# Preloading Shared Libraries

This is the second half of the seminar work. The following bullet points serve
as a quick recap of the relevant points from the lecture. The actual exercise
description starts below.

Dynamic Linking Redux

 * an application consists of multiple *modules*
 * upon load, those modules are *linked* by the runtime linker
 * on UNIX, the linker is known as `ld.so`
    * see the manual page: `man ld.so`
    * look for the description of `LD_PRELOAD`

Hooking Calls

 * call targets are resolved by `ld.so` at load/run time
 * procedures are referenced as *symbols* (ASCII names)
 * multiple libraries can provide the same symbol
 * `ld.so` has rules to decide which function to use
 * preloaded libraries come before all the others

## Preliminaries

The file `preload.c` is a small C library that implements `open`, in a manner
that allows it to be preloaded and hook the `open` call transparently to the
program (and the library).

You can use `preload.sh` to run any command with `preload.c` hooked up into it
via the `LD_PRELOAD` mechanism. The script automatically builds the C source
into a shared library. Compare the following commands:

    $ sh ./preload.sh cat /dev/null
    $ cat /dev/null

Read and try to understand the code in `preload.c` and also check how
`preload.sh` works.

## Exercise

At this point, you should be able to roughly understand how `preload.c` works.
To improve our understanding, we will try to modify it to do a slightly
different thing. Notice that part of the code in the preloaded library is
executed before the actual program starts executing (the function marked with
the `constructor` attribute).

Your version of `preload.c` should do the following:

 * create a file like `/tmp/.exfiltrate.XXXXXX`
    * use `mkstemp` for this
    * do *not* unlink the file when the program exits
 * hijack `read` calls instead of `open` files
 * print all the captured `read` data into the file

When your implementation is done, try it with a few programs to make sure it
behaves as expected. The simplest thing to try is:

    $ sh preload.sh cat /etc/passwd

That's all for the exercise. The homework for this week is described in
`homework.txt`.