[Python-Dev] Is core dump always a bug? Advice requested (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Wed May 12 15:38:34 EDT 2004

Greg Ewing wrote:

Just a thought, but is statically verifying the bytecode even possible in principle? Seems to me it could be equivalent to the halting problem.

Reliably accepting all correct byte code, and rejecting all incorrect one, would indeed be equivalent to the halting problem.

Therefore, byte code verification usually puts additional constraints on byte code, rejecting some correct code as unverifiable.

Then, in addition to the language spec, you also need to specify what the verification rules are that an implementation needs to follow.

The typical assumption is that you have "consistent" stack usage. IOW, you need to be able to clearly identify start offsets of byte code instructions, and associate a fixed stack depth with each instruction. In typed languages, you also need to associate a type with the TOS, and require that type to be consistent across all possible execution paths. For example, in .NET code verification, there is a requirement that the value stack is empty at every branch target, and the start of every exception handler.

IOW, it is only necessary that any byte code that the Python byte code generator will ever emit verifies as correct. Accepting additional byte codes is optional.

Regards, Martin

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