Can you edit address values in “/proc/$pid/mem” to reflect changes in that process

/proc/$pid/maps

/proc/$pid/mem shows the contents of $pid's memory mapped the same way as in the process, i.e., the byte at offset x in the pseudo-file is the same as the byte at address x in the process. If an address is unmapped in the process, reading from the corresponding offset in the file returns EIO (Input/output error). For example, since the first page in a process is never mapped (so that dereferencing a NULL pointer fails cleanly rather than unintendedly accessing actual memory), reading the first byte of /proc/$pid/mem always yield an I/O error.

The way to find out what parts of the process memory are mapped is to read /proc/$pid/maps. This file contains one line per mapped region, looking like this:

08048000-08054000 r-xp 00000000 08:01 828061     /bin/cat
08c9b000-08cbc000 rw-p 00000000 00:00 0          [heap]

The first two numbers are the boundaries of the region (addresses of the first byte and the byte after last, in hexa). The next column contain the permissions, then there's some information about the file (offset, device, inode and name) if this is a file mapping. See the proc(5) man page or Understanding Linux /proc/id/maps for more information.

Here's a proof-of-concept script that dumps the contents of its own memory.

#! /usr/bin/env python
import re
maps_file = open("/proc/self/maps", 'r')
mem_file = open("/proc/self/mem", 'rb', 0)
output_file = open("self.dump", 'wb')
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
        output_file.write(chunk)  # dump contents to standard output
maps_file.close()
mem_file.close()
output_file.close()

/proc/$pid/mem

[The following is for historical interest. It does not apply to current kernels.]

Since version 3.3 of the kernel, you can access /proc/$pid/mem normally as long as you access only access it at mapped offsets and you have permission to trace it (same permissions as ptrace for read-only access). But in older kernels, there were some additional complications.

If you try to read from the mem pseudo-file of another process, it doesn't work: you get an ESRCH (No such process) error.

The permissions on /proc/$pid/mem (r--------) are more liberal than what should be the case. For example, you shouldn't be able to read a setuid process's memory. Furthermore, trying to read a process's memory while the process is modifying it could give the reader an inconsistent view of the memory, and worse, there were race conditions that could trace older versions of the Linux kernel (according to this lkml thread, though I don't know the details). So additional checks are needed:

• The process that wants to read from /proc/$pid/mem must attach to the process using ptrace with the PTRACE_ATTACH flag. This is what debuggers do when they start debugging a process; it's also what strace does to a process's system calls. Once the reader has finished reading from /proc/$pid/mem, it should detach by calling ptrace with the PTRACE_DETACH flag.
• The observed process must not be running. Normally calling ptrace(PTRACE_ATTACH, …) will stop the target process (it sends a STOP signal), but there is a race condition (signal delivery is asynchronous), so the tracer should call wait (as documented in ptrace(2)).

A process running as root can read any process's memory, without needing to call ptrace, but the observed process must be stopped, or the read will still return ESRCH.

In the Linux kernel source, the code providing per-process entries in /proc is in fs/proc/base.c, and the function to read from /proc/$pid/mem is mem_read. The additional check is performed by check_mem_permission.

Here's some sample C code to attach to a process and read a chunk its of mem file (error checking omitted):

sprintf(mem_file_name, "/proc/%d/mem", pid);
mem_fd = open(mem_file_name, O_RDONLY);
ptrace(PTRACE_ATTACH, pid, NULL, NULL);
waitpid(pid, NULL, 0);
lseek(mem_fd, offset, SEEK_SET);
read(mem_fd, buf, _SC_PAGE_SIZE);
ptrace(PTRACE_DETACH, pid, NULL, NULL);

I've already posted a proof-of-concept script for dumping /proc/$pid/mem on another thread.

The "memory used by a process" is not a clear cut concept in modern operating systems. What can be measured is the size of the address space of the process (SIZE) and resident set size (RSS, how many of the pages in the address space are currently in memory). Part of RSS is shared (most processes in memory share one copy of glibc, and so for assorted other shared libraries; several processes running the same executable share it, processes forked share read-only data and possibly a chunk of not-yet-modified read-write data with the parent). On the other hand, memory used for the process by the kernel isn't accounted for, like page tables, kernel buffers, and kernel stack. In the overall picture you have to account for the memory reserved for the graphics card, the kernel's use, and assorted "holes" reserved for DOS and other prehistoric systems (that isn't much, anyway).

The only way of getting an overall picture is what the kernel reports as such. Adding up numbers with unknown overlaps and unknown left outs is a nice exercise in arithmetic, nothing more.