Published on November 18, 2011 by Alexandru Juncu
Tagged: process, procfs, /proc, file descriptor

A process is an instance of a binary executable file. This means that when you ‘run’ a binary, the code from the storage media is copied into the system’s memory, more precisely, into the process’ virtual memory space. From a single binary, several processes can be spawned.

The virtual memory of a process, made up of pages, is mapped to several things, like shared objects(libraries), shared memory, stack and heap space, read-only space and executable space. A good way to view what is mapped to what is with the pmap utility, or by just looking in the /proc directory hierarchy. The /proc/$PID/maps file (where $PID is the process ID of the targeted process) has the page mappings. Also in /proc/$PID, you can find other useful files, like the exe file that contains a symlink to the executable or the fd directory that contains symlinks to all the files opened as file descriptors in a process.

Except useful information, what can we get out of the procfs? Here is a situation that has been known to happen. You are in a console, with your bash shell, and you manage to delete some important files, like /bin/bash. Without that executable, you cannot run new shells and on a restart, your system will be inaccessible. What can you do?

The code of your bash is no longer on the hard drive, but it is in the virtual memory of the process you are currently running. You can find out what’s the PID of the current shell instance using $$ enviroment variable . Knowing that, you can cd to the /proc/$$ and access the content of the exe file there.

Although the exe file is shown as a link to the original file that is now deleted (thus the link should be broken), if you cat it, you will get its binary content. In fact, all the original binary file. Here is the step by step process:

/bin # md5sum bash
e116963c760727bf9067e1cb96bbf7d3  bash
/bin # rm bash
/bin # echo $$
5051
/bin # cd /proc/$$
/proc/5051 # ls -la exe
lrwxrwxrwx 1 root root 0 2011-11-15 23:47 exe -> /bin/bash (deleted)
/proc/5051 # cat maps
[snip]
00f9e000-00f9f000 rw-p 0001c000 08:01 263123     /lib/i386-linux-gnu/ld-2.13.so
08048000-0810c000 r-xp 00000000 08:01 284760     /bin/bash (deleted)
0810c000-0810d000 r--p 000c3000 08:01 284760     /bin/bash (deleted)
0810d000-08112000 rw-p 000c4000 08:01 284760     /bin/bash (deleted)
[snip]

/proc/5051 # cat exe>/bin/bash_rescued
/proc/5051 # cd -
/bin # md5sum bash_rescued
e116963c760727bf9067e1cb96bbf7d3  bash_rescued
/bin # chmod +x bash_rescured
/bin # mv bash_rescured bash

What other things can we rescue? How about a file that was opened by a process? For example, a video file, opened by a player:

alexj@hathor ~ $ md5sum movie.ogv
9f701e645fd55e1ae8d35b7671002881  movie.ogv
alexj@hathor ~ $ vlc movie.ogv &
[1] 6487
alexj@hathor ~ $ cd /proc/6487/fd
alexj@hathor /proc/6487/fd $ ls -la |grep movie
lr-x------ 1 alexj alexj 64 2011-11-16 00:11 23 -> /home/alexj/movie.ogv
alexj@hathor /proc/6487/fd $ rm /home/alexj/movie.ogv
alexj@hathor /proc/6487/fd $ ls -la |grep movie
lr-x------ 1 alexj alexj 64 2011-11-16 00:11 23 -> /home/alexj/movie.ogv (deleted)
alexj@hathor /proc/6487/fd $ cp 23 /home/alexj/movie_rescued.ogv
alexj@hathor /proc/6487/fd $ md5sum /home/alexj/movie_rescued.ogv
9f701e645fd55e1ae8d35b7671002881  /home/alexj/movie_rescued.ogv

These things are possible because the instances of the files are still kept and used by the kernel. The VFS (the Virtual File System) still has references to the inodes of the files. They won’t be released until the processes will be finished.

Thanks to Răzvan Deaconescu and Vlad Dogaru for the idea of this article.


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