Processes, Threads, and Jobs in the Windows Operating System

6/17/2009

Contents

In this chapter from Windows Internals, 5th Edition, learn the data structures and algorithms that deal with processes, threads, and jobs in the Windows operating system. The first section focuses on the internal structures that make up a process. The second section outlines the steps involved in creating a process (and its initial thread). The internals of threads and thread scheduling are then described. The chapter concludes with a description of the job object.

Where relevant performance counters or kernel variables exist, they are mentioned. Although this book isn’t a Windows programming book, the pertinent process, thread, and job Windows functions are listed so that you can pursue additional information on their use.

Because processes and threads touch so many components in Windows, a number of terms and data structures (such as working sets, objects and handles, system memory heaps, and so on) are referred to in this chapter but are explained in detail elsewhere in the book. To fully understand this chapter, you need to be familiar with the terms and concepts explained in Chapter 1 and Chapter 2, such as the difference between a process and a thread, the Windows virtual address space layout, and the difference between user mode and kernel mode.

Process Internals

This section describes the key Windows process data structures. Also listed are key kernel variables, performance counters, and functions and tools that relate to processes.

Data Structures

Each Windows process is represented by an executive process (EPROCESS) block. Besides containing many attributes relating to a process, an EPROCESS block contains and points to a number of other related data structures. For example, each process has one or more threads represented by executive thread (ETHREAD) blocks. (Thread data structures are explained in the section Thread Internals later in this chapter.) The EPROCESS block and its related data structures exist in system address space, with the exception of the process environment block (PEB), which exists in the process address space (because it contains information that needs to be accessed by user-mode code).

In addition to the EPROCESS block and the PEB, the Windows subsystem process (Csrss) maintains a parallel structure for each process that is executing a Windows program. Finally, the kernel-mode part of the Windows subsystem (Win32k.sys) will also maintain a per-process data structure that is created the first time a thread calls a Windows USER or GDI function that is implemented in kernel mode.

Figure 5-1 is a simplified diagram of the process and thread data structures. Each data structure shown in the figure is described in detail in this chapter.

Figure 5-1. Data structures associated with processes and threads

First let’s focus on the process block. (We’ll get to the thread block in the section Thread Internals later in the chapter.) Figure 5-2 shows the key fields in an EPROCESS block.

Figure 5-2. Structure of an executive process block

EXPERIMENT: Displaying the Format of an EPROCESS Block

For a list of the fields that make up an EPROCESS block and their offsets in hexadecimal, type dt _eprocess in the kernel debugger. (See Chapter 1 for more information on the kernel debugger and how to perform kernel debugging on the local system.) The output (truncated for the sake of space) on a 32-bit system looks like this:

lkd> dt _eprocess
nt!_EPROCESS
  +0x000 Pcb              : _KPROCESS
  +0x080 ProcessLock      : _EX_PUSH_LOCK
  +0x088 CreateTime       : _LARGE_INTEGER
  +0x090 ExitTime         : _LARGE_INTEGER
  +0x098 RundownProtect   : _EX_RUNDOWN_REF
  +0x09c UniqueProcessId  : Ptr32 Void
  +0x0a0 ActiveProcessLinks : _LIST_ENTRY
  +0x0a8 QuotaUsage       : [3] Uint4B
  +0x0b4 QuotaPeak        : [3] Uint4B
  +0x0c0 CommitCharge     : Uint4B
  +0x0c4 PeakVirtualSize  : Uint4B
  +0x0c8 VirtualSize      : Uint4B
  +0x0cc SessionProcessLinks : _LIST_ENTRY
  +0x0d4 DebugPort        : Ptr32 Void
  +0x0d8 ExceptionPortData : Ptr32 Void
  +0x0d8 ExceptionPortValue : Uint4B
  +0x0d8 ExceptionPortState : Pos 0, 3 Bits
  +0x0dc ObjectTable      : Ptr32 _HANDLE_TABLE
  +0x0e0 Token            : _EX_FAST_REF
  +0x0e4 WorkingSetPage   : Uint4B
  +0x0e8 AddressCreationLock : _EX_PUSH_LOCK
  +0x0ec RotateInProgress : Ptr32 _ETHREAD
  +0x0f0 ForkInProgress   : Ptr32 _ETHREAD
  +0x0f4 HardwareTrigger  : Uint4B
  +0x0f8 PhysicalVadRoot  : Ptr32 _MM_AVL_TABLE
  +0x0fc CloneRoot        : Ptr32 Void
  +0x100 NumberOfPrivatePages : Uint4B
  +0x104 NumberOfLockedPages : Uint4B
  +0x108 Win32Process     : Ptr32 Void
  +0x10c Job              : Ptr32 _EJOB
  +0x110 SectionObject    : Ptr32 Void
  +0x114 SectionBaseAddress : Ptr32 Void
  +0x118 QuotaBlock       : Ptr32 _EPROCESS_QUOTA_BLOCK

Note that the first field (Pcb) is actually a substructure, the kernel process block (KPROCESS), which is where scheduling-related information is stored. To display the format of the kernel process block, type dt _kprocess:

lkd> dt _kprocess
nt!_KPROCESS
  +0x000 Header           : _DISPATCHER_HEADER
  +0x010 ProfileListHead  : _LIST_ENTRY
  +0x018 DirectoryTableBase : Uint4B
  +0x01c Unused0          : Uint4B
  +0x020 LdtDescriptor    : _KGDTENTRY
  +0x028 Int21Descriptor  : _KIDTENTRY
  +0x030 IopmOffset       : Uint2B
  +0x032 Iopl             : UChar
  +0x033 Unused           : UChar
  +0x034 ActiveProcessors : Uint4B
  +0x038 KernelTime       : Uint4B
  +0x03c UserTime         : Uint4B
  +0x040 ReadyListHead    : _LIST_ENTRY
  +0x048 SwapListEntry    : _SINGLE_LIST_ENTRY
  +0x04c VdmTrapcHandler  : Ptr32 Void
  +0x050 ThreadListHead   : _LIST_ENTRY
  +0x058 ProcessLock      : Uint4B
  +0x05c Affinity         : Uint4B
  +0x060 AutoAlignment    : Pos 0, 1 Bit
  +0x060 DisableBoost     : Pos 1, 1 Bit
  +0x060 DisableQuantum   : Pos 2, 1 Bit
  +0x060 ReservedFlags    : Pos 3, 29 Bits
  +0x060 ProcessFlags     : Int4B
  +0x064 BasePriority     : Char
  +0x065 QuantumReset     : Char
  +0x066 State            : UChar
  +0x067 ThreadSeed       : UChar
  +0x068 PowerState       : UChar
  +0x069 IdealNode        : UChar
  +0x06a Visited          : UChar
  +0x06b Flags            : _KEXECUTE_OPTIONS
  +0x06b ExecuteOptions   : UChar
  +0x06c StackCount       : Uint4B
  +0x070 ProcessListEntry : _LIST_ENTRY
  +0x078 CycleTime        : Uint8B

An alternative way to see the KPROCESS (and other substructures in the EPROCESS) is to use the recursion (–r) switch of the dt command. For example, typing dt _eprocess –r1 will recurse and display all substructures one level deep.

The dt command shows the format of a process block, not its contents. To show an instance of an actual process, you can specify the address of an EPROCESS structure as an argument to the dt command. You can get the address of all the EPROCESS blocks in the system by using the !process 0 0 command. An annotated example of the output from this command is included later in this chapter.

Table 5-1 explains some of the fields in the preceding experiment in more detail and includes references to other places in the book where you can find more information about them. As we’ve said before and will no doubt say again, processes and threads are such integral parts of Windows that it’s impossible to talk about them without referring to many other parts of the system. To keep the length of this chapter manageable, however, we’ve covered those related subjects (such as memory management, security, objects, and handles) elsewhere.

Table 5-1. Contents of the EPROCESS Block

Element	Purpose	Additional Reference
Kernel process (KPROCESS) block	Common dispatcher object header, pointer to the process page directory, list of kernel thread (KTHREAD) blocks belonging to the process, default base priority, affinity mask, and total kernel and user time and CPU clock cycles for the threads in the process.	Thread scheduling (Chapter 5)
Process identification	Unique process ID, creating process ID, name of image being run, window station process is running on.
Quota block	Limits on processor usage, nonpaged pool, paged pool, and page file usage plus current and peak process nonpaged and paged pool usage. (Note: Several processes can share this structure: all the system processes in session 0 point to a single systemwide quota block; all other processes in interactive sessions share a single quota block.)
Virtual address descriptors (VADs)	Series of data structures that describes the status of the portions of the address space that exist in the process.	Virtual address descriptors (Chapter 9)
Working set information	Pointer to working set list (MMWSL structure); current, peak, minimum, and maximum working set size; last trim time; page fault count; memory priority; outswap flags; page fault history.	Working sets (Chapter 9)
Virtual memory information	Current and peak virtual size, page file usage, hardware page table entry for process page directory.	Chapter 9
Exception legacy local procedure call (LPC) port	Interprocess communication channel to which the process manager sends a message when one of the process’s threads causes an exception.	Exception dispatching (Chapter 3)
Debugging object	Executive object through which the user-mode debugging infrastructure sends notifications when one of the process’s threads causes a debug event.	User-mode debugging (Chapter 3)
Access token (TOKEN)	Executive object describing the security profile of this process.	Chapter 6
Handle table	Address of per-process handle table.	Object handles and the process handle table (Chapter 3)
Device map	Address of object directory to resolve device name references in (supports multiple users).	Object names (Chapter 3)
Process environment block (PEB)	Image information (base address, version numbers, module list), process heap information, and thread-local storage utilization. (Note: The pointers to the process heaps start at the first byte after the PEB.)	Chapter 5
Windows subsystem process block (W32PROCESS)	Process details needed by the kernel-mode component of the Windows subsystem.

The kernel process (KPROCESS) block, which is part of the EPROCESS block, and the process environment block (PEB), which is pointed to by the EPROCESS block, contain additional details about the process object. The KPROCESS block (which is sometimes called the PCB or process control block) is illustrated in Figure 5-3. It contains the basic information that the Windows kernel needs to schedule the threads inside a process. (Page directories are covered in Chapter 9, and kernel thread blocks are described in more detail later in this chapter.)

The PEB, which lives in the user process address space, contains information needed by the image loader, the heap manager, and other Windows system DLLs that need to access it from user mode. (The EPROCESS and KPROCESS blocks are accessible only from kernel mode.) The basic structure of the PEB is illustrated in Figure 5-4 and is explained in more detail later in this chapter.

Figure 5-3. Structure of the executive process block

Figure 5-4. Fields of the process environment block

EXPERIMENT: Examining the PEB

You can dump the PEB structure with the !peb command in the kernel debugger. To get the address of the PEB, use the !process command as follows:

lkd> !process
PROCESS 8575f030  SessionId: 1  Cid: 08d0    Peb: 7ffd9000  ParentCid: 0360
    DirBase: 1a81b000  ObjectTable: e12bd418  HandleCount:  66.
    Image: windbg.exe

Then specify that address to the !peb command as follows:

lkd> !peb 7ffd9000
PEB at 7ffd9000
    InheritedAddressSpace:    No
    ReadImageFileExecOptions: No
    BeingDebugged:            No
    ImageBaseAddress:         002a0000
    Ldr                       77895d00
    Ldr.Initialized:          Yes
    Ldr.InInitializationOrderModuleList: 00151c38 . 00191558
    Ldr.InLoadOrderModuleList:           00151bb8 . 00191548
    Ldr.InMemoryOrderModuleList:         00151bc0 . 00191550
            Base TimeStamp                     Module
          2a0000 4678a41e Jun 19 23:50:54 2007 C:\Program Files\Debugging Tools for
              Windows\windbg.exe
        777d0000 4549bdc9 Nov 02 05:43:37 2006 C:\Windows\system32\Ntdll.dll
        764c0000 4549bd80 Nov 02 05:42:24 2006 C:\Windows\system32\kernel32.dll
    SubSystemData:     00000000
    ProcessHeap:       00150000
    ProcessParameters: 001512e0
    WindowTitle:  'C:\Users\Alex Ionescu\Desktop\WinDbg.lnk'
    ImageFile:    'C:\Program Files\Debugging Tools for Windows\windbg.exe'
    CommandLine:  '"C:\Program Files\Debugging Tools for Windows\windbg.exe" '
    DllPath:      'C:\Program Files\Debugging Tools for Windows;C:\Windows        system32;C:\Windows\system;C:\Windows;.;C:\Windows\system32;C:\Windows;
        C:\Windows\System32\Wbem;C:\Program Files\Common Files\Roxio Shared        DLLShared\;C:\Program Files\Common Files\Roxio Shared\DLLShared\;C:\Program
        Files\Common Files\Roxio Shared\9.0\DLLShared\;c:\sysint;C:\Program Files        QuickTime\QTSystem\'
    Environment:  001850a8
        ALLUSERSPROFILE=C:\ProgramData
        APPDATA=C:\Users\Alex Ionescu\AppData\Roaming
     .
     .
     .

Kernel Variables

A few key kernel global variables that relate to processes are listed in Table 5-2. These variables are referred to later in the chapter, when the steps in creating a process are described.

Table 5-2. Process-Related Kernel Variables

Element	Purpose	Additional Reference
PsActiveProcessHead	Doubly linked list	List head of process blocks
PsIdleProcess	Pointer to EPROCESS	Idle process block
PsInitialSystemProcess	Pointer to EPROCESS	Pointer to the process block of the initial system process that contains the system threads
PspCreateProcessNotifyRoutine	Array of executive callback objects	Array of callback objects describing the routines to be called on process creation and deletion (maximum of eight)
PspCreateProcessNotifyRoutineCount	32-bit integer	Count of registered process notification routines
PspCreateProcessNotifyRoutineCountEx	32-bit integer	Count of registered extended process notification routines
PspLoadImageNotifyRoutine	Array of executive callback objects	Array of callback objects describing the routines to be called on image load (maximum of eight)
PspLoadImageNotifyRoutineCount	32-bit integer	Count of registered image-load notification routines
PspNotifyEnableMask	32-bit integer	Mask for quickly checking whether any extended or standard notification routines are enabled
PspCidTable	Pointer to HANDLE_TABLE	Handle table for process and thread client IDs

Performance Counters

Windows maintains a number of counters with which you can track the processes running on your system; you can retrieve these counters programmatically or view them with the Performance tool. Table 5-3 lists the performance counters relevant to processes.

Table 5-3. Process-Related Performance Counters

Object: Counter	Function
Process: % Privileged Time	Describes the percentage of time that the threads in the process have run in kernel mode during a specified interval.
Process: % Processor Time	Describes the percentage of CPU time that the threads in the process have used during a specified interval. This count is the sum of % Privileged Time and % User Time.
Process: % User Time	Describes the percentage of time that the threads in the process have run in user mode during a specified interval.
Process: Elapsed Time	Describes the total elapsed time in seconds since this process was created.
Process: ID Process	Returns the process ID. This ID applies only while the process exists because process IDs are reused.
Process: Creating Process ID	Returns the process ID of the creating process. This value isn’t updated if the creating process exits.
Process: Thread Count	Returns the number of threads in the process.
Process: Handle Count	Returns the number of handles open in the process.

Relevant Functions

For reference purposes, some of the Windows functions that apply to processes are described in Table 5-4. For further information, consult the Windows API documentation in the MSDN Library.

Table 5-4. Process-Related Functions

Function	Description
CreateProcess	Creates a new process and thread using the caller’s security identification
CreateProcessAsUser	Creates a new process and thread with the specified alternate security token
CreateProcessWithLogonW	Creates a new process and thread to run under the credentials of the specified username and password
CreateProcessWithTokenW	Creates a new process and thread with the specified alternate security token, with additional options such as allowing the user profile to be loaded
OpenProcess	Returns a handle to the specified process object
ExitProcess	Ends a process, and notifies all attached DLLs
TerminateProcess	Ends a process without notifying the DLLs
FlushInstructionCache	Empties the specified process’s instruction cache
FlushProcessWriteBuffers	Empties the specified process’s write queue
GetProcessTimes	Obtains a process’s timing information, describing how much time the threads inside the process spent in user and kernel mode
QueryProcessCycleTimeCounter	Obtains a process’s CPU timing information, describing how many clock cycles the threads inside the process have spent in total
Query/SetProcessAffinityUpdateMode	Defines whether the process’s affinity is automatically updated if new processors are added to the running system
Get/SetProcessDEPPolicy	Returns or sets the DEP (Data Execution Protection) policy for the process
GetExitCodeProcess	Returns the exit code for a process, indicating how and why the process shut down
GetCommandLine	Returns a pointer to the command-line string passed to the current process
QueryFullProcessImageName	Returns the full name of the executable image associated with the process
GetCurrentProcess	Returns a pseudo handle for the current process
GetCurrentProcessId	Returns the ID of the current process
GetProcessVersion	Returns the major and minor versions of the Windows version on which the specified process expects to run
GetStartupInfo	Returns the contents of the STARTUPINFO structure specified during CreateProcess
GetEnvironmentStrings	Returns the address of the environment block
Get/SetEnvironmentVariable	Returns or sets a specific environment variable
Get/SetProcessShutdownParameters	Defines the shutdown priority and number of retries for the current process
SetProcessDPIAware	Specifies whether the process is aware of dots per inch (DPI) settings
GetGuiResources	Returns a count of User and GDI handles

EXPERIMENT: Using the Kernel Debugger !process Command

The kernel debugger !process command displays a subset of the information in an EPROCESS block. This output is arranged in two parts for each process. First you see the information about the process, as shown here (when you don’t specify a process address or ID, !process lists information for the active process on the current CPU):

lkd> !process
PROCESS 85857160  SessionId: 1  Cid: 0bcc    Peb: 7ffd9000  ParentCid: 090c
    DirBase: b45b0820  ObjectTable: b94ffda0  HandleCount:  99.
    Image: windbg.exe
    VadRoot 85a1c8e8 Vads 97 Clone 0 Private 5919. Modified 153. Locked 1.
    DeviceMap 9d32ee50
    Token                             ebaa1938
    ElapsedTime                       00:48:44.125
    UserTime                          00:00:00.000
    KernelTime                        00:00:00.000
    QuotaPoolUsage[PagedPool]         166784
    QuotaPoolUsage[NonPagedPool]      4776
    Working Set Sizes (now,min,max)  (8938, 50, 345) (35752KB, 200KB, 1380KB)
    PeakWorkingSetSize                8938
    VirtualSize                       106 Mb
    PeakVirtualSize                   108 Mb
    PageFaultCount                    37066
    MemoryPriority                    BACKGROUND
    BasePriority                      8
    CommitCharge                      6242

After the basic process output comes a list of the threads in the process. That output is explained in the EXPERIMENT: Using the Kernel Debugger !thread Command section later in the chapter. Other commands that display process information include !handle, which dumps the process handle table (which is described in more detail in the section Object Handles and the Process Handle Table in Chapter 3). Process and thread security structures are described in Chapter 6.

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