Hacky script to check for the set_fast_math constructor in an executable/shared library using objdump

check_for_ffast_math.py

#!/usr/bin/env python

import subprocess
import re
import sys

def get_init_array(filename):
    # Call objdump -s -j .init_array <filename> to get the contents of the .init_array section
    try:
        objdump_output = subprocess.check_output(['objdump', '-s', '-j', '.init_array', filename], stderr=subprocess.STDOUT)
    except subprocess.CalledProcessError as e:
        return []
    objdump_output = objdump_output.decode('utf-8')
    objdump_output = objdump_output.split('\n')
    found_contents = False
    constructors = []
    for line in objdump_output:
        if line.startswith("Contents of section .init_array:"):
            found_contents = True
            continue
        if found_contents:
            if not line.strip():
                break
            line_re = re.compile(r'^ *[0-9a-f]+ (([0-9a-f]+) ([0-9a-f]+)) ?(([0-9a-f]+) ([0-9a-f]+))?')
            m = line_re.match(line)
            if not m: continue
            addr = m.group(1).replace(' ', '')
            addr = int.from_bytes(bytes.fromhex(addr), 'little')
            constructors.append(addr)
            try:
                addr = m.group(4)
                if not addr: continue
                addr = addr.replace(' ', '')
                addr = int.from_bytes(bytes.fromhex(addr), 'little')
                constructors.append(addr)
            except IndexError:
                pass
    return constructors

def check_for_ffast_math(filename, addr):
    # Call objdump to disassemble filename at addr
    objdump_process = subprocess.Popen(
        ['objdump', f'--start-address={hex(addr)}', '-d', filename],
        stdout=subprocess.PIPE
    )
    found_stmxcsr, found_8040, found_rdmxcsr = False, False, False

    for line in iter(lambda: objdump_process.stdout.readline(), b""):
        line = line.decode('utf-8')
        if "stmxcsr" in line:
            found_stmxcsr = True
        if "0x8040" in line:
            found_8040 = True
        if "ldmxcsr" in line:
            found_rdmxcsr = True
        if "retq" in line:
            objdump_process.kill()
            break
    return found_stmxcsr and found_8040 and found_rdmxcsr

for filename in sys.argv[1:]:
    print(f"{filename} ", end="", flush=True)
    constructors = get_init_array(filename)
    i = len(constructors)
    print(f"({i} constructor{'s'[:i^1]}) ", end="", flush=True)
    for addr in constructors:
        if check_for_ffast_math(filename, addr):
            print(f"contains ffast-math constructor at {hex(addr)}")
            break
    else:
        print("is clean")