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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
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Loading the Kernel Debugger
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Checking Creation Time with !drivers
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Using Link.exe to Check Creation Time
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Unassembling a Function
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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:\symbolskd: crash dump initialized [c:\dump\memory.dmp]could not determine the current processor, using zeroKernel Version 1057 Free loaded @ 0x80100000Bugcheck 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:\symbolskd: crash dump initialized [c:\dump\memory.dmp]Kernel Version 1057 Free loaded @ 0x80100000Bugcheck 0000001e : c0000005 80151d5b 00000000 00000001re-loading all kernel symbolsKD: unloading symbols for "ntoskrnl.exe"KD: deferring symbol load for "ntoskrnl.exe" at 80100000KD: 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 fc810000KD: deferring symbol load for "diskdump.sys" at fc800000KD: deferring symbol load for "hal.dll" at 80400000KD: deferring symbol load for "atapi.sys" at 80010000KD: deferring symbol load for "SCSIPORT.SYS" at 80013000KD: deferring symbol load for "Atdisk.sys" at 80001000KD: deferring symbol load for "Scsidisk.sys" at 8001b000KD: deferring symbol load for "Fastfat.sys" at 80372000Unable to read image header for Floppy.SYS at fc820000 - status c0000001*** Contents Deleted ***KD: deferring symbol load for "srv.sys" at fc9e0000KD: deferring symbol load for "ntdll.dll" at 77f80000finished re-loading all kernel symbolsNT!_PspUnhandledExceptionInSystemThread+0x18:80131ff8 b801000000 mov eax,0x1kd> 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> !driversBase Code Size Data Size Driver Name Creation Time80100000 Ntoskrnl.exe Fri May 26 18:18:36 199580400000 Hal.dll Thu May 11 13:54:18 199580010000 Atapi.sys Tue May 23 21:01:41 199580013000 Scsiport.sys Fri May 05 23:11:06 199580001000 Atdisk.sys Fri May 05 23:10:40 19958001b000 Scsidisk.sys Fri May 05 23:11:01 199580372000 Fastfat.sys Mon May 22 23:57:13 1995fc820000 Floppy.sys Header Paged Outfc830000 Scsicdrm.sys Wed May 10 21:57:03 1995fc840000 Fs_Rec.sys Header Paged Outfc850000 Null.sys Header Paged Outfc860000 Beep.sys Header Paged Outfc870000 I8042prt.sys Fri May 05 23:10:42 1995fc880000 Mouclass.sys Fri May 05 23:10:45 1995fc890000 Kbdclass.sys Fri May 05 23:10:44 1995fc8b0000 Videoprt.sys Fri May 05 23:10:05 1995fc8a0000 S3.sys Fri May 19 21:18:06 1995fc8c0000 Vga.sys Fri May 05 23:10:10 1995fc8d0000 Msfs.sys Fri May 05 23:11:57 1995fc8e0000 Npfs.sys Fri May 05 23:11:40 1995fc900000 Ndis.sys Mon May 22 20:23:18 1995fc8f0000 El59x.sys Fri Feb 10 16:18:09 1995fc940000 Tdi.sys Fri May 05 23:13:09 1995fc920000 Nbf.sys Mon May 08 15:00:47 1995fc950000 Netbios.sys Fri May 05 23:13:19 1995fc960000 Parport.sys Header Paged Outfc970000 Parallel.sys Header Paged Outfc980000 Serial.sys Fri May 05 23:11:20 1995fc990000 Afd.sys Header Paged Outfc9a0000 Rdr.sys Wed May 17 17:18:16 1995fc9e0000 Srv.sys Wed May 24 21:56:59 1995TOTAL: 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.
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List loaded modules x *!
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List symbols for a particular module (for example, Ntoskrnl.exe). x nt!*
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Select a function to unassemble that is not compiler optimized. u NT!_NtUnlockFile
Good Symbols
kd> u NT!_NtUnlockFileu NT!_NtUnlockFileNT!_NtUnlockFile:80156bd8 64a100000000 mov eax,fs:[00000000]80156bde 55 push ebp80156bdf 8bec mov ebp,esp80156be1 6aff push 0xff80156be3 68200d1080 push 0x80100d2080156be8 68304f1380 push 0x80134f3080156bed 50 push eax80156bee 64892500000000 mov fs:[00000000],espkd>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!_NTUnlockFileu NT!_NTUnlockFileNT!_NtUnlockFile:801574a4 7520 jnz NT!_NtUnlockFile+0x22 (801574c6)801574a6 6a00 push 0x0801574a8 8d45dc lea eax,[ebp-0x24]801574ab 50 push eax801574ac 53 push ebx801574ad ff356c361480 push dword ptr [NT!_CcNoDelay+0x4 (8014366c)]801574b3 6a01 push 0x1801574b5 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:
