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Toggle hardware data/read/execute breakpoints programmatically

, 23 Jul 2008 CPOL
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Simple code to introduce a hardware breakpoint mechanism.

Introduction

I decided to write this article about hardware breakpoints for the following reasons:

  • Visual C++ only supports write-only data breakpoints. You might want to trigger a break when data is read as well.
  • You might not be using Visual C++, so chances are that your debugger uses some slow software-based mechanism.
  • You might want to set/remove a breakpoint programmatically.
  • You may be interested in low level CPU stuff!

Features

  • Works for x86 and x64.
  • Supports upto 4 hardware breakpoints per thread.

Debug Registers

x86/x64 contains a set of debug registers, named DR0, DR1, DR2, DR3, DR6, and DR7. These registers are 32-bit when in 32-bit mode, and 64-bit when in long mode. DR0, DR1, DR2, and DR3 contain the linear addresses of the breakpoint, and DR7 contains the bits explained here:

Bits Meaning
0-7 Flags for each of the 4 debug registers (2 for each). The first flag is set to specify a local breakpoint (so the CPU resets the flag when switching tasks), and the second flag is set to specify a global breakpoint. In Windows, obviously, you can only use the first flag (although I haven't tried the second).
16-23

2 bits for each register, defining when the breakpoint will be triggered:

  • 00b - Triggers when code is executed
  • 01b - Triggers when data is written
  • 10b - Reserved
  • 11b - Triggers when data is read or written
24-31

2 bits for each register, defining the size of the breakpoint:

  • 00b - 1 byte
  • 01b - 2 bytes
  • 10b - 8 bytes
  • 11b - 4 bytes

We use SetThreadContext to set the necessary flags for the thread. After that, when the breakpoint is triggered, an exception of the value EXCEPTION_SINGLE_STEP is raised.

Setting the Breakpoint

HANDLE SetHardwareBreakpoint(HANDLE hThread,HWBRK_TYPE Type,HWBRK_SIZE Size,void* s);
  • hThread - Handle to the thread for which the breakpoint is to be set.
  • Type - Type of the breakpoint:
    • HWBRK_TYPE_CODE
    • HWBRK_TYPE_READWRITE
    • HWBRK_TYPE_WRITE
  • Size - Size of the breakpoint:
    • HWBRK_SIZE_1
    • HWBRK_SIZE_2
    • HWBRK_SIZE_4
    • HWBRK_SIZE_8
  • addr - The address of the breakpoint.

The function returns a handle to the breakpoint, to be used later in RemoveHardwareBreakpoint. It can return 0 if:

  • You do not have access to the thread.
  • You have set the maximum number of breakpoints for that thread (4).

Removing the Breakpoint

bool RemoveHardwareBreakpoint(HANDLE hBrk);

Removes the breakpoint, returning true on success.

Sample

int __stdcall WinMain(HINSTANCE,HINSTANCE,LPSTR,int)
{
 char c1[100] = {0};
 lstrcpyA(c1,"Hello 1");
 HANDLE hX1 = 0;

 hX1 = SetHardwareBreakpoint(GetCurrentThread(), 
       HWBRK_TYPE_READWRITE,HWBRK_SIZE_4,c1);
 __try
 {
   volatile char a1 = c1[2];
   // To ensure that it won't be optimized out.
 }
 __except(GetExceptionCode() == STATUS_SINGLE_STEP)
 {
   MessageBoxA(0,"Breakpoint hit!",0,MB_OK);
 }
 RemoveHardwareBreakpoint(hX1);
 return 0; 
}

I wait for your comments and questions!

History

  • July 24, 2008 - First post.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Michael Chourdakis
Engineer
Greece Greece
I'm working in C++, PHP , Flash and DSP Programming, currently experimenting with Windows 7 technologies and professional audio applications.
 
I 've a PhD in Digital Signal Processing.
 
My home page: http://www.michaelchourdakis.com

Comments and Discussions

 
QuestionWould it have to disrupt the thread? Pinmembersupercat926-Jul-08 17:59 
AnswerRe: Would it have to disrupt the thread? PinmemberMichael Chourdakis26-Jul-08 22:15 

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