Rethinking Backdoor Repair Evaluation: Distinguishing Aggregate Clean Utility from Benign Performance Preservation

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Backdoor repair aims to suppress malicious behavior in compromised models while preserving benign task performance. Existing studies typically evaluate these objectives using Attack Success Rate (ASR) and Overall Clean Accuracy, but aggregate clean accuracy can obscure substantial degradation concentrated in a small portion of the label space. We revisit benign-performance evaluation from a preservation perspective by distinguishing aggregate clean utility from the preservation of previously available class-wise performance. We define class-wise preservation loss by comparing clean performance before and after repair and show that aggregation can hide localized degradation through localized-loss dilution and cross-class compensation. To complement Overall Clean Accuracy, we characterize localized preservation loss using Worst-Class Preservation Loss and Tail Preservation Loss. We conduct a systematic empirical study across representative backdoor attacks, repair methods, datasets, attack targets, and model architectures, with additional validation under clean-label attacks. Results show that effective attack suppression and favorable aggregate clean performance do not necessarily imply uniform preservation of previously available benign performance across classes. Substantial localized preservation losses can remain, and their severity and class-wise structure vary across repair conditions. These findings motivate preservation-oriented class-wise evaluation alongside ASR and Overall Clean Accuracy.