IntroductionIn 2025, the only known copy of UNIX v4 surfaced on a magnetic tape1. This version marks a pivotal moment in computer history: the rewriting of UNIX into C. Enthusiasts quickly recovered the data and successfully ran the system on a PDP-11 simulator2.Fascinated by this artifact, I set up an instance to explore it. Because the distribution includes the source code, I examined the implementation of several core utilities. While auditing the su(1) program, I identified a bug. Let’s fix ...