Extracting structure information from pdbs of unloaded modules

Question

I'm trying to write a WinDbg debugger extension that works on both live remote targets and crash dumps. This extension analyzes an opaque block of memory by walking down it via structure offsets and casting different regions of it to known objects.

The structures change fields/field ordering between versions, so I can't hard code it (or include the headers) in my debugger extension itself. Instead I'd like to extract the structure information from the pdbs for which I have private symbols.

When using this on a live target where the pdb/image is in the loaded module list, this works great, and I can use functions like GetFieldOffset to get a field in a structure in a class.

GetFieldOffset("MyClass!MyNestedClass", "m_Struct", &offsetInClass);

GetFieldData(offsetInClass + classAddr, "MyClass!_MY_STRUCT", "FieldInStruct",
             sizeof(ULONG), &myFieldValue);

My problem: When I don't have the module in the loaded module list (either in the wrong context, or analyzing a crash dump), I can't use the above functions.

At the beginning of the memory region I'm analyzing, I've stored the pdb GUID and age. Using that I can find the path to my pdb in my symbol path/symbol cache using SymFindFileInPath.

char symbolPath[MAX_SYMBOL_PATH] = "";
PSTR pdbPath = NULL;

hr = ExtSymbols->lpVtbl->GetSymbolPath(ExtSymbols,
                                       symbolPath,
                                       sizeof(symbolPath),
                                       NULL);

SymSetOptions(SYMOPT_IGNORE_CVREC | SYMOPT_FAIL_CRITICAL_ERRORS | 
              SYMOPT_CASE_INSENSITIVE);

result = SymFindFileInPath(hSymbols,
                           symbolPath,
                           Name,
                           &GUID,
                           Age,
                           0,
                           SSRVOPT_GUIDPTR,
                           pdbPath,
                           NULL,
                           NULL);

So I have the path to my specific pdb instance, but I'm not sure where to go from here. Looking through the Sym* functions exposed by DbgHelp.dll, I don't see any obvious way to use this pdb file to get type information. Functions such as SymGetTypeInfo require a module base, and my module is not and cannot be loaded. All I need is the byte offset of fields within a structure. Any ideas?

Thanks!

Answer 1

The POC CODE below show how to extract TypeInfo from a pdb using dia sdk

//fragile code handle with care

#include "typefrompdb.h"
int main(int argc, char* argv[]) {
    USAGE;
    swprintf(pdb, MAX_PATH,L"%S",argv[1]);
    swprintf(type, MAX_PATH,L"%S",argv[2]);
    result = CoInitialize(NULL);
    result = CoCreateInstance( CLSID_DiaSource,NULL, 
        CLSCTX_INPROC_SERVER,__uuidof( IDiaDataSource ),(void **) &pSource);
    result = pSource->loadDataFromPdb(pdb);
    SHOUT("%s 2find %S %d\n",(result==S_OK)?"succeded":"failed",pdb,__LINE__);
    result = pSource->openSession(&pSession);
    result = pSession->get_globalScope(&pSymbol);
    result = pSymbol->findChildren(SymTagUDT,type,nsNone,&pEnumsymbols);
    result = pEnumsymbols->get_Count(&count);   
    result = pEnumsymbols->Next(1,&pSymudt,&noofsymret);
    SHOUT("%s 2find %S %d\n",(result==S_OK)?"succeded":"failed",type,__LINE__);
    result = pSymudt->get_name(udtname);
    result = pSymudt->findChildren(SymTagNull,NULL,nsNone,&pEnumsymbols);
    result = pEnumsymbols->get_Count(&count);
    SHOUT("no of members in struct %S is  0X%X %d\n",type,count,__LINE__);
    wprintf(L"\nstruct %s {\nType Leng Tags Name  \n",*udtname);
    for (LONG i =0 ; i< count; i++)     {
        result = pEnumsymbols->Next(1,&pSymchild,&noofsymret);            
        result = pSymchild->get_name(childname);
        result = pSymchild->get_type(&pSymtags);
        result = pSymtags->get_symTag(&dwtag);
        result = pSymtags->get_length(&len);
        result = pSymtags->get_baseType(&basetype);
        wprintf(L"0x%.2X 0x%.2I64X 0x%.2X %s\n",basetype,len,dwtag,*childname);
    }    return 0; }

contents of header file typefrompdb.h

/* handling errors/releasing memory BSTRS pointers  closing handles using 
sensible coding standards using dynamic allocations replacing ansi with unicode
etc etc etc should be implemented POC CODE not meant for blind copy pasting 
typical test case is typefrompdb.exe ntdll.pdb _DRIVER_OBJECT */
#include <stdio.h>
#include <Windows.h>
#include <Dia2.h>    // set INCLUDE=diasdkdir\inc
#include <atlbase.h> // vs 2010 express edition used with wdk 7600 
#include <atlcom.h>  // set INCLUDE=C:\WinDDK\7600.16385.1\inc\atl71
#include <dbghelp.h> // set INCLUDE=windbg\sdk\inc
#define SHOUT(...) if(result!=S_OK){printf(__VA_ARGS__);exit(0);\
        }else{printf(__VA_ARGS__);}
#define USAGE if (argc != 3) { printf( \
    "usage %s %s %s\n",argv[0],"file.pdb","typename"); return 0;}
HRESULT                 result              = E_FAIL;
IDiaDataSource          *pSource            = NULL;
IDiaSession             *pSession           = NULL;
IDiaSymbol              *pSymbol,*pSymchild = NULL;
IDiaEnumSymbols         *pEnumsymbols       = NULL;
wchar_t                 pdb[500], type[500] = {0};
BSTR        childname[0x100],udtname[0x100] = {0};
LONG                    count               = 0;
ULONG                   noofsymret          = 0;
DWORD               dwtag,basetype          = 0;
ULONGLONG               len                 = 0;
CComPtr< IDiaSymbol >   pSymudt;
CComPtr< IDiaSymbol >   pSymtags;

compiled and linked with

@call "C:\Program Files\Microsoft Visual Studio 10.0\VC\vcvarsall.bat" x86
set INCLUDE=XXXX;XXXX;XXXX;%INCLUDE%
set LIB=YYYY;YYYY;YYYY;%LIB%
cl /c /Zi /nologo /W4 /wd6387 /analyze %1% 
link /DEBUG /nologo /RELEASE /IGNORE:4254 diaguids.lib  *.obj
pause

results for test case

typefrompdb.exe
usage typefrompdb.exe file.pdb typename  
typefrompdb.exe ntd dr
failed 2find ntd 11

typefrompdb.exe ntdll.pdb _driver_object
succeded 2find ntdll.pdb 11
failed 2find _driver_object 17

typefrompdb.exe ntdll.pdb _DRIVER_OBJECT
succeded 2find ntdll.pdb 11
succeded 2find _DRIVER_OBJECT 17
no of members in struct _DRIVER_OBJECT is  0XF 21    
struct _DRIVER_OBJECT {
Type Leng Tags Name
0x06 0x02 0x10 Type
0x06 0x02 0x10 Size
0x00 0x04 0x0E DeviceObject
0x0E 0x04 0x10 Flags
0x00 0x04 0x0E DriverStart
0x0E 0x04 0x10 DriverSize
0x00 0x04 0x0E DriverSection
0x00 0x04 0x0E DriverExtension
0x00 0x08 0x0B DriverName
0x00 0x04 0x0E HardwareDatabase
0x00 0x04 0x0E FastIoDispatch
0x00 0x04 0x0E DriverInit
0x00 0x04 0x0E DriverStartIo
0x00 0x04 0x0E DriverUnload
0x00 0x70 0x0F MajorFunction

interpretation of results for the typical test case

grep -iE "btint|btulong|btnotype" cvconst.h
    btNoType = 0,
    btInt = 6,
    btULong = 14,

grep -i -A 35 "enum symtagenum" cvconst.h 
| awk "{ if ( NR==0x0B || NR==0x0E || NR==0x0F || NR==0x10 ) print $0 }"
    SymTagAnnotation,
    SymTagUDT,
    SymTagEnum,
    SymTagFunctionType,

edit
I was never sure this was an answer this was more of a test or experiment in using dia sdk SymLoadModule uses an arbitrary address hardcoded as 0x1000000 you can see that in DBH src in platform sdk samples the compiled binary of the same src is available in windbg installation

C:\Program Files\Microsoft SDKs\Windows\v7.1\Samples\winbase\debug\dbh


//global
DWORD64 gDefaultBaseForVirtualMods;

BOOL init()
{
    int i;

    *gModName = 0;
    gBase = 0;;
    gDefaultBaseForVirtualMods = 0x1000000;

else if (!_tcsicmp(ext, _T(".pdb"))) 
{
    addr = gDefaultBaseForVirtualMods;
    dontopen = true;
} else {
    addr = gDefaultBase;
}


C:\>dbh XXXXXXX\ntdll.pdb t _DRIVER_OBJECT

   name : _DRIVER_OBJECT
   addr :        0
   size : a8
  flags : 0
   type : 1
modbase :  1000000
  value :        0
    reg : 0
  scope : SymTagNull (0)
    tag : SymTagUDT (b)
  index : 1

C:\>

anyway i would reiterate that you are not supposed to use dbghelp functions in windbg extensions read the notes on following links (writing wdbg extensions/ dbgeng extensions / engextcpp function which states that it is not supported to use dbghelp functions in windbg extension

Note   You must not attempt to call any DbgHelp or ImageHlp routines from any debugger extension. Calling these routines is not supported and may cause a variety of problems. 

writing wdbgexts windbg extensions

writing dbgeng windbg extensions

writing engextcpp windbg extensions

Answer 2

The Debug Interface Access SDK provides APIs for working with the PDB directly:

http://msdn.microsoft.com/en-us/library/x93ctkx8.aspx

The DIA2Dump sample is full featured and demonstrates how to extract type information.