Debug Visual C++ memory allocation problems

Question

I'm debugging a software which crashes eventually with one of the following messages:

1. DAMAGE: after normal block (#24729280) at 0x00D710E0
2. Debug Assertion Failed
   Program: D:\Soft\Test.exe
   File: dbgheap.c
   Line: 1017

   Expression: _BLOCK_TYPE_IS_VALID(phead->nBlockUse)

This software is really old but changing it now is not an option. It's written on Visual C++ 6.0. We are guessing it's some kind of buffer overflow, so we are trying to find ways to detect where it is happening.

I have found information about PageHeap (which seems to be able to tell me what I want) and GFlags, but it seems I can't make it work.

I created a test program:

char* test;
test = new char[5];
test[5] = 'a';
delete[] test;

which raises an error:

DAMAGE: after normal block (#55) at 0x1671920

Then, I tried attaching PageHeap to it by running:

gflags.exe /p /enable MemoryTest.exe /full

and then rerunning it (both through Visual C++ 6.0 interface and through the windows explorer), which resulted on the same error.

Then I tried to compile the release version, and ran it through the Visual C++ 6.0 interface to get the error:

User breakpoint called from code at 0x7c90120e

And from the windows explorer, I just got the windows dialog asking me to send an error report.

What am I missing?

Answer 1

Before using gFlags/PageHeap I suggest you to check for Access Violation exception. First attach the process by using Build->Start Debug->Attach to process option. Once it is attached enable the access violation exception by going to Debug->Exceptions select Access Violation and select the check box Stop Always. Then check whether your debugger catches any access violation exceptions.

Answer 2

You can run your application in release mode by attaching to Windbg.

• Enable the gflags ( As you mentioned)
• Start the application in release mode.
• Attach it to Windbg using Attach to process option in Windbg.
• Configure the correct path for release PDBs.
• Reload the PDB manually using .reload /f in case of automatic loading fails.
• Perform the use case.

WinDbg would stop the execution whenever an exception occurs. For every first chance exception, analyze the reasons. It could be one of the error for crash.