How to Verify Windows Debug Symbols

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Summary

The Windows debug symbols must be verified after starting the kernel debugger (I386kd.exe | Windbg.exe). The debugger may load and present a prompt, but if the symbols are incorrect, future debugging commands do not reference proper functions and variables, which leads to sporadic results. Listed below are a few red flags and a couple of methods to spot check validity of the symbols.

More Information

This article assumes that the kernel debugger is running and has loaded a Memory.dmp file or is connected to a remote machine. Commands presented are not fully documented and only excerpts from the output are shown.

Article Contents

  • Loading the Kernel Debugger
  • Checking Creation Time with !drivers
  • Using Link.exe to Check Creation Time
  • Unassembling a Function
  • Service Pack Symbols

Loading the Kernel Debugger

A normal load of the kernel debugger displays one of the following prompts:
kd> Uniprocessor System

0: kd> Multiprocessor System
The prompt shown below indicates that the symbol file for the Ntoskrnl.exe file is incorrect. Check to make sure that the appropriate multiprocessor or uniprocessor kernel and HAL are copied into the symbol tree.


Example:

Symbol search path is: C:\symbols
kd: crash dump initialized [c:\dump\memory.dmp]
could not determine the current processor, using zero
Kernel Version 1057 Free loaded @ 0x80100000
Bugcheck 0000001e : c0000005 80151d5b 00000000 00000001
*** Contents Deleted ***
16kd>
When the kernel debugger loads the Memory.dmp file, the Kernel Version as well as the Processor Count is displayed in the first few lines. Confirm that the processor count corresponds with the appropriate kernel and HAL. Driver symbol information is displayed as either the symbols are loaded or deferred.

Sample Load:

Symbol search path is: C:\symbols
kd: crash dump initialized [c:\dump\memory.dmp]
Kernel Version 1057 Free loaded @ 0x80100000
Bugcheck 0000001e : c0000005 80151d5b 00000000 00000001
re-loading all kernel symbols
KD: unloading symbols for "ntoskrnl.exe"
KD: deferring symbol load for "ntoskrnl.exe" at 80100000
KD: Loaded (2248) fpo entries for image (ntoskrnl.exe)
KD: "ntoskrnl.exe" loaded 4074 symbols (80100000-801bbb80)
KD: loaded symbols for "ntoskrnl.exe"
KD: deferring symbol load for "atapi.sys" at fc810000
KD: deferring symbol load for "diskdump.sys" at fc800000
KD: deferring symbol load for "hal.dll" at 80400000
KD: deferring symbol load for "atapi.sys" at 80010000
KD: deferring symbol load for "SCSIPORT.SYS" at 80013000
KD: deferring symbol load for "Atdisk.sys" at 80001000
KD: deferring symbol load for "Scsidisk.sys" at 8001b000
KD: deferring symbol load for "Fastfat.sys" at 80372000
Unable to read image header for Floppy.SYS at fc820000 - status c0000001
*** Contents Deleted ***
KD: deferring symbol load for "srv.sys" at fc9e0000
KD: deferring symbol load for "ntdll.dll" at 77f80000
finished re-loading all kernel symbols
NT!_PspUnhandledExceptionInSystemThread+0x18:
80131ff8 b801000000 mov eax,0x1
kd>
Note that the kernel debugger was "Unable to read image header for Floppy.sys at fc820000 - status c0000001." This message is normal because the particular driver header is not currently in memory.

Checking Creation Time with !drivers

Use the !drivers command to list the drivers currently loaded in memory. Make note of the dates and times of the drivers and whether or not they could be loaded. (The Code Size and Data Size columns in the following example are removed to fit the width of the article.)

kd> !drivers
Base Code Size Data Size Driver Name Creation Time
80100000 Ntoskrnl.exe Fri May 26 18:18:36 1995
80400000 Hal.dll Thu May 11 13:54:18 1995
80010000 Atapi.sys Tue May 23 21:01:41 1995
80013000 Scsiport.sys Fri May 05 23:11:06 1995
80001000 Atdisk.sys Fri May 05 23:10:40 1995
8001b000 Scsidisk.sys Fri May 05 23:11:01 1995
80372000 Fastfat.sys Mon May 22 23:57:13 1995
fc820000 Floppy.sys Header Paged Out
fc830000 Scsicdrm.sys Wed May 10 21:57:03 1995
fc840000 Fs_Rec.sys Header Paged Out
fc850000 Null.sys Header Paged Out
fc860000 Beep.sys Header Paged Out
fc870000 I8042prt.sys Fri May 05 23:10:42 1995
fc880000 Mouclass.sys Fri May 05 23:10:45 1995
fc890000 Kbdclass.sys Fri May 05 23:10:44 1995
fc8b0000 Videoprt.sys Fri May 05 23:10:05 1995
fc8a0000 S3.sys Fri May 19 21:18:06 1995
fc8c0000 Vga.sys Fri May 05 23:10:10 1995
fc8d0000 Msfs.sys Fri May 05 23:11:57 1995
fc8e0000 Npfs.sys Fri May 05 23:11:40 1995
fc900000 Ndis.sys Mon May 22 20:23:18 1995
fc8f0000 El59x.sys Fri Feb 10 16:18:09 1995
fc940000 Tdi.sys Fri May 05 23:13:09 1995
fc920000 Nbf.sys Mon May 08 15:00:47 1995
fc950000 Netbios.sys Fri May 05 23:13:19 1995
fc960000 Parport.sys Header Paged Out
fc970000 Parallel.sys Header Paged Out
fc980000 Serial.sys Fri May 05 23:11:20 1995
fc990000 Afd.sys Header Paged Out
fc9a0000 Rdr.sys Wed May 17 17:18:16 1995
fc9e0000 Srv.sys Wed May 24 21:56:59 1995
TOTAL: 1ab460 (1709 kb) 3b7c0 ( 237 kb) ( 0 kb 0 kb)
kd>
Note the Creation Time of the file, which is the date the file is compiled. This date is close to the Date\Time Stamp on the actual file when viewing the Winnt\System32\Drivers subfolder using File Manager.

If you subscribe to the Microsoft Developers Network (MSDN) or have the Windows Device Driver Kit (DDK), the Link.exe tool can be used to confirm that the Dbg file is the same file that was created when the driver was compiled. The "time date stamp" in the example below matches the "Create Time" of Netbios.sys in the output above.

Use Link.exe to check the "time date stamp" of the Dbg file:
LINK -dump -headers \symbols\sys\netbios.dbg
Microsoft (R) COFF Binary File Dumper Version 3.00.5270
Copyright (C) Microsoft Corp 1992-1995. All rights reserved.

Dump of file Netbios.dbg
4944 signature
0 flags
14C machine (i386)
306 characteristics
2FAAE94F time date stamp Fri May 05 23:13:19 1995
8D0B checksum of image

10000 base of image

70A0 size of image
** Contents Deleted **

If the !drivers command produces any lines that look like the following, an important table in the dump is corrupt. Further analysis of the dump may not be possible.
Unable to read DosHeader at 0583002a - status 00000000
Unable to read DosHeader at 0587002a - status 00000000
Unable to read DosHeader at 0588002a - status 00000000
Unable to read DosHeader at 058a002a - status 00000000
Unable to read DosHeader at 058c002a - status 00000000

Unassembling a Function

The symbol files enable the kernel debugger to reference functions and global variables by name. Spot check a couple of unassembled functions to make sure they look proper. Many functions are compiler optimized and do not necessarily conform to the examples below. Compiler optimized functions are identified by the FPO in parenthesis in the function name.


Functions normally begin with either "push ebp" or "mov eax, fs[000000000]." Basic knowledge of machine language and experience will enable one to recognize these functions.
  • List loaded modules
    x *!
  • List symbols for a particular module (for example, Ntoskrnl.exe).
    x nt!*
  • Select a function to unassemble that is not compiler optimized.
    u NT!_NtUnlockFile

Good Symbols


kd> u NT!_NtUnlockFile
u NT!_NtUnlockFile
NT!_NtUnlockFile:
80156bd8 64a100000000 mov eax,fs:[00000000]
80156bde 55 push ebp
80156bdf 8bec mov ebp,esp
80156be1 6aff push 0xff
80156be3 68200d1080 push 0x80100d20
80156be8 68304f1380 push 0x80134f30
80156bed 50 push eax
80156bee 64892500000000 mov fs:[00000000],esp
kd>
The unassembly of the function should look similar to the text above; however, the unassembly is subject to change. If the unassembly looks proper, you can assume that the symbols have loaded properly for this module.


The following unassembly begins with "jnz NT!_NTUnlockFile+0x22." Obviously, the beginning of a function would not start with a jump instruction code. Check the symbol tree once again.

Bad Symbols


kd> u NT!_NTUnlockFile
u NT!_NTUnlockFile
NT!_NtUnlockFile:
801574a4 7520 jnz NT!_NtUnlockFile+0x22 (801574c6)
801574a6 6a00 push 0x0
801574a8 8d45dc lea eax,[ebp-0x24]
801574ab 50 push eax
801574ac 53 push ebx
801574ad ff356c361480 push dword ptr [NT!_CcNoDelay+0x4 (8014366c)]
801574b3 6a01 push 0x1
801574b5 ff7508 push dword ptr [ebp+0x8]

Service Pack Symbols

There are times when it is difficult to match the symbols with a particular installation. At some point in time, a service could have been added that required copying files from the Windows Installation CD-ROM, which copied over files that were updated with a service pack. Be sure to reapply the service pack and any third-party support disks after changing system setup, which copies files from the Windows Installation CD-ROM. The symbols should be applied in the same order as the installation.

ADDITIONAL INFORMATION

To find the Kernel Debug How To series of the articles, search on the keyword: debugref.

Windbg 2.0.x Symbol Techniques

In debugger version 2.0.x and greater of the Microsoft Debuggers there are additional symbol verification steps that can be used. For more information, refer to the following Microsoft Web site:
Properties

Article ID: 148660 - Last Review: Jun 8, 2011 - Revision: 1

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