At first I thought this amounted to a claim that the halting problem had been solved,
That's not what the halting problem means! There is no rule that says you cannot prove a program is safe: the rule is only that you cannot prove any arbitrary program is safe. NaCl gets around that by adding checks to the code (bounds checks, etc) to anywhere that it can't prove is safe.
That's not what the halting problem means! There is no rule that says you cannot prove a program is safe: the rule is only that you cannot prove any arbitrary program is safe. NaCl gets around that by adding checks to the code (bounds checks, etc) to anywhere that it can't prove is safe.