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Noah Hacking Guide

Build from Source

$ mkdir build
$ cd build
$ ccmake ..
$ make && make install

You can find an install prefix field in the graphical UI of ccmake. The commands above install the noah command in your system. The noah command is a simple perl script that checks if a Linux system is ready on your system and install it otherwise before executing the "real" noah binary.

The "real" noah binary, which is made from src/main.c, requires some mandatory command line options. If you want to execute it directly, you will need to start the binary with the following manner:

$ INSTALL_PREFIX/libexec/noah -m ROOT_PATH PATH_TO_INIT

where ROOT_PATH is a path to the directory that is treated as the root mount point in the Linux box, and PATH_TO_INIT is a path to the first command to be run in the boot sequence, like /bin/bash.

noahstrap helps you set up a Linux environment on your local machine. It retrieves a ready-to-use distro image from the Internet and extracts it to a specified directory. noahstrap is installed via homebrew.

$ brew install linux-noah/noah/noahstrap
$ noahstrap --help  # prints help message

Debugging

There are several methods to debug noah.

The first option is to use lldb, of course. Since the noah command indirectly invokes the noah binary through a perl script, it would be convenient to modify the script to start up the binary with lldb.

The second option is to use meta-strace. Because we still don't have implemented the ptrace system call, we cannot use strace to trace the log inside the Linux box. To tackle this problem, we provide a feature called meta-strace, that logs system call entering and retirement at the meta level, which means outside virtual machines. This feature is enabled via --strace OUTFILE option for the noah command. When you add a new system call and check the behavior, it might be helpful to add new strace {pre,post} hooks in the src/meta_strace.c. This should greatly improve the appearance of the trace logs.

The third option is to use the output option. This is mere debug logs that are emit using the printk function in noah's source codes. This feature is enabled with --output OUTFILE option.

Source Structure

Sources are placed in the following rules:

  • bin/: the noah perl script inhabits
  • lib/: sources for the VMM components
  • src/foo/*: Linux subsystem emulations put in the corresponding directories
  • src/meta_strace.c: meta-strace, see Debugging section
  • src/base.c: general virtual kernel primitives, like copy_from_user
  • src/main.c: the entry point