Understanding Paths

• What is PATH?

Challenges in pwn.college are in the challenge directory that is, in turn, right in the root directory (/). The path to the challenge the directory is, thus, /challenge, and the path to the run challenge program is /challenge/run. This style of path, one that starts with the root directory, is referred to as an "absolute path". From the commandline, you can execute programs by refering to them by their absolute paths.

This challenge requires you to execute it by invoking its absolute path. You'll want to execute the run file that is in the challenge directory that is, in turn, in the / directory. If you invoke the challenge correctly, it will give you the flag. Good luck!

The Linux filesystem has tons of directories with tons of files. You can navigate around directories by using the cd (change directory) command and passing a path to it, as so:

hacker@dojo:~$ cd /some/new/directory
hacker@dojo:/some/new/directory$ cd /some/new/directory

This affects the "current working directory" of your process (in this case, the bash shell). Each process has a directory in which it's currently hanging out. The reasons for this will become clear later in the module.

This challenge will require you to execute the /challenge/run program from a specific path (which it will tell you). You'll need to cd to that directory before rerunning the challenge program. Good luck!

The Linux filesystem has tons of directories with tons of files. You can navigate around directories by using the cd (change directory) command and passing a path to it, as so:

hacker@dojo:~$ cd /some/new/directory
hacker@dojo:/some/new/directory$ cd /some/new/directory

This affects the "current working directory" of your process (in this case, the bash shell). Each process has a directory in which it's currently hanging out. The reasons for this will become clear later in the module.

This challenge will require you to execute the /challenge/run program from a specific path (which it will tell you). You'll need to cd to that directory before rerunning the challenge program. Good luck!

The Linux filesystem has tons of directories with tons of files. You can navigate around directories by using the cd (change directory) command and passing a path to it, as so:

hacker@dojo:~$ cd /some/new/directory
hacker@dojo:/some/new/directory$ cd /some/new/directory

This affects the "current working directory" of your process (in this case, the bash shell). Each process has a directory in which it's currently hanging out. The reasons for this will become clear later in the module.

This challenge will require you to execute the /challenge/run program from a specific path (which it will tell you). You'll need to cd to that directory before rerunning the challenge program. Good luck!

Now you're familiar with the concept of refering to absolute paths and changing directories. If you put in absolute paths everywhere, then it really doesn't matter what directory you are in, as you likely found out in the previous three challenges.

However, the current working directory matters for relative paths. If a path does not start with a /, it is not an absolute, but a relative path. Relative paths are interpreted relative to the current working directory, rather than starting from /. When used by programs, relative paths will be resolved into absolute paths. For example, if you are in the /var directory, and your relative path is asdf, the full resolved path is /var/asdf. If your relative path is asdf/fdsa, the full resolved path will be /var/asdf/fdsa.

Let's try it here! You'll need to run /challenge/run using a relative path while having a current working directory of /. For this level, I'll give you a hint. Your relative path starts with the letter c 😊

Previously, your relative path was "naked": it directly specified the directory to descend into from the current directory. In this level, we're going to explore more explicit relative paths.

In most operating systems, including Linux, every directory has two implicit entries that you can reference in paths: . and ... The first, ., refers right to the same directory, so the following absolute paths are all identical to each other:

• /challenge
• /challenge/.
• /challenge/./././././././././
• /./././challenge/././

The following relative paths are also all identical to each other:

• challenge
• ./challenge
• ./././challenge
• challenge/.

Of course, if your current working directory is /, the above relative paths are equivalent to the above absolute paths.

This challenge will get you using . in your relative paths. Get ready!

In this level, we'll practice refering to paths using . a bit more. This challenge will need you to run it from the /challenge directory. Here, things get slightly tricky.

Linux explicitly avoids automatically looking in the current directory when you provide a "naked" path. Consider the following:

hacker@dojo:~$ cd /challenge
hacker@dojo:/challenge$ run

This will not invoke /challenge/run. This is actually a safety measure: if Linux searched the current directory for programs every time you entered a naked path, you could accidentally execute programs in your current directory that happened to have the same names as core system utilities! As a result, the above commands will yield the following error:

bash: run: command not found

We'll explore the mechanisms behind this concept later, but in this challenge, will learn how to explicitly use relative paths to launch run in this scenario. The way to do this is to tell Linux that you explicitly want to execute a program in the current directory, using . like in the previous levels. Give it a try now!

Every user has a home directory, typically under /home in the filesystem. In the dojo, you are the hacker user, and your home directory is /home/hacker. The home directory is typically where users store most of their personal files. As you make your way through pwn.college, this is where you'll store most of your solutions.

Typically, your shell session will start with your home directory as your current working directory. Consider the initial prompt:

hacker@dojo:~$

The ~ in this prompt is the current working directory, with ~ being shorthand for /home/hacker. Bash provides and uses this shorthand because, again, most of your time will be spent in your home directory. Thus, whenever bash sees ~ provided as the start of an argument in a way consistent with a path, it will expand it to your home directory. Consider:

hacker@dojo:~$ echo LOOK: ~
LOOK: /home/hacker
hacker@dojo:~$ cd /
hacker@dojo:/$ cd ~
hacker@dojo:~$ cd ~/asdf
hacker@dojo:~/asdf$ cd ~/asdf
hacker@dojo:~/asdf$ cd ~
hacker@dojo:~$ cd /home/hacker/asdf
hacker@dojo:~/asdf$

Note that the expansion of ~ is an absolute path, and only the leading ~ is expanded. This means, for example, that ~/~ will be expanded to /home/hacker/~ rather than /home/hacker/home/hacker.

Fun fact: cd will use your home directory as the default destination:

hacker@dojo:~$ cd /tmp
hacker@dojo:/tmp$ cd
hacker@dojo:~$

Now it's your turn to play! In this challenge, /challenge/run will write a copy of the flag to any file you specify as an argument on the commandline, with these constraints:

1. Your argument must be an absolute path.
2. The path must be inside your home directory.
3. Before expansion, your argument must be three characters or less.