When even Process Monitor isn’t enough

I was recently tasked to investigate why an App-V 5.1 application was giving a license error at launch on XenApp 7.8 (on Server 2008R2) when the same application installed locally worked fine. I therefore ran up the trusty Process Monitor (procmon) tool to get traces on the working and non-working systems so I could look for differences. As I knew what the licence file was called, I honed in quickly on this in the traces. In the working trace, you could see it open the licence file, via a CreateFile operation, and then read from the file. However, in the App-V version it wasn’t reading from the file (a ReadFile operation) but no CreateFile operation was failing so I couldn’t understand why it wasn’t even attempting to read from the file when it didn’t appear to be unable to access it. The same happened when running as an administrator so it didn’t look like a file permission issue.

Now whilst procmon is a simply awesome tool, such that life without it would be an unimaginably difficult place, it does unfortunately only tell you about a few of the myriad of Microsoft API calls. In order to understand even more of what a process is doing under the hood, you need to use an API monitor program that has the ability to hook any API call available. To this end I used WinAPIOverride (available here). What I wanted was to find the calls to CreateFile for the licence file and then see what happened after that, again comparing good and bad procmon traces.

WinAPIOverride can launch a process but it needs to be inside the App-V bubble for the app in order for it to be able to function correctly. We therefore run the following PowerShell to get a PowerShell prompt inside the bubble for our application which is called “Medallion”:

$app = Get-AppvClientPackage | ?{ $_.Name -eq 'Medallion' };
Start-AppvVirtualProcess -AppvClientObject $app powershell.exe

We can then launch WinAPIOverride64.exe in this new PowerShell prompt, tell it what executable to run and then run it:


Note that you may not be able to browse to the executable name so you may have to type it in manually.

Once we tell it to run, it will allow us to specify what APIs we want to get details on by clicking on the “Monitoring Files Library” button before we click “Resume”.


You need to know the module (dll) which contains the API that you want to monitor. In this case it is kernel32.dll which we can glean from the MSDN manual page for the CreateFile API call (see here).


Whilst you can use the search facility to find the specific APIs that you want to monitor and just tick those, I decided initially to monitor everything in kernel32.dll, knowing that it would generate a lot of data but we can search for what we want if necessary.

So I resumed the process, saw the usual error about the licence file being corrupt, stopped the API monitor trace and set about finding the CreateFile API call for the licence file to see what it revealed. What I actually found was that CreateFile was not being called for the licence file but when I searched for the licence file in the trace, it revealed that it was being opened by a legacy API called OpenFile instead. Looking at the details for this API (here), it says the following:

you cannot use the OpenFile function to open a file with a path length that exceeds 128 characters

Guess how long the full path for our licence file is? 130 characters! So we’re doomed it would seem with this API call which we could see was failing in API monitor anyway:


I suspect that we don’t see this in procmon as the OpenFile call fails before it gets converted to a CreateFile call and thence hits the procmon filter driver.

The workaround, as we found that the installation wouldn’t work in any other folder than c:\Medallion so we couldn’t install it to say C:\M, was to shorten the package installation root by running the following as an admin:

Set-AppvClientConfiguration -PackageInstallationRoot '%SystemDrive\A'

This changes the folder where App-V packages are cached from “C:\ProgramData\App-V” to “C:\A” which saves us 18 characters. The C:\A folder needed to be created and given the same permissions and owner (system) as the original folder. I then unloaded and reloaded the App-V package so it got cached to the \A folder whereupon it all worked properly.