Windows® Internals, Sixth Edition, Part 2

SmpLoadDataFromRegistry calls SmpConfigureSharedSessionData to initialize the list of subsystems that will be started in each session (both immediately and deferred) as well as the Session 0 initialization command (which, by default, is to launch the Wininit.exe process). The initialization command can be overridden by creating a string value called S0InitialCommand in HKLM\SYSTEM\CurrentControlSet\Control\Session Manager and setting it as the path to another program.

SmpLoadDataFromRegistry calls SmpInitializeKnownDlls to open known DLLs, and creates section objects for them in the \Knowndlls directory of the object manager namespace. The list of DLLs considered known is located in HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\KnownDLLs, and the path to the directory in which the DLLs are located is stored in the DllDirectory value of the key. On 64-bit systems, 32-bit DLLs used as part of Wow64 are stored in the DllDirectory32 value.

Finally, SmpLoadDataFromRegistry calls SmpTranslateSystemPartitionInformation to convert the SystemPartition value stored in HKLM\SYSTEM\Setup, which is stored in native NT object manager path format, to a volume drive letter stored in the BootDir value. Among other components, Windows Update uses this registry key to figure out what the system volume is.

At this point, SmpLoadDataFromRegistry returns to SmpInit, which returns to the main thread entry point. Smss then creates the number of initial sessions that were defined (typically, only one, session 0, but you can change this number through the NumberOfInitialSessions registry value in the Smss registry key mentioned earlier) by calling SmpCreateInitialSession, which creates an Smss process for each user session. This function’s main job is to call SmpStartCsr to start Csrss in each session.

As part of Csrss’s initialization, it loads the kernel-mode part of the Windows subsystem (Win32k.sys). The initialization code in Win32k.sys uses the video driver to switch the screen to the resolution defined by the default profile, so this is the point at which the screen changes from the VGA mode the boot video driver uses to the default resolution chosen for the system.

Meanwhile, each spawned Smss in a different user session starts the other subsystem processes, such as Psxss if the Subsystem for Unix-based Applications feature was installed. (See Chapter 3 in Part 1 for more information on subsystem processes.)

The first Smss from session 0 executes the Session 0 initialization command (described in step 14), by default launching the Windows initialization process (Wininit). Other Smss instances start the interactive logon manager process (Winlogon), which, unlike Wininit, is hardcoded. The startup steps of Wininit and Winlogon are described shortly.

Pending File Rename Operations

The fact that executable images and DLLs are memory-mapped when they are used makes it impossible to update core system files after Windows has finished booting (unless hotpatching technology is used, which is only for Microsoft patches to the operating system). The MoveFileEx Windows API has an option to specify that a file move be delayed until the next boot. Service packs and hotfixes that must update in-use memory-mapped files install replacement files onto a system in temporary locations and use the MoveFileEx API to have them replace otherwise in-use files. When used with that option, MoveFileEx simply records commands in the PendingFileRenameOperations and PendingFileRenameOperations2 keys under HKLM\SYSTEM\CurrentControlSet\Control\Session Manager. These registry values are of type MULTI_SZ, where each operation is specified in pairs of file names: the first file name is the source location, and the second is the target location. Delete operations use an empty string as their target path. You can use the Pendmoves utility from Windows Sysinternals (http://www.microsoft.com/technet/sysinternals) to view registered delayed rename and delete commands.

After performing these initialization steps, the main thread in Smss waits forever on the process handle of Winlogon, while the other ALPC threads wait for messages to create new sessions or subsystems. If either Wininit or Csrss terminate unexpectedly, the kernel crashes the system because these processes are marked as critical. If Winlogon terminates unexpectedly, the session associated with it is logged off.