guess_runtime.py

#! /usr/bin/env python
#-*-coding: utf-8 -*-

import sys
import ctypes
import struct

ELF_MAGIC = [0x7F, ord('E'), ord('L'), ord('F')]
PT_INTERP = 3
ELFCLASS32 = 1
ELFCLASS64 = 2

def string_compat(s):
    return s.decode('ascii') if not isinstance(s, str) else s

def sized_read(format, handle):
    value = struct.unpack(format, handle.read(struct.calcsize(format)))

    return value[0] if len(format) == 1 else value

def extract_interpreter(elf):
    # read header
    ei_ident = sized_read('16B', elf)
    ei_magic = ei_ident[:4]
    ei_class, ei_data, ei_version, ei_pad = ei_ident[4:8]

    # validate initial header
    if not all([ ei_magic[i] == b for i, b in enumerate(ELF_MAGIC) ]):
        raise RuntimeError('Executable is not ELF')
    if ei_class not in [ ELFCLASS32, ELFCLASS64 ]:
        raise RuntimeError('Invalid ELF class')

    # skip file type, machine, version
    sized_read('HHI', elf)

    # continue parsing Elf32/64 header:
    # get program and section headers info
    if ei_class == ELFCLASS32:
        ei_entry, e_phoff, e_shoff, e_flags, e_ehsize, e_phentsize, e_phnum = sized_read('IIIIHHH', elf)
    elif ei_class == ELFCLASS64:
        ei_entry, e_phoff, e_shoff, e_flags, e_ehsize, e_phentsize, e_phnum = sized_read('QQQIHHH', elf)
    else:
        raise RuntimeError('Invalid ELF class')

    # iterate sections of program header
    for i in range(0, e_phnum):
        elf.seek(e_phoff + e_phentsize * i)

        # parse section
        if ei_class == ELFCLASS32:
            p_type, p_offset, p_vaddr, p_paddr, p_filesz, p_memsz, p_flags, p_align = sized_read('IIIIIIII', elf)
        elif ei_class == ELFCLASS64:
            p_type, p_flags, p_offset, p_vaddr, p_paddr, p_filesz, p_memsz, p_align = sized_read('IIQQQQQQ', elf)
        else:
            raise RuntimeError('Invalid ELF class')

        # interpreter section found
        if p_type == PT_INTERP:
            elf.seek(p_offset)

            return string_compat(elf.read(p_filesz))

    return None

def have_compatible_glibc():
    process_namespace = ctypes.CDLL(None)

    try:
        gnu_get_libc_version = process_namespace.gnu_get_libc_version

        # Call gnu_get_libc_version, which returns a string like "2.5".
        gnu_get_libc_version.restype = ctypes.c_char_p

        return string_compat(gnu_get_libc_version())

    except AttributeError:
        # Symbol doesn't exist -> therefore, we are not linked to
        # glibc.
        return False

def guess_runtime():
    with open(sys.executable, 'rb') as elf:
        print('Python {}'.format(sys.version))

        env = extract_interpreter(elf)

        if env:
            print('Interpreter extracted: {}'.format(env))

            if 'musl' in env:
                print('Running on musl')
            else:
                gtest = have_compatible_glibc()

                if gtest:
                    print('Running on glibc version: {}'.format(gtest))
                else:
                    print('Not linked against glibc')
        else:
            print('Failed to extract interpreter. Static python?')

if __name__ == '__main__':
    guess_runtime()