OSDev.org

Posted: **Fri Mar 27, 2009 6:30 pm**

Hello, new to the board here but I just had an exciting method of debugging boot code that I thought I'd share with the community. Debugging the boot code from 32-bit to 64-bit isn't a technique I've seen mentioned before.

There is one bug with this method. You CANNOT stepi through the jmp that lands in 64-bit code as this will cause VMware to crash; you can however continue to a breakpoint on the other side. I have not tested a breakpoint in the higher-half yet as I haven't had time.

If someone could test that I will update this post.

I'll be using the xomb bare bones kernel as an example although I have modified mine to be assembly only; essentially I removed the call to kmain and only link boot.S and load.S.
The most helpful documentation for the boot.S/load.S files is at: XBB From Start to Finish

Tools used:

Ubuntu 64-bit host 8.10
Ubuntu 64-bit 8.10 Desktop installation CD image
VMware Workstation 6.5
GParted Live CD 0.4.3-2 image
apt-get install build-essential git-svn git-gui gcc-multilib ddd

First steps:

create a 64-bit virtual machine with VMware
partition the disk using GParted
install GRUB on the disk image with the Ubuntu install CD
add debugStub32/64 and hideBreakpoints to the vmx configuration file
git clone git://github.com/xomboverlord/xomb-bare-bones.git
modify load.S to not call kmain and linker.ld to remove paths

VMware

Machine: I will install the operating system later, OS Other 64-bit, 1024MB memory, 12GB IDE hard disk single file
Set the Virtual CD to the GParted Live CD image

Add the following lines to the virtual machine *.vmx file:

Code: Select all

bios.bootDelay = "3000"
debugStub.hideBreakpoints = "TRUE"              # allows gdb breakpoints to work
debugStub.listen.guest32 = "TRUE"                 # (gdb) target remote localhost:8832
debugStub.listen.guest64 = "TRUE"                 # (gdb) target remote localhost:8864
#debugStub.listen.guest32.remote = "TRUE"   # for debugging from another machine
#debugStub.listen.guest64.remote = "TRUE"   # for debugging from another machine
#monitor.debugOnStartGuest32 = "TRUE"       # This will halt the VM at the very first instruction at 0xFFFF0, you could set the next breakpoint to break *0x7c00 to break when the bootloader is loaded by the BIOS

GParted Live CD

Partition the 12GB into: 100MB label=boot ext3, 9.90GB label=root ext3, 2.0GB swap

Ubuntu Installation CD

Set the Virtual CD to the Ubuntu CD image
When VMware boots, hit ESC when the BIOS appears to get the boot menu and select CD-ROM Drive
Boot into the 'Try Ubuntu' portion

Perform the following commands to install GRUB on the disk:

Code: Select all

ls -l /dev/disk/by-label   # list drive e.g. boot -> ../../sda1
sudo fdisk -l                 # if you did not assign a label
sudo mount /dev/sda1 /mnt/
sudo grub-install --root-directory=/mnt/ /dev/sda
sudo gedit /mnt/boot/grub/device.map    # change (hd0) /dev/sda to (hd0) /dev/hda

Reboot and verify GRUB shell loads.

Code: Select all


/* Debug 32-bit boot code */
# Boot to GRUB shell
# run ddd, make sure the View | Machine Code Window and View | Command Tool are available 
set architecture i386
target remote localhost:8832
# Break where the kernel will be loaded by GRUB
break *0x100000
continue

/* transition to 64-bit */
# before jumping to 64-bit code gdb needs to be placed into x86-64 mode and the target switched to the debugStub64 remote
# first, set a breakpoint at the linear address in 64-bit space your kernel will be jumping into, this can be seen in the 32-bit disassembly
break *0x118000
continue
# gdb will hang as the debugStub32 stops responding. Hit Ctrl-C twice or click Interrupt twice in the ddd Command Tool to revive the (gdb) prompt
# The vmware.log will contain something like: Mar 27 19:43:33.359: vcpu-0| Waiting for Guest 64-bit debugger to connect!
set architecture i386:x86-64
target remote localhost:8864
# we should now be in 64-bit space at the breakpoint we set earlier.
# when the kernel jumps to the higher-half, ddd (gdb) follows along flawlessly

/* Debug 64-bit boot code without transtion from 32-bit */
# Boot to GRUB shell
# in GRUB
kernel /boot/xomb
boot
# in ddd
set architecture i386:x86-64
break *<known address>
target remote localhost:8864

This is a work in progress; occasionally I will save the post and edit it to add more content. If this information ends up in the wiki eventually, I will be very happy.

OSDev.org

32/64-bit transition boot code debugging w/VMware [GUIDE]

32/64-bit transition boot code debugging w/VMware [GUIDE]