linux - Reading living process memory without interrupting it

Question

I would like to explore the memory of a living process, and when I do so, the process must not get disturbed - so attaching gdb to the process (which would stop it) is not an option. Therefore I would like to get this info from /proc/kcore (if you know of another way to do this please let me know). So I made a little experiment. I created a file called TEST with only "EXTRATESTEXTRA" inside. Then I opened it with less

$ less TEST

I got the PID of this process with

$ ps aux | grep TEST
user    7785  0.0  0.0  17944   992 pts/8    S+   16:15   0:00 less TEST
user    7798  0.0  0.0  13584   904 pts/9    S+   16:16   0:00 grep TEST

And then I used this script to create a dump of all files :

#!/bin/bash
grep rw-p /proc/$1/maps | sed -n 's/^([0-9a-f]*)-([0-9a-f]*) .*$/1 2/p' | while read start stop; do gdb --batch --pid $1 -ex "dump memory $1-$start-$stop.dump 0x$start 0x$stop"; done

(I found it on this site https://serverfault.com/questions/173999/dump-a-linux-processs-memory-to-file)

$ sudo ./dump_all_pid_memory.sh 7785

After this, I looked for "TRATESTEX" in all dumped files :

$ grep -a -o -e '...TRATESTEX...' ./*.dump
./7785-00624000-00628000.dump:HEXTRATESTEXTRA
./7785-00b8f000-00bb0000.dump:EXTRATESTEXTRA
./7785-00b8f000-00bb0000.dump:EXTRATESTEXTRA

So I concluded that there must be an occurance of this string somewhere between 0x00624000 and 0x00628000 . Therefore I converted the offsets into decimal numbers and used dd to get the memory from /proc/kcore :

$ sudo dd if="/proc/kcore" of="./y.txt" skip="0" count="1638400" bs=1

To my surprise, the file y.txt was full of zeros (I didn't find the string I was looking for in it).

As a bonus surprise, I ran a simmilar test at the same time with a different test file and found that the other test string i was using (both processes with less were running at the same time) should be found at the same location (the dumping and greping gave the same offset). So there must be something I don't understand clearly.

• Isn't the /proc/pid/maps supposed to show the offset of the memory (i.e. : if it would say "XXX" is at offset 0x10, another program could not be using the same offset am I right? - this is the source of my second surprise)

• How can I read /proc/kmap to get the memory that belongs to a process which's pid I know ?

Answer 1

If you have root access and are on a linux system, you can use the following linux script (adapted from Gilles' excellent unix.stackexchange.com answer and the answer originally given in the question above but including SyntaxErrors and not being pythonic):

#!/usr/bin/env python

import re
import sys

def print_memory_of_pid(pid, only_writable=True):
    """ 
    Run as root, take an integer PID and return the contents of memory to STDOUT
    """
    memory_permissions = 'rw' if only_writable else 'r-'
    sys.stderr.write("PID = %d" % pid)
    with open("/proc/%d/maps" % pid, 'r') as maps_file:
        with open("/proc/%d/mem" % pid, 'r', 0) as mem_file:
            for line in maps_file.readlines():  # for each mapped region
                m = re.match(r'([0-9A-Fa-f]+)-([0-9A-Fa-f]+) ([-r][-w])', line)
                if m.group(3) == memory_permissions: 
                    sys.stderr.write("
OK : 
" + line+"
")
                    start = int(m.group(1), 16)
                    if start > 0xFFFFFFFFFFFF:
                        continue
                    end = int(m.group(2), 16)
                    sys.stderr.write( "start = " + str(start) + "
")
                    mem_file.seek(start)  # seek to region start
                    chunk = mem_file.read(end - start)  # read region contents
                    print chunk,  # dump contents to standard output
                else:
                    sys.stderr.write("
PASS : 
" + line+"
")

if __name__ == '__main__': # Execute this code when run from the commandline.
    try:
        assert len(sys.argv) == 2, "Provide exactly 1 PID (process ID)"
        pid = int(sys.argv[1])
        print_memory_of_pid(pid)
    except (AssertionError, ValueError) as e:
        print "Please provide 1 PID as a commandline argument."
        print "You entered: %s" % ' '.join(sys.argv)
        raise e

If you save this as write_mem.py, you can run this (with python2.6 or 2.7) or early in python2.5 (if you add from __future__ import with_statement) as:

sudo python write_mem.py 1234 > pid1234_memory_dump

to dump pid1234 memory to the file pid1234_memory_dump.