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App-V 4 Application Troubleshooting: Breaking Down Virtualization Issues Pt. II

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App-V 4.5 and 4.6 virtualize at the user mode layer. One of the most identifying factors of seeing that a thread stack is that of a virtualized application is the presence of the SFTLDR.DLL file. This is what is injected into every process a virtual application will create. This file is responsible for ensuring proper redirections and translations necessary to make virtualization function properly by:

  • File changes to included virtual directory and file paths are redirected to the VFS
  • Registry changes hooked and redirected to the virtual registry
  • The spoofing of objects
  • The spoofing of COM GUIDS

In addition to the common troubleshooting methods such as disabling local interaction and disabling object spoofing, you can also take things further by disabling various virtualization components using the System Guard Overrides in App-V 4.x. These are not meant to be solutions but isolation factors in case you need to modify mappings. Many of these can be set at the registry level affecting the entire client or at the application level using the OSD file.

All of the registry values mentioned are located under HKLM\SOFTWARE\Microsoft\SoftGrid\4.5\SystemGuard\Overrides:

Disabling Virtual Services

You can disable virtual services on a per package basis by adding in the <VIRTUAL_SERVICES_DISABLED> tag under the <POLICY> XML element in the OSD file. You can disable the subsystem for the entire client by going adding the DisableVirtualServices DWORD value with a value of 1. If this is enabled, the sftldr.dll will not hook the service APIs.

Disabling the Virtual Registry

You can disable the virtual registry on a per package basis by adding in the <VIRTUAL_REGISTRY_DISABLED> tag under the <POLICY> XML element in the OSD file. You can disable the subsystem for the entire client by going adding the DisableVreg DWORD value with a value of 1. If this is enabled, the sftldr.dll will not hook the virtual registry calls.

 Disabling the Virtual File System

You can disable the virtual file system on a per package basis by adding in the <VIRTUAL_FILE_SYSTEM_DISABLED> tag under the <POLICY> XML element in the OSD file. You can disable the subsystem for the entire client by going adding the DisableVFS DWORD value with a value of 1. If this is enabled, the sftldr.dll will not hook virtual file system calls.

Finally, if you are really interested in going to the extreme . . .

You can disable ALL hooking. Can be useful when you are launching an application that is locally installed but still being brought into the virtual bubble. This allows you to turn it on and off if troubleshooting odd behavior. This is done at the client level which is why it is definitely only a troubleshooting option. You can disable hooking by adding in the registry value DisableSftldr DWORD value with a value of 42. Why 42? Well because that is the answer to everything in the universe. This basically makes the sftldr.dll (which is the primary hook DLL) dormant. MAVINJECT32 (or MAVINJECT64 if a 64-bit system) will still inject this DLL though. It will just remain dormant. This is a last resort.