VMware kernel debugging

Since I'm mostly travelling around between my different customers my laptop is the only computer I have around. To setup test environments for the different projects I use the free VMware player and WMware server products to create virtual machines, in which I can test different solutions.

One of my last projects was to find a memory leak in a kernel mode driver. To do this I needed to do some kernel mode debugging again, something that normaly involves two computers connected by a serial cable. But not any more. It's a simple thing to setup a kernel mode debugging session using VMware.

First we need to get VMware to export a COM serial port to the host. This can be done through a named pipe and some lines added to the VMware configuration file for the virtual machine.

serial0.present = "TRUE"
serial0.fileType = "pipe"
serial0.fileName = "\\.\pipe\com_1"
serial0.tryNoRxLoss = "TRUE"
serial0.pipe.endPoint = "server"

We also need to enable kernel mode debugging in the target OS. This is done by editing the c:\boot.ini file for the virtual machine OS. Start by copying the current startup line, and add the /debug, /debugport and /baudrate startup arguments. My boot.ini looks like this:

[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows Server 2003, Enterprise" /noexecute=optout /fastdetect
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows Server 2003, Enterprise DEBUG" /noexecute=optout /fastdetect /debug /debugport=com1 /baudrate=115200


Now boot the virtual machine and select to boot into the DEBUG version. During the boot windows will stop and wait for the debugger. To start the debugger we give the arguments to connect to the named pipe that the VMware player now has created.

windbg.exe -b -k com:pipe,port=\\.\pipe\com_1,resets=0"

The boot will stop on a breakpoint during bootup, just enter g and press enter to continue.

To shorten the round-trip then doing driver development it's very nice to be able to change the driver directly on the target machine. This is since all bugs in kernel mode ends up in a BSOD (Blue Screen of Death). To do this a VMware utility called DiskMount comes very handy. With this utility it's possible to mount the virtual machine hard disk on your host computer and change the driver before the next boot.

Java Web Start connection problem

Anyone who has deployed any large application using Java Web Start 1.5 or 1.6 over a slow WAN or through some kind of SSL proxy has noticed the extreme number of connections that can be initiated during application startup. The first deploy of the application works as expected and all the JAR files is downloaded. But the sencond time the user clicks to start the application, Java Web Start will flood the network with connections to the server.

Investigation the problem revelas an interesting piece of code in the class com.sun.javaw.LaunchDownload and the method updateCheck, this function is called once for every JAR file in the JNLP description file. The code does the update check by starting a new thread and doing a HTTP request to the web server to check if the JAR file has been updated.
This means that if we have a JNLP file with 200 JAR files, the code will start 200 threads, creating 200 individual connections to the server!

The solution can be found in the same function as well, as the first lines of the function reads:

// no update check for versioned resource
if (version != null) return;


This means that if we put version attributes on the JAR files we will not invoke the badly written code. To add the version attribute is simple, just add it to the JAR element of the JNLP file:

<jar href="/application/foo.jar" version="”1.0” />


When Java Web Start finds this attribute on a JAR element in the JNLP file it will send this version string along with the GET request to the server. Like:

GET /application/foo.jar?version-id=1.0

To handle this on the server we have two options, if we are running a J2EE environment the JDK contains a JNLP servlet that can respond to this request and also return the required x-java-jnlp-version-id custom header, or we could implement the same functionality in some other kind of server side language.

I choose to implement the JAR server in PHP/Apache since we have noticed that the J2EE container we are using is not very good at serving large amounts of data.

To have Apache invoke my script for every JAR file requested from the server, I added the following lines to the httpd.conf file.

AddType application/java-archive .jar

Action application/java-archive /cgi-bin/jar_send.php

The script is very simple; we have chosen to store the versioned JAR archives
as /application/foo_x_y.jar for version x.y. This makes the script very simple to implement, and make deploying very simple as well. Another strategy could be to store a complete version of the application under /x.y/application/foo.jar and have all JAR archives in the JNLP file reference the same version.

Here is the code for the jar_send.php script:

<?

 // Make sure we have got a version-id argument
 if (isset($_GET["version-id"])) {
   $version = $_GET["version-id"];
 } else {
   $version = null;
 }
  
 // Retreive the requested file
 $file = $_SERVER["PATH_TRANSLATED"];
 
 // If not version is requested, or version is 1.0, send foo.jar
 if ($version == null || $version == "1.0") {
   $path = $file;
 } else {
   // If version 1.1 is requested, send foo_1_1.jar
   $x = strrpos($file,".jar");
   $path = substr($file,0,$x);
   $path .= "_";
   $path .= str_replace(".","_",$version);
   $path .= ".jar";
 }
 
 // Make sure the file exists
 if (!is_file($path)) {
   header("HTTP/1.0 404 NOT FOUND");
   print $path;
   die;
 }
  
 // Open the file
 $f = fopen($path, 'rb');
 if ($f == null) {
   header("HTTP/1.0 404 NOT FOUND");
   print $path;
   die;
 }

 // Send the JNLP custom header
 header("x-java-jnlp-version-id: $version");

 // Inform Apache about how much data we are going to send
 header("Content-Length: ".(string)(filesize($path)));
 
 // Send the data, 8K blocks
 while(!feof($f) &&(connection_status()==0)) {
   print(fread($f, 1024*8));
   flush();
 }

 // Close the file
 fclose($f);

?>