Adversarial training based on the minimax formulation is necessary for
obtaining adversarial robustness of trained models. However, it is conservative
or even pessimistic so that it sometimes hurts the natural generalization. In
this paper, we raise a fundamental question---do we have to trade off natural
generalization for adversarial robustness? We argue that adversarial training
is to employ confident adversarial data for updating the current model. We
propose a novel approach of friendly adversarial training (FAT): rather than
employing most adversarial data maximizing the loss, we search for least
adversarial (i.e., friendly adversarial) data minimizing the loss, among the
adversarial data that are confidently misclassified. Our novel formulation is
easy to implement by just stopping the most adversarial data searching
algorithms such as PGD (projected gradient descent) early, which we call
early-stopped PGD. Theoretically, FAT is justified by an upper bound of the
adversarial risk. Empirically, early-stopped PGD allows us to answer the
earlier question negatively---adversarial robustness can indeed be achieved
without compromising the natural generalization.

本研究提出了友好对抗训练 (FAT) 的新方法，其中通过提前停止最严格的对抗数据搜索算法，即早停止的 PGD，来最小化损失并利用自信的对抗数据更新当前模型，理论上可以通过对抗风险的上限来证明，实验证明不需要以自然泛化为代价也可以实现对抗强度。