dwarf-assembly/generate_eh_elf.py

#!/usr/bin/env python3

"""
Generates the `.eh_frame` equivalent in C code of the given ELF file, and
all the shared objects it depends upon.
"""


import os
import sys
import subprocess
import re
import tempfile
import argparse


DWARF_ASSEMBLY_BIN = os.path.join(
    os.path.dirname(os.path.abspath(sys.argv[0])),
    'dwarf-assembly')
C_BIN = (
    'gcc' if 'C' not in os.environ
    else os.environ['C'])


def elf_so_deps(path):
    ''' Get the list of shared objects dependencies of the given ELF object.
    This is obtained by running `ldd`. '''

    deps_list = []

    try:
        ldd_output = subprocess.check_output(['/usr/bin/ldd', path]) \
            .decode('utf-8')
        ldd_re = re.compile(r'^.* => (.*) \(0x[0-9a-fA-F]*\)$')

        ldd_lines = ldd_output.strip().split('\n')
        for line in ldd_lines:
            line = line.strip()
            match = ldd_re.match(line)
            if match is None:
                continue  # Just ignore that line — it might be eg. linux-vdso
            deps_list.append(match.group(1))

        return deps_list

    except subprocess.CalledProcessError as e:
        raise Exception(
            ("Cannot get dependencies for {}: ldd terminated with exit code "
             "{}.").format(path, e.returncode))


def gen_dw_asm_c(obj_path, out_path):
    ''' Generate the C code produced by dwarf-assembly from `obj_path`, saving
    it as `out_path` '''

    try:
        with open(out_path, 'w') as out_handle:
            # TODO enhance error handling
            dw_asm_output = subprocess.check_output(
                [DWARF_ASSEMBLY_BIN, obj_path]) \
                .decode('utf-8')
            out_handle.write(dw_asm_output)
    except subprocess.CalledProcessError as e:
        raise Exception(
            ("Cannot generate C code from object file {} using {}: process "
             "terminated with exit code {}.").format(
                 obj_path,
                 DWARF_ASSEMBLY_BIN,
                 e.returncode))


def gen_eh_elf(obj_path, out_dir=None):
    ''' Generate the eh_elf corresponding to `obj_path`, saving it as
    `out_dir/$(basename obj_path).eh_elf.so` (or in the current working
    directory if out_dir is None) '''

    if out_dir is None:
        out_dir = '.'

    print("> {}...".format(os.path.basename(obj_path)))

    out_base_name = os.path.basename(obj_path) + '.eh_elf'
    out_so_path = os.path.join(out_dir, (out_base_name + '.so'))

    try:
        so_mtime = os.path.getmtime(out_so_path)
        obj_mtime = os.path.getmtime(obj_path)

        if obj_mtime < so_mtime:
            return  # The object is recent enough, no need to recreate it
    except OSError:
        pass


    with tempfile.TemporaryDirectory() as compile_dir:
        # Generate the C source file
        print("\tGenerating C…")
        c_path = os.path.join(compile_dir, (out_base_name + '.c'))
        gen_dw_asm_c(obj_path, c_path)

        # Compile it into a .o
        print("\tCompiling into .o…")
        o_path = os.path.join(compile_dir, (out_base_name + '.o'))
        call_rc = subprocess.call(
            [C_BIN, '-o', o_path, '-c', c_path, '-O3', '-fPIC'])
        if call_rc != 0:
            raise Exception("Failed to compile to a .o file")

        # Compile it into a .so
        print("\tCompiling into .so…")
        call_rc = subprocess.call(
            [C_BIN, '-o', out_so_path, '-shared', o_path])
        if call_rc != 0:
            raise Exception("Failed to compile to a .so file")


def gen_all_eh_elf(obj_path, out_dir=None):
    ''' Call `gen_eh_elf` on obj_path and all its dependencies '''
    if out_dir is None:
        out_dir = '.'

    deps = elf_so_deps(obj_path)
    deps.append(obj_path)
    for dep in deps:
        gen_eh_elf(dep, out_dir)


def process_args():
    ''' Process `sys.argv` arguments '''
    parser = argparse.ArgumentParser(
        description="Compile ELFs into their related eh_elfs",
    )

    parser.add_argument('--deps', action='store_const',
                        const=gen_all_eh_elf, default=gen_eh_elf,
                        dest='gen_func',
                        help=("Also generate eh_elfs for the shared objects "
                              "this object depends on"))
    parser.add_argument('-o', '--output', metavar="path",
                        help=("Save the generated objects at the given path "
                              "instead of the current working directory"))
    parser.add_argument('object', nargs='+',
                        help="The ELF object(s) to process")
    return parser.parse_args()


def main():
    args = process_args()

    for obj in args.object:
        args.gen_func(obj, args.output)


if __name__ == "__main__":
    main()