How to benefit from heap tagging by DLL?

StackOverflow https://stackoverflow.com/questions/23264272

  •  08-07-2023
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Question

How do I use and benefit from the GFlags setting Enable heap tagging by DLL?

I know how to activate the setting for a process, but I did not find useful information in the output of !heap -t in WinDbg. I was expecting some output like this:

0:000> !heap -t
Index   Address   Allocated by 
1:      005c0000  MyDll.dll
2:      006b0000  AnotherDll.dll

so that I can identify which heap was created by which DLL and then e.g. identify the source of a memory leak.

Is this a misunderstanding of the term "heap tagging by DLL" or do I need some more commands to get to the desired result?

My research so far:

  • I googled for a tutorial on this topic, but I couldn't find a detailed description
  • I read WinDbg's .hh !heap but it's not explained there in detail as well. Tag is only used in !heap -b
Was it helpful?

Solution

again a very late answer

to benefit from HeapTagging you need to create a tag first in your code.
as far as i know (that is upto xp-sp3) there were no Documented APIS to Create a tag

(I havent mucked with heap since then so i am not aware of latest apis in os > vista Rewrites were done to heap manager so probably many of the ^^^features^^^ that i post below might have been corrected or bettered or bugs removed )

in xp-sp3 you can use undocumented RtlCreateTagHeap to create a new tag to either Process Heap or Private Heap

and after you create tha tag you need to set the global flag 8000 | 800 

htg - Enable heap tagging
htd - Enable heap tagging by DLL

and theoratically all allocs and frees must get tagged .

but practically only allocations > 512 kB gets tagged in xp-sp3 with these basic steps

it either is a bug or a feature that limits tagging to allocations and frees > 512 kB
HeapAlloc goes through ZwAllocateVirtualMemory in case of Allocations > 512 kB in 32 bit process refer HeapCreate / HeapAlloc Documentation in msdn

and as a debuging aid you can patch ntdll.dll on the fly to enable tagging for all Allocations and frees .

below is a sample code that demonstrates the tagging and how to view it all in windbg

compile using cl /Zi /analyze /W4 <src> /link /RELEASE

use windbg to execute the app and watch tagging with !heap * -t command

#include <windows.h>
#include <stdio.h>

//heaptags are kinda broken or they are intentionally 
//given only to allocations > 512 kb // allocation > 512 kb
//go through VirtualAlloc Route for Heap created with maxsize 
//set to 0 uncomment ALLOCSIZE 0xfdfd2 and recompile to watch 
// tagging increase by 100% with ALLOCSIZE  0xfdfd1 only 50 allocs 
// and frees that are > 512 kB will be tagged these magic numbers 
// are related to comment in HeapCreate Documentation that state 
// slightly less than 512 kB will be allocated for 32 bit process 
// tagging can be dramatically increased by patching ntdll when 
// stopped on system breakpoint patch 7c94b8a4 (xpsp3 ntdll.dll) 
// use the below command in windbg for finding the offset of pattern
// command must be in single line no line breaks
// .foreach /pS 4 /ps 4 ( place  { !grep -i -e call -c 
// "# call*RtlpUpdateTagEntry 7c900000 l?20000" } ) { ub place }
// the instruction we are searching to patch is 
//7c94b8a1 81e3ff0fffff    and     ebx,0FFFF0FFFh 
// patch 0f to 00 at system breakpoint with eb 7c94b8a1+3 00 

#define BUFFERSIZE 100
#define ALLOCSIZE  0xfdfd1
//#define ALLOCSIZE  0xfdfd2

typedef int ( __stdcall *g_RtlCreateTagHeap) ( 
    HANDLE hHeap ,
    void * unknown, 
    wchar_t * BaseString, 
    wchar_t * TagString 
    );

void HeapTagwithHeapAllocPrivate()
{
    PCHAR pch[BUFFERSIZE] = {};
    HANDLE hHeap    = 0;
    ULONG tag1      = 0;
    ULONG tag2      = 0;
    ULONG tag3      = 0;
    ULONG tag4      = 0;
    ULONG tag5      = 0;
    g_RtlCreateTagHeap RtlCreateTagHeap = 0;
    HMODULE hMod = LoadLibrary("ntdll.dll");
    if(hMod)
    {
        RtlCreateTagHeap = (g_RtlCreateTagHeap) 
            GetProcAddress( hMod,"RtlCreateTagHeap");
    }
    if (hHeap == 0)
    {
        hHeap = HeapCreate(0,0,0);
        if (RtlCreateTagHeap != NULL)
        {
            tag1 = RtlCreateTagHeap (hHeap,0,L"HeapTag!",L"MyTag1");
            tag2 = RtlCreateTagHeap (hHeap,0,L"HeapTag!",L"MyTag2"); 
            tag3 = RtlCreateTagHeap (hHeap,0,L"HeapTag!",L"MyTag3");
            tag4 = RtlCreateTagHeap (hHeap,0,L"HeapTag!",L"MyTag4");
        }
    }
    HANDLE DefHeap = GetProcessHeap();
    if ( (RtlCreateTagHeap != NULL)  && (DefHeap != NULL ))
    {
        tag5 = RtlCreateTagHeap (DefHeap,0,L"HeapTag!",L"MyTag5");
        for ( int i = 0; i < BUFFERSIZE ; i++ )
        {
            pch[i]= (PCHAR) HeapAlloc( DefHeap,HEAP_ZERO_MEMORY| tag5, 1 );
            HeapFree(DefHeap,NULL,pch[i]);
        }

    }
    if(hHeap)
    {
        for ( int i = 0; i < BUFFERSIZE ; i++ )
        {
            pch[i]= (PCHAR) HeapAlloc( hHeap,HEAP_ZERO_MEMORY| tag1, 1 );
            //lets leak all allocs patch ntdll to see the tagging details
            //HeapFree(hHeap,NULL,pch[i]);
        }
        for ( int i = 0; i < BUFFERSIZE ; i++ )
        {
            pch[i]= (PCHAR) HeapAlloc( hHeap,HEAP_ZERO_MEMORY| tag2, 100 );
            // lets leak 40% allocs patch ntdll to see the tagging details
            if(i >= 40)
                HeapFree(hHeap,NULL,pch[i]);
        }
        // slightly less than 512 kb no tagging
        for ( int i = 0; i < BUFFERSIZE / 2 ; i++ ) 
        {
            pch[i]= (PCHAR) HeapAlloc( 
                hHeap,HEAP_ZERO_MEMORY| tag3, ALLOCSIZE / 2 );
        }
        // > 512 kb  default tagging 
        for ( int i = BUFFERSIZE / 2; i < BUFFERSIZE ; i++ ) 
        {
            pch[i]= (PCHAR) HeapAlloc( 
                hHeap,HEAP_ZERO_MEMORY | tag4 ,ALLOCSIZE );
        }
        for (int i =0 ; i < BUFFERSIZE ; i++)
        {
            HeapFree(hHeap,NULL,pch[i]);
        }
    }
}
void _cdecl main()
{
    HeapTagwithHeapAllocPrivate();
}

the compiled exe to be run with windbg as below

DEFAULT execution and inspection
**only 50 tags will be visible all of them are > 512 kB Allocations

cdb -c "g;!heap * -t;q" newheaptag.exe | grep Tag**

heaptag:\>cdb -c "g;!heap * -t;q" newheaptag.exe | grep Tag
 Tag  Name                   Allocs    Frees   Diff  Allocated
 Tag  Name                   Allocs    Frees   Diff  Allocated
 Tag  Name                   Allocs    Frees   Diff  Allocated
0004: HeapTag!MyTag4             50       50      0        0

patching ntdll on system breakpoint should make all tags visible

eb = write byte patch and run the exe on exit inspect heaps with tags cdb -c "eb 7c94b8a1+3 00;g;!heap * -t;q" newheaptag.exe | grep Tag

heaptag:\>cdb -c "eb 7c94b8a1+3 00;g;!heap * -t;q" newheaptag.exe | grep Tag
 Tag  Name                   Allocs    Frees   Diff  Allocated
0012: HeapTag!MyTag5            100      100      0        0  <-our tag in process heap
 Tag  Name                   Allocs    Frees   Diff  Allocated
 Tag  Name                   Allocs    Frees   Diff  Allocated
0001: HeapTag!MyTag1            100        0    100     3200  <--- leak all
0002: HeapTag!MyTag2            100       60     40     5120  <--- leak 40 %
0003: HeapTag!MyTag3             50       50      0        0  <--- clean < 512 kB
0004: HeapTag!MyTag4             50       50      0        0  <----clean > 512 kB
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