Monday, September 24, 2007

Application debugging with Record/Replay

My previous article explained how to use Replay debugger with the kernel. The key benefit of Replay is that you get 100% reproducibility of bugs with low overhead, and can analyze the recording using existing debuggers.

You can debug Linux processes running in the Virtual Machine as well. The approach is similar. First, you need to set up a replayable Virtual Machine. The easiest way is to convert existing Virtual Appliance. Here is how to do it in four steps:

Download VMware Workstation 6.0.1. Free evaluation version is here. The speed of recording and replaying is improved considerably in this release, especially with Intel Core 2 CPUs. Also, a few bugs here and there were taken care of.

Download Ubuntu 7.04 Virtual Appliance. (If you use different distro, check out this).

Unzip the appliance and edit the Ubuntu-7.04-desktop-i386.vmx. Add the following lines (they enable record/replay, add toolbar buttons to control the recording and enable remote debugging):

replay.logging = "TRUE"
pref.view.toolbars.vplay = "TRUE"
pref.view.toolbars.view = "FALSE"
debugStub.listen.guest32 = "TRUE"
debugStub.listen.guest32.remote = "TRUE"

Delete these lines (LSILogic and CD-ROM are not supported with Replay):

scsi0.virtualDev = "lsilogic"
ide1:0.startConnected = "TRUE"

... and add this line:

ide1:0.startConnected = "FALSE"

Power on the VM. It will ask you if you copied the VM - tell that you did. It will ask if you want to convert the SCSI to BusLogic type, click "yes".

Congratulations! You virtual machine is ready. For extra convenience, you may want to start /usr/bin/vmware-toolbox in a VM. This will automatically grab and release mouse when you enter or leave the VM.

The process of debugging is iterative. First you copy your application and tests inside the virtual machine, then you run the test scenario in recording mode until you get a crash. Once you've got a crash recorded, you can replay it as many times as you want, and inspect your application with debugger running on the Host. Let me go through these steps in details.

Copying the application and tests is easy. The appliance has networking enabled, so use you favorite network protocol: scp, ftp, rsync, etc. You need to make sure that you have same copy of the application running in the VM, and being passed to the debugger on the Host.

The VM has three new buttons on the Toolbar now: Record, Replay and Stop.

You can press Record, start the test inside the virtual machine and when it is done you can press Stop. This will record the execution of the test.

When you press Replay, the last recording is going to be replayed. If you want to replay some other recording, use Snapshot Manager to chose and replay it. You can also use Snapshot Manager to delete the recordings that you do not need anymore.

While you are replaying an application you can attach the gdb running on the Host to a virtual machine. Launch gdb on the Host machine. Assuming that your test application is MyApp:

% gdb MyApp

If you run gdb on the same Host, this line will attach gdb to the VM:

(gdb) target remote localhost:8832

By default our debugger is in system mode, that is, it doesn't know anything about processes in the virtual machine. To switch to process mode, you need to tell the debugger a little about the offsets of different kernel data structures. Here is how you can do it for Ubuntu 7.04:

(gdb) monitor linuxoffsets 0x20614,0x80,0,0x68,0x194,0xa4,0x1b0,\


Different kernel versions require different line. I will post instructions on how compute it in next article. When debugger is in process mode, you can use "info threads" command to get a list of all processes:

If you do not see your process in the listing, then it either hasn't started yet, or exited already. If it didn't start, you can just issue "continue" and then "^C" a little later. For short-living processes, you may want to add "sleep(5)" in the beginning of "main()" to simplify attaching.

To attach to the process you are interested in, you can use "thread" commands. The argument of the command is the first number in the "info threads" output corresponding to your process. For example, to attach to MyApp in the above listing you should issue this:

(gdb) thread 22

Once attached, you can inspect memory, insert breakpoints, step over instructions, etc. When you are done, you can either issue "quit" in the debugger to shut down Virtual Machine and debugger, or issue "detach" to let the Virtual Machine continue running without debugger. Of course, you can attach the debugger again anytime.

It is convenient to combine all gdb steps into a macro, e.g.:

--- cut here: attach.gdb ---

target remote localhost:8832
monitor linuxoffsets 0x20614,0x80,0,0x68,0x194,0xa4,0x1b0,\
info threads
--- cut here ---

Then you can attach gdb and get a list of processes with a single line:

% gdb --command=attach.gdb MyApp

The Replay feature remains experimental in the VMware Workstation 6.0.1. If you have a question or suggestion, or if you discovered a bug, please post here. The engineers working on Replay are checking the forum and will be happy to talk to you.

This note explained how to use replay debugging with processes running inside Ubuntu 7.04 Virtual Appliance. I will describe how to use it with different Linux kernels next.


hieu said...

Hi Salva,

Very interesting entry, thanks.

However, please may you explain how did you get the below numbers for the "monitor command" ?


This confused me, as I tried to follow your steps.


Slava said...

Hi Hieu,

The list of hex numbers gives gdb offsets of different fields in kernel data structures. If you do "monitor help linuxoffsets" it will tell you what these numbers are.

I plan to post another article shortly with script computing the numbers for any given Linux kernel.


hieu said...

Salva, thanks so much for the help.

I am out of reach of VMWare now, so please could you post the result of "monitor help linuxoffset" here?


Slava said...

Hi Hieu. I just posted a new article that has the format of the "monitor linuxoffsets" string as well as a few scripts that compute offsets line automatically. Please post on the forum if you have a question or suggestion about it. Thank you!

Slava said...

I had to disable additional comments due to spam added to some other posts. Please post your comments and suggestions at our forum:

Thank you.