Q:

Python read Linux memory process error (/proc/$pid/mem)

I have the following code tested on some Linux distros (Debian, Linux Mint...) and working, but under CentOS I get an error even I run it as root:
#!/usr/bin/env python
import re
maps_file = open("/proc/18396/maps", 'r')
mem_file = open("/proc/18396/mem", 'r', 0)
for line in maps_file.readlines():  # for each mapped region
    m = re.match(r'([0-9A-Fa-f]+)-([0-9A-Fa-f]+) ([-r])', line)
    if m.group(3) == 'r':  # if this is a readable region
        start = int(m.group(1), 16)
        end = int(m.group(2), 16)
        mem_file.seek(start)  # seek to region start
        chunk = mem_file.read(end - start)  # read region contents
        print chunk,  # dump contents to standard output
maps_file.close()
mem_file.close()

The script reads the process' memory and dumps the readable region. Under CentOS 5.4 x64 I get the following error:
Traceback (most recent call last):
  File "./mem.py", line 11, in ?
    chunk = mem_file.read(end - start)  # read region contents
IOError: [Errno 3] No such process

The process is alive and readable:
[root@localhost ~]# ps xa|grep 18396
18396 ?        S      0:00 /usr/sbin/httpd
[root@localhost ~]# ls -al /proc/18396/maps && ls -al /proc/18396/mem
-r--r--r-- 1 root root 0 Jan 31 17:26 /proc/18396/maps
-rw------- 1 root root 0 Jan 31 17:26 /proc/18396/mem

any idea? I tried it under Python 2.4 and Python 2.7 works on Debian-like distros but not under CentOS.

A:

Found the answer myself after some digging:
#!/usr/bin/env python
import ctypes, re, sys

## Partial interface to ptrace(2), only for PTRACE_ATTACH and PTRACE_DETACH.
c_ptrace = ctypes.CDLL("libc.so.6").ptrace
c_pid_t = ctypes.c_int32 # This assumes pid_t is int32_t
c_ptrace.argtypes = [ctypes.c_int, c_pid_t, ctypes.c_void_p, ctypes.c_void_p]
def ptrace(attach, pid):
    op = ctypes.c_int(16 if attach else 17) #PTRACE_ATTACH or PTRACE_DETACH
    c_pid = c_pid_t(pid)
    null = ctypes.c_void_p()
    err = c_ptrace(op, c_pid, null, null)
    if err != 0: raise SysError, 'ptrace', err

pid = "18396"

ptrace(True, int(pid))
maps_file = open("/proc/"+pid+"/maps", 'r')
mem_file = open("/proc/"+pid+"/mem", 'r', 0)
for line in maps_file.readlines():  # for each mapped region
    m = re.match(r'([0-9A-Fa-f]+)-([0-9A-Fa-f]+) ([-r])', line)
    if m.group(3) == 'r':  # if this is a readable region
        start = int(m.group(1), 16)
        end = int(m.group(2), 16)
        mem_file.seek(start)  # seek to region start
        chunk = mem_file.read(end - start)  # read region contents
        print chunk,  # dump contents to standard output
maps_file.close()
mem_file.close()
ptrace(False, int(pid))

