LiDAR sensors measure the environment by emitting lasers and, when combined with deep neural networks (DNNs), can effectively identify surrounding obstacles such as vehicles and pedestrians. Given its crucial role in autonomous driving perception, the security of LiDAR is closely tied to driving safety. Some studies have explored its vulnerabilities to physical-world attacks, such as laser-based attacks or adversarial objects. However, these methods are either extremely difficult to execute or lack stealth and flexibility. In this paper, we propose a novel attack method called EMTrig, which leverages common roadside objects combined with controlled intentional electromagnetic interference (IEMI) targeting specific LiDARs to create flexible and covert adversarial attacks against designated vehicles. This causes the victim vehicle to misidentify roadside objects as obstacles, such as other vehicles, leading to dangerous driving behaviors like sudden stops and lane changes. Unlike conventional adversarial examples, our deployed objects are common items (e.g., signboards) that are harmless without the IEMI trigger but pose a threat only under IEMI attacks, providing better stealthiness and flexibility. Extensive experiments in both digital and physical domains validate the effectiveness of EMTrig, demonstrating its significant threat to LiDAR perception.