %d\n", new->branches, insn_idx);

	cur->parent = new;
	cur->first_insn_idx = insn_idx;
	clear_jmp_history(cur);
	new_sl->next = *explored_state(env, insn_idx);
	*explored_state(env, insn_idx) = new_sl;
	/* connect new state to parentage chain. Current frame needs all
	 * registers connected. Only r6 - r9 of the callers are alive (pushed
	 * to the stack implicitly by JITs) so in callers' frames connect just
	 * r6 - r9 as an optimization. Callers will have r1 - r5 connected to
	 * the state of the call instruction (with WRITTEN set), and r0 comes
	 * from callee with its full parentage chain, anyway.
	 */
	/* clear write marks in current state: the writes we did are not writes
	 * our child did, so they don't screen off its reads from us.
	 * (There are no read marks in current state, because reads always mark
	 * their parent and current state never has children yet.  Only
	 * explored_states can get read marks.)
	 */
	for (j = 0; j <= cur->curframe; j++) {
		for (i = j < cur->curframe ? BPF_REG_6 : 0; i < BPF_REG_FP; i++)
			cur->frame[j]->regs[i].parent = &new->frame[j]->regs[i];
		for (i = 0; i < BPF_REG_FP; i++)
			cur->frame[j]->regs[i].live = REG_LIVE_NONE;
	}

	/* all stack frames are accessible from callee, clear them all */
	for (j = 0; j <= cur->curframe; j++) {
		struct bpf_func_state *frame = cur->frame[j];
		struct bpf_func_state *newframe = new->frame[j];

		for (i = 0; i < frame->allocated_stack / BPF_REG_SIZE; i++) {
			frame->stack[i].spilled_ptr.live = REG_LIVE_NONE;
			frame->stack[i].spilled_ptr.parent =
						&newframe->stack[i].spilled_ptr;
		}
	}
	return 0;
}