Computer Bugs

Configuring Kernel Debugging with WinDbg and a NULL modem

Lately, I have been digging deep into Windows to get really low level with the the I/O path SQL Server takes (yep, there is an even deeper layer to understand fully).

Once you start playing around with the Windows Kernel, you will at some point need kernel level debugging set up. Traditionally, this is something I have used a Windows machine for, and even there, it can be painful to get working. As you may know, I have switched to Mac as my client machine and my Windows utilities (including WinDbg) now run in VMWare Fusion – it looked like I was heading into an interop nightmare…

imageWindows 8 allows kernel debugging directly over the network card, but how does one configure kernel debugging with a Windows 2008R2 target from a Mac with VM Ware?

I found a very cheap solution today. You need:

  • A USB to Serial (RS-232) converter
  • A NULL modem (I would recommend getting a long one, so you don’t have to sit next to the server)
  • A  target server that you want to debug
  • A serial port on the target server
  • WinDbg from the Windows SDK in a virtual machine on the Mac
  • Symbols set up as per my previous post

A USB converter and a NULL modem is a dirt cheap way to get the required hardware for kernel debugging. I sourced my cables from Maplins (Thanks @SQLServerMonkey) in the UK – for a total of around 20 GBP.

Step 1: Prepare the Client/Debugger

A Macbook air, like most other lightweight laptops, does not have a serial port. So, we have to use a USB/Serial converter. It is possible to debug directly over USB, but good luck with that from a Mac, I didn’t have the courage.

Make sure VMWare routes the USB device to the Windows Guest OS and not the Mac. It will look something like this in VMWare Fusion 5:


Now, install WinDbg and set up your symbol paths if they are not set up already.

Check your Device Manager in the client to see which COM port the USD device created. As you can see below, my laptop mounted the USB/Serial converter as COM3



After installing the device, I had to restart my virtual machine before VMWare would let me mount it – but what can you expect from a 10 GBP component? Your mileage may vary depending on the serial/USB driver you have.

Step 2: Connect Client and Server

Using the NULL modem, connect the USB/Serial converter to the server’s Serial port.

Make sure the server has the serial port enabled in the BIOS (my Dell box had it disabled, had to re-enable).

Step 3: Configure Server/Debugee for kernel debugging

Log into the server, start a command line as administrator

First, copy the default startup options into a new boot option:

  • BCDEDIT /copy {current} /d DebugMode

This will create a new entry in the boot list when the server starts. Make a note of the GUID returned or copy it to the clipboard (using the ever so annoying copy/paste function in the Windows command prompt)

Next, configure the parameters for serial cable debugging:

  • BCDEDIT /set <GUID> debugport <port #>
  • BCDEDIT /set <GUID> debugtype serial
  • BCDEDIT /set <GUID> baudrate 115200

Replace the <GUID> the guid returned previously. Set <port #> to the COM port the NULL modem is connected to in your server (NOT the COM port of the client). For example, if the server has the NULL modem in COM2, set <port #> to 2.

Finally, enable debugging for on the newly created boot option:

  • BCDEDIT /debug <GUID> ON

Validate that your configuration works by running:

  • BCDEDIT /v.

You should get an output somewhat like this (this is also how you find the GUID if you didn’t note it down before):


If you want to make sure debugging is always turned on (great for a sandbox machines where you explore stuff in the kernel) you can use BCDEDIT /default <GUID> to make debugging the default startup option.

Step 3: Start Debugging

You are now ready to start debugging the windows kernel on the server. Here is how:

On the client, start WinDbg and choose File—>Kernel Debug (or CTRL+K) and set up the com port you got in step 1:

. image

Press OK, and reboot the server. If you didn’t select the debug configuration as the default boot option, make sure you pick it when the server starts.

If you have done things right, you will get something like this in WinDbg (below, I broke execution with CTRL+C)


One thing to note when you are debugging the kernel: Not all your typical WinDbg commands work as they normally do (for some good reasons), but that is outside of scope for this blog entry.

Time to dig in even deeper… my Macbook Air to a Windows box :-)…Happy hacking everyone.