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xdis

A Cross-Python bytecode disassembler, bytecode/wordcode and magic-number manipulation library/package.

Introduction

The Python dis module allows you to disassemble bytecode from the same version of Python that you are running on. But what about bytecode from different versions?

That's what this package is for. It can "marshal load" Python bytecodes from different versions of Python. The command-line routine pydisasm will show disassembly output using the most modern Python disassembly conventions in a variety of user-specified formats. Some of these formats like extended and extended-format are the most advanced of any Python disassembler I know of because they can show expression-tree on operarators. See the [Disasembler Example][#disasembler-example] below.

Also, if you need to modify and write bytecode, the routines here can be of help. There are routines to pack and unpack the read-only tuples in Python's Code type. For interoperability between Python 2 and 3 we provide our own versions of the Code type, and we provide routines to reduce the tedium in writing a bytecode file.

This package also has an extensive knowledge of Python bytecode magic numbers, including Pypy and others, and how to translate from sys.sys_info major, minor, and release numbers to the corresponding magic value.

So if you want to write a cross-version assembler, or a bytecode-level optimizer this package may also be useful. In addition to the kinds of instruction categorization that dis` offers, we have additional categories for things that would be useful in such a bytecode assembler, optimizer, or decompiler.

The programs here accept bytecodes from Python version 1.0 to 3.11 or so. The code requires Python 2.4 or later and has been tested on Python running lots of Python versions.

When installing, except for the most recent versions of Python, use the Python egg or wheel that matches that version, e.g. xdis-6.0.2-py3.3.egg, xdis-6.0.2-py33-none-any.whl. Of course for versions that pre-date wheel's, like Python 2.6, you will have to use eggs.

To install older versions for from source in git use the branch python-2.4-to-2.7 for Python versions from 2.4 to 2.7, python-3.1-to-3.2 for Python versions from 3.1 to 3.2, python-3.3-to-3.5 for Python versions from 3.3 to 3.5. The master branch handles Python 3.6 and later.

Installation

The standard Python routine:

$ pip install -e .
$ pip install -r requirements-dev.txt

A GNU makefile is also provided so make install (possibly as root or sudo) will do the steps above.

Disassembler Example

The cross-version disassembler that is packaged here, can produce assembly listing that are superior to those typically found in Python's dis module. Here is an example:

pydisasm --show-source -F extended bytecode_3.8/pydisasm-example.pyc
byte-compiling simple_source/pydisasm-example.py to bytecode_3.8/pydisasm-example.py.pyc
/src/external-vcs/github/rocky/python-xdis/test
# pydisasm version 6.1.0.dev0
# Python bytecode 3.8.0 (3413)
# Disassembled from Python 3.8.17 (default, Jun 21 2023, 08:20:16)
# [GCC 12.2.0]
# Timestamp in code: 1693155156 (2023-08-27 12:52:36)
# Source code size mod 2**32: 320 bytes
# Method Name:       <module>
# Filename:          simple_source/pydisasm-example.py
# Argument count:    0
# Position-only argument count: 0
# Keyword-only arguments: 0
# Number of locals:  0
# Stack size:        3
# Flags:             0x00000040 (NOFREE)
# First Line:        4
# Constants:
#    0: 0
#    1: None
#    2: ('version_info',)
#    3: 1
#    4: (2, 4)
#    5: 'Is small power of two'
# Names:
#    0: sys
#    1: version_info
#    2: print
#    3: version
#    4: len
#    5: major
#    6: power_of_two
             # import sys
  4:           0 LOAD_CONST           (0)
               2 LOAD_CONST           (None)
               4 IMPORT_NAME          (sys)
               6 STORE_NAME           (sys) | sys = import(sys)

             # from sys import version_info
  5:           8 LOAD_CONST           (0)
              10 LOAD_CONST           (('version_info',))
              12 IMPORT_NAME          (sys)
              14 IMPORT_FROM          (version_info)
              16 STORE_NAME           (version_info) | version_info = import(version_info)
              18 POP_TOP

             # print(sys.version)
  7:          20 LOAD_NAME            (print)
              22 LOAD_NAME            (sys)
              24 LOAD_ATTR            (version) | sys.version
              26 CALL_FUNCTION        (1 positional argument) | print(sys.version)
              28 POP_TOP

             # print(len(version_info))
  8:          30 LOAD_NAME            (print)
              32 LOAD_NAME            (len)
              34 LOAD_NAME            (version_info)
              36 CALL_FUNCTION        (1 positional argument) | len(version_info)
              38 CALL_FUNCTION        (1 positional argument) | print(len(version_info))
              40 POP_TOP

             # major = sys.version_info[0]
  9:          42 LOAD_NAME            (sys)
              44 LOAD_ATTR            (version_info) | sys.version_info
              46 LOAD_CONST           (0)
              48 BINARY_SUBSCR        sys.version_info[0]
              50 STORE_NAME           (major) | major = sys.version_info[0]

             # power_of_two = major & (major - 1)
 10:          52 LOAD_NAME            (major)
              54 LOAD_NAME            (major)
              56 LOAD_CONST           (1)
              58 BINARY_SUBTRACT      major - 1
              60 BINARY_AND           major & (major - 1)
              62 STORE_NAME           (power_of_two) | power_of_two = major & (major - 1)

             # if power_of_two in (2, 4):
 11:          64 LOAD_NAME            (power_of_two)
              66 LOAD_CONST           ((2, 4))
              68 COMPARE_OP           (in) | power_of_two in (2, 4)
              70 POP_JUMP_IF_FALSE    (to 80)

             # print("Is small power of two")
 12:          72 LOAD_NAME            (print)
              74 LOAD_CONST           ('Is small power of two')
              76 CALL_FUNCTION        (1 positional argument) | print('Is small power of two')
              78 POP_TOP
         >>   80 LOAD_CONST           (None)
              82 RETURN_VALUE         return None

Note in the above that some operand interpretation is done on items that are in the stack. For example in

24 LOAD_ATTR            (version) | sys.version

from the instruction see that sys.version is the resolved attribute that is loaded.

Similarly in:

68 COMPARE_OP           (in) | power_of_two in (2, 4)

we see that we can resolve the two arguments of the in operation. Finally in some CALL_FUNCTIONS we can figure out the name of the function and arguments passed to it.

Testing

$ make check

A GNU makefile has been added to smooth over setting running the right command, and running tests from fastest to slowest.

If you have remake installed, you can see the list of all tasks including tests via remake --tasks.

Usage

Run

$ ./bin/pydisasm -h

for usage help.

As a drop-in replacement for dis

xdis also provides some support as a drop in replacement for the the Python library dis module. This is may be desirable when you want to use the improved API from Python 3.4 or later from an earlier Python version.

For example:

>>> # works in Python 2 and 3
>>> import xdis.std as dis
>>> [x.opname for x in dis.Bytecode('a = 10')]
['LOAD_CONST', 'STORE_NAME', 'LOAD_CONST', 'RETURN_VALUE']

There may some small differences in output produced for formatted disassembly or how we show compiler flags. We expect you'll find the xdis output more informative though.

rocky/python-xdis

rocky

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