<- .languages[Python Reversing] ->

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PVM (Python Virtual Machine)

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• PVM is a stack-based virtual machine that stores operands in an upwardly-growing stack
• In stack-based execution, an operation is performed by popping operands from the stack, operates on them, and then storing the result back to the stack
• Advantages of Stack-Based Virtual Machine
  • Instructions are shorter than instructions in register-based virtual machine since operands don’t need to be explicitly stated because they are on the stack, thus resulting in shorter overall bytecode
  • Easier to implement since it doesn’t have to worry about register allocation

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The 3 Tuples Associated With Function Object

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• Local variables and parameters are stored in <object name>.__code__.co_varnames
• Global variables that it uses are stored in <object name>.__code__.co_names
• Constants it uses are stored in <object name>.__code__.co_consts

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Python Bytecode Instructions

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• For full instruction set, check out the Official Python Documentation
• An instruction consists of an opcode and possibly an oparg. Opcode is the instruction and oparg is the index that resolves to the actual operands
  • Some instructions, such as BINARY_ADD, doesn't require an oparg since the operands it needs are on the stack
  • Other instructions, such as those for LOAD/STORE, require an oparg to index a tuple for the operand. The specific tuple is referenced in the latter half of the opcode
    • LOAD/STORE_CONST: uses oparg to index the <object name>.__code__.co_consts tuple
    • LOAD/STORE_FAST: uses oparg to index the <object name>.__code__.co_varnames tuple
    • LOAD/STORE_GLOBAL: uses oparg to index the <object name>.__code__.co_names tuple
    • Load will push the value it indexed onto the stack and store will store the value at the top of the stack into the indexed object

C++ Reversing <- RERM[.languages] -> ELF Files