xkdb

Xinu Kernel DeBugger is a project that allows you to use GDB on remote xinu backends.

How to Use

Firstly, you need to find the right stub for your architecture. If you don't know the architecture of your Xinu backend board maybe pay more attention in your OS class :) For reference, here is a cheat sheet for Purdue's boards in 2019.

• Galilelo - x86 / i386

• Beaglebone Black - ARM

If a stub doesn't exist for your board, please read the instructions in PORTING.

1. Compile appropriate stub into your xinu build

   Move stub/<arch>-stub.c into system/<arch>-stub.c.

   Make the following change in your system/initialize.c file, firstly add these prototypes on the top of the file with the other externs:

   extern void set_debug_traps(); // Add these two function
   extern void breakpoint();      // prototypes

   Next add these two function calls near the bottom of the sysinit() function (changes marked with // comments):

   static  void    sysinit()
   {
   ...
       /* Create a ready list for processes */

       readylist = newqueue();

       set_debug_traps(); // Add these two
       breakpoint();      // function calls here
        
       /* Initialize the real time clock */
    
       clkinit();
   ...
   }

2. Add debug flag to your xinu Makefile

   Open up compile/Makefile and find the line with the compiler flags, it should look like this: CFLAGS = -march=i586 ...

   Add -g to the CFLAGS variable so it looks along the lines of: CFLAGS = -g -march=i586 -m32 -fno-builtin...

3. Recompile

   Run make clean, make rebuild and make in order to compile the stub and debug symbols into your xinu image.

4. Use xkdb.py to connect to a backend board

   Change into your Xinu /compile directory. You can then run xkdb with ~/path/to/xkdb/py-console/xkdb.py

   This will automatically upload the xinu image file and power cycle the backend. Use the --help option view all the options available for xkdb.py

   Tip: Add $HOME/path/to/xkdb/py-console to your PATH variable so you can simply use the command xkdb.py instead of specifying the full path.

   To do that edit your ~/.bashrc and add the line export PATH=${PATH}:$HOME/path/to/xkdb/py-console . If you already have this line to add another path, just add the path next to your list of path seperate by a colon :.

   Then just run . ~/.bashrc or source ~/.bashrc to update your bash profile.

5. Connect GDB to the backend

   Open up another terminal and run gdb -x ~/.xkdb

   GDB will be unresponsive until the backend is fully booted, then you should see the breakpoint be hit.

Project Structure

• stub/ - The gdb stub and changes on the Xinu side to act as a remote debugging target.

• py-console/ - A Python version of the cs-console command, used to establish a connection to the Xinu backends and pipe data in and out through to GDB.

Running Tests

cd py-console

python -m pytest

How does it work?

This project leverages the ability of GDB to debug remote targets using a thin text protocol. The primary purpose of this protocol is to allow the use of GDB on systems where compiling the full debugger is too difficult. A remote host can then drive the debugging session by sending simple commands to the small gdb server.

By equipping a board running Xinu with the GDB stub, we allow a remote host running GDB to debug it. The main problems this project solves are:

1. Getting the GDB stub to compile on the Xinu source tree.

2. Augmenting the cs-console command to patch through the GDB protocol to a local instance of GDB.

The final architecture to allow remote debugging then looks like:

+---------+         +------------+
| Galileo | Serial  |    Xinu    |
|  Board  <--------->   Server   |
|         |         |            |
+---------+         +-----^------+
                          |  Network
      +-------------------|-------+
      |  +-----+    +-----v----+  |
      |  | GDB |    |cs-console|  |
      |  |     <---->          |  |
      |  +-----+    +----------+  |
      |      Xinu Workstation     |
      +---------------------------+

In order to allow regular output and the GDB protocol to both be sent through serial, we prefix each GDB message with <STX>G where STX is the ASCII start of text character (\x02). Each message is terminated with <EOT> (end of transmission \x04).

Our re-written version of cs-console does the following:

• Opens up a server socket that gdb can connect to as a target.

• Prints output as the original cs-console normally would but upon encountering <STX>G will stop printing and start piping the messages through to gdb.

• Forward any data sent by gdb through to the galileo board.

Known Issues

1. Caution when using GDB Plugins with this project

If you choose to do so, expect that they may potentially cause problems that xinu doesn't like. gdb-peda (https://github.com/longld/peda) calls getpid() in the remote process when it starts up, which makes it look like xinu is crashing there. It's not :)