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Methods

The second method is to debug on a virtual machine. There are several available, including the commercial VMWare and the open-source projects User Mode Linux and bochs.

VMware

1. Install VMware. The configuration I used when writing this document was VMware 3.2 for Linux. I'm running a custom 2.4.19 kernel on the host (the physical hardware), and RedHat 7.3 on the guest (the VMware virtual machine).
2. Configure VMware serial port(s).
   • Configure VMware to use /dev/ptysX (gdb will use the corresponding /dev/ttysX). The kgdb examples use ttyS1, so configure VMware COM2 to use '/dev/ptys1' (Window uses COM1-COM4, Linux uses ttyS0-ttyS3; see table below to use other ports).
   • VMware 3.2 contains a rewritten serial port interface, so you may need to exit VMware, add 'serial.v2=False' to your configuration file, and restart VMware. (This obscure fact is hidden here in their troubleshooting section.) Then the configuration editor should allow you to sellect 'TTY' as the type for the serial port. Note:
     

     VWware TTY Configuration	kgdb gdbttyS	gdb remote target
     COM1 = /dev/ptys0	0	/dev/ttys0
     COM2 = /dev/ptys1	1	/dev/ttys1
     COM3 = /dev/ptys2	2	/dev/ttys2
     COM4 = /dev/ptys3	3	/dev/ttys3

   • The relevant secion of my VMware configuration file looks like this:

     serial.v2=False
     serial1.yieldOnMsrRead = "TRUE"
     serial1.present = "TRUE"
     serial1.fileName = "/dev/ptys1"
     serial1.tryNoRxLoss = "true"
     serial1.fileType = "tty"
     

3. Build a kgdb enabled kernel. I used the RedHat 2.4.18-3 kernel, and built it on the host.
   • The kgdb patch (linux-2.4.18-kgdb-1.5.patch, as of when I wrote this document) didn't apply cleanly, but it was pretty easy to sort out. After you apply the patch, search for any '.rej' files and merge them by hand. If you can't figure out how to do this, then you probably shouldn't be debugging kernel anyways.
   • kgdb recommends that you compile everything you need into the kernel, instead of using modules. Here are some modules the VMware uses with my RedHat 7.3: pcnet32, mii, ide-cd, cdrom, usb-uhci, usbcore. If you want to use NFS, add nfs, lockd, sunrpc.
   • There was one remaining compilation problem, here is a fix from the kgdb mailing list: do_BUG symbol.
   • See the kgdb kernel build page for more details.
4. Install the kgdb kernel (and any necessary modules) on the virtual machine, per the kgdb docs. When you reboot, the kernel will pause an wait for gdb to attach.
5. Attach gdb to the virtual machine, again, as described in the kgdb connecting to a debug kernel document. You should hit a break point right away. Set any breakpoints youd like, and enter 'c' to continue booting the virtual machine.
6. Debug the kernel.

User Mode Linux

1. Install User Mode Linux per the documents at their web site.
2. Attach a debugger to a kernel as described in their debugging page.
3. Debug the kernel.

Modules

VMware

1. Get rsh working.
   • add 'rsh' to /etc/securetty
   • set 'disabled = no' in /etc/xinetd.d/rsh
   • run 'service xinetd restart'

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