I believe you've nailed it.  I was overthinking the problem (see previous comment), then failing to consider that my first setuid($u) was negating my setuid/sgid bits that were allowing me to setuid(0) in the first place.

In reply to Re^2: An odd failure of setuid(0) by Llew_Llaw_Gyffes
in thread An odd failure of setuid(0) by Llew_Llaw_Gyffes

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