Toggle hardware data/read/execute breakpoints programmatically

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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.