Task 1: ELFs and aliens

Announced at: 20.03.2018

Due at: 10.04.2018 (final 24.04.2018)

Contents

• Task 1: ELFs and aliens
  • Extra materials
  • Alien spaceship
  • Task
  • Solution format
  • Scoring
  • Hints

Extra materials

• alienos.h
• prog
• a template for making own testcases
• Zadanie 1: ELFy i kosmici
• The AlienOS operating system

Alien spaceship

Recently, an alien spaceship has crashed near the faculty. We managed to extract data carriers containing programs for the very technically advanced AlienOS operating system, but unfortunately no alien computer system survived – so we are not able to run their programs. However, after many months of reverse engineering, we finally managed to find out how the interfaces between the AlienOS kernel and user programs work.

Task

Write a program that loads AlienOS programs and emulates system calls, allowing you to run AlienOS programs under Linux. For testing, we attach an example program extracted from the spacecraft (unfortunately, we have no idea what this program does). Your loader should of course work with any alien program using the AlienOS interface.

Solution format

As the solution, you should deliver an archive containing:

• any number of source files with the solution code
• a Makefile that can build the solution, or a corresponding file from another sensible build system (eg cmake)
• a readme file with a brief description of the solution and compilation instructions for the qemu image from first class

The solution code should be written entirely in C (or C++ if you really want to) and can use any sane libraries available in the Debian system. The solution should compile to a program called emu that can be launched as follows:

./emu <alien program> [<parameters for alien program>]

After the alien program finishes running, the emu program should end with the same exit code as the alien program.

The loader should check correctness of the input data to a reasonable degree. In particular, it should be remembered that a crashed alien ship is not a trusted source of code – you must ensure that the code of the emulated program can not do anything except using known system calls (eg writing / reading arbitrary files). If an error is detected by the emu program, the program should end with the exit code 127.

The program will be tested inside qemu, running with the image from the first class. We recommend making sure the solution compiles inside this image.

Please send the solutions to the address marmarek@mimuw.edu.pl with a copy to mwk@mimuw.edu.pl.

Scoring

You can get up to 10 points for the task. The score is a sum of two parts:

• the result of testing (from 0 to 10 points)
• code assessment (from 0 to -10 points)

Hints

Based on the documentation found on the spacecraft, our team of extraterrestrial linguistics found that the example program is probably controlled by the arrow keys, space bar, and enter. It takes one parameter whose sensible values are in the range of 50-400. It requires a terminal size of at least 80×24.

The easiest way to emulate AlienOS system calls is to use ptrace with the PTRACE_SYSEMU option (defined in asm/ptrace.h – a different place than most ptrace definitions).

To implement the getrand system call from the AlienOS system, we recommend using the Linux system call getrandom, or a sensible PRNG initiated with such a call. Unfortunately, the glibc in our image doesn’t provide this function, but it can still be called directly via syscall function.

You can use the ncurses library to handle the terminal, or issue the appropriate codes yourself (man console_codes).

The colors of the characters given in the documentation are just an approximation (aliens see colors differently than humans) and you can use any reasonably similar colors in the Linux terminal.