What does DebugView's "Enable Verbose Kernel Output" do? - windows

Question

I've followed Microsoft's "Write a Hello World Windows Driver (KMDF)" and wasn't sure how to see the output on WinDbg. After trying to set the Debug Print Filter registry to 0xFFFFFFFF, rebooting and other rain dance solutions, the one thing that worked was enabling DebugView's "Enable Verbose Kernel Output" option. Now, WinDbg shows debug outputs. Its too verbose but at least it's there.
So what did DebugView modify for WinDbg to show more verbose debug output?
I'm running WinDbg attached to a VM from my Windows host with a bridged connection.

Answer 1

TL;DR: it calls a driver to repeatedly call NtSetDebugFilterState on all kernel components, so that they are all able to print something on the debug output.
Program
Let start with the program itself; there's only one occurrence of the sentence "Enable Verbose Kernel Output":
mov [rsp+78h+mi.wID], 9C7Ch
lea rax, aEnableVerboseK ; "Enable &Verbose Kernel Output"
sbb ecx, ecx
mov [rsp+78h+mi.dwTypeData], rax
and ecx, 8
mov [rsp+78h+mi.fState], ecx
mov rcx, cs:hMenu ; hMenu
call cs:GetSubMenu
mov rcx, rax ; hmenu
lea r9, [rsp+78h+mi] ; lpmi
lea edx, [rdi+3] ; item
lea r8d, [rdi+1] ; fByPosition
call cs:InsertMenuItemA
The above code insert the sub-menu into the main menu. What's important here is the the menu ID, namely 0x9C7C.
This menu ID is used only once more here:
movzx edx, al ; al can either be 0 or 1
xor edi, edi
mov qword ptr [rsp+830h+iNumButtons], rdi ; lpOverlapped
lea rax, [rsp+830h+BytesReturned]
mov [rsp+830h+lpButtons], rax ; lpBytesReturned
xor edx, 1
mov dword ptr [rsp+830h+wBMID], edi ; nOutBufferSize
xor r9d, r9d ; nInBufferSize
xor r8d, r8d ; lpInBuffer
mov [rsp+830h+dwInitParam], rdi ; lpOutBuffer
lea edx, ds:0FFFFFFFF8305003Ch[rdx*4] ; dwIoControlCode
call cs:DeviceIoControl
movzx eax, cs:byte_1400935A3
mov edx, 9C7Ch ; uIDCheckItem
mov rcx, cs:hMenu ; hMenu
mov cs:byte_1400A2776, al
neg al
sbb r8d, r8d
and r8d, 8 ; uCheck
call cs:CheckMenuItem
The above code calls DeviceIoControl and then checks the menu item. The former means the program is actually talking with a device driver.
If we remove a bit of code we can see which IOCTL can be sent to the driver:
movzx edx, al ; al can either be 0 or 1
; snip
xor edx, 1 ; invert AL
; snip
lea edx, ds:0FFFFFFFF8305003Ch[rdx*4] ; dwIoControlCode
call cs:DeviceIoControl
Since RDX can be either 0 or 1 we end up with (base 10):
[rdx*4-2096824260]
Thus:
4 - 2096824260 = -2096824256
0 - 2096824260 = -2096824260
Looking at the handles opened by dbgview64.exe we can see a \Device\dbgv is currently opened.
0: kd> !devobj \Device\dbgv
Device object (ffffd58a97007630) is for:
Dbgv \Driver\DBGV DriverObject ffffd58a8688aaa0
Current Irp 00000000 RefCount 0 Type 00008305 Flags 00000048
SecurityDescriptor ffffe58fb8bdeea0 DevExt 00000000 DevObjExt ffffd58a97007780
ExtensionFlags (0x00000800) DOE_DEFAULT_SD_PRESENT
Characteristics (0000000000)
Device queue is not busy.
0: kd> dt _driver_object ffffd58a8688aaa0
nt!_DRIVER_OBJECT
+0x000 Type : 0n4
+0x002 Size : 0n336
+0x008 DeviceObject : 0xffffd58a`97007630 _DEVICE_OBJECT
+0x010 Flags : 0x12
+0x018 DriverStart : 0xfffff800`dcf90000 Void
+0x020 DriverSize : 0x9000
+0x028 DriverSection : 0xffffd58a`a3ba9be0 Void
+0x030 DriverExtension : 0xffffd58a`8688abf0 _DRIVER_EXTENSION
+0x038 DriverName : _UNICODE_STRING "\Driver\DBGV"
+0x048 HardwareDatabase : 0xfffff800`8372e990 _UNICODE_STRING "\REGISTRY\MACHINE\HARDWARE\DESCRIPTION\SYSTEM"
+0x050 FastIoDispatch : (null)
+0x058 DriverInit : 0xfffff800`dcf97058 long +0
+0x060 DriverStartIo : (null)
+0x068 DriverUnload : (null)
+0x070 MajorFunction : [28] 0xfffff800`dcf91b80 long +0
0: kd> dt nt!_LDR_DATA_TABLE_ENTRY 0xffffd58a`a3ba9be0 Full*
+0x048 FullDllName : _UNICODE_STRING "\??\C:\WINDOWS\system32\Drivers\Dbgv.sys"
So the driver is currently loaded from C:\WINDOWS\system32\Drivers\Dbgv.sys (or you can extract it from the .rsrc section...).
Driver
Looking at the driver, in the driver entry we spot the function used for IRP_MJ_DEVICE_CONTROL:
lea rax, sub_180001B80
mov [rdi+0E0h], rax ; IRP_MJ_DEVICE_CONTROL
mov [rdi+80h], rax
mov [rdi+70h], rax
Inside that function we have the usual setup before calling the right IOCTL:
movzx eax, [rcx+_IO_STACK_LOCATION.MajorFunction]
mov r9d, [rcx+_IO_STACK_LOCATION.Parameters.DeviceIoControl.OutputBufferLength]
mov r10d, [rcx+_IO_STACK_LOCATION.Parameters.DeviceIoControl.IoControlCode]
test al, al ; IRP_MJ_CREATE
jz loc_180001C6C
cmp al, 2 ; IRP_MJ_CLOSE
jz short loc_180001C0C
cmp al, 0Eh ; IRP_MJ_DEVICE_CONTROL
jnz ##CompleteRequest
mov eax, r10d
and eax, 3
cmp al, METHOD_NEITHER
jnz short loc_180001BDF
mov rdx, [rdi+_IRP.UserBuffer]
loc_180001BDF:
mov [rsp+98h+do], r11 ; _DEVICE_OBJECT*
mov [rsp+98h+IoStatus], rbx ; IoStatus
mov [rsp+98h+ioctl], r10d ; IoCtl
mov [rsp+98h+OutputBufferLength], r9d ; OuputBufferLength
mov r9d, [rcx+_IO_STACK_LOCATION.Parameters.DeviceIoControl.InputBufferLength] ; int
mov rcx, [rcx+_IO_STACK_LOCATION.FileObject]
mov qword ptr [rsp+98h+Buffer], rdx ; Buffer
mov dl, 1 ; int
call sub_1800017E0
jmp ##CompleteRequest
Inside the call (sub_1800017E0) we have a big switch for the IOCTL, here's the case -2096824260 (case -2096824256 is slightly different):
loc_1800018B9:
call sub_180002470 ; jumptable 000000018000182F case -2096824260
jmp loc_180001AEB
This function is mostly comprised of two loops:
loc_1800024A0:
xor ebx, ebx
##LoopQuerySetDebugFilter:
mov edx, ebx
mov ecx, esi
call cs:qword_180005438 ; DbgQueryDebugFilterState
mov r8b, 1 ; State
mov edx, ebx ; Level (keeps incrementing up to 0x1E)
mov ecx, esi ; ComponentId (keeps incrementing up to 0x82)
mov [rdi], al ; save current state.
call cs:qword_180005440 ; DbgSetDebugFilterState
inc ebx
inc rdi
cmp ebx, 1Eh
jb short ##LoopQuerySetDebugFilter
inc esi
cmp esi, 82h ; '‚'
jb short loc_1800024A0
Both calls are on DbgQueryDebugFilterState and DbgSetDebugFilterState (reactos source)
which is just a minimal wrapper around NtSetDebugFilterState (reactos source).
As far as we can see the debug filter state is queried, saved, and then set for all kernel components (following is the component tables from the kernel, there are a lot of them):
.rdata:00000001400073E0 KdComponentTable dq offset Kd_SYSTEM_Mask
.rdata:00000001400073E0 ; DATA XREF: NtQueryDebugFilterState+36↓o
.rdata:00000001400073E0 ; NtSetDebugFilterState+43↓o ...
.rdata:00000001400073E8 dq offset Kd_SMSS_Mask
.rdata:00000001400073F0 dq offset Kd_SETUP_Mask
.rdata:00000001400073F8 dq offset Kd_NTFS_Mask
.rdata:0000000140007400 dq offset Kd_FSTUB_Mask
.rdata:0000000140007408 dq offset Kd_CRASHDUMP_Mask
.rdata:0000000140007410 dq offset Kd_CDAUDIO_Mask
.rdata:0000000140007418 dq offset Kd_CDROM_Mask
.rdata:0000000140007420 dq offset Kd_CLASSPNP_Mask
....
Which finally means that all kernel components are able to print something to the debug output.
Note that the other IOCTL just reset the components masks to what they were before checking the menu in the main program.