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How do breakpoints work in debuggers?
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Vineel Kovvuri - http://vineelkovvuri.com

Contents
========

    1 - Introduction
    2 - Breakpoints Theory
    3 - Breakpoints Practice

1. Introduction
===============

    It's been a while, I have got a chance to blog about low-level stuff. In
    this article, I am going to explain how breakpoints work in debuggers. I am
    assuming the reader is already familiar with what a breakpoint is? and how
    to set it in your debugger of choice. The goal of this post is to explain
    the interplay between Debugger, Debuggee, Operating System and the CPU.

2. Breakpoints Theory
=====================

    To get there, we have to ask ourselves  What does it mean by debugging a
    program/process?. To keep it simple, It's the controlled execution of a
    program by other program. The devil is in the word controlled. Whenever you
    open a debugger and launch a program or attach to a running process, the OS
    and the CPU guarantees that any event (like dll loading or interrupts or
    exceptions etc) happening on behalf of debuggee process are notified to
    debugger process for it to take necessary action. The important thing to
    understand here is, it's the execution of debuggee which is causing the
    debugger to take actions on behalf of it, not the other way around.  So
    there should be something in the debuggee's code that should cause CPU to do
    the heavy lifting.  This is where CPU's breakpoint instruction comes in.
    Every x86/amd64 CPU provides a special instruction called breakpoint
    instruction Int 3 whose mnemonic is 0xCC. When a piece of code executes this
    instruction, the CPU triggers a breakpoint interrupt and notifies it to OS
    and asks it what needs to be done next. Now this event from OS gets
    propagated to debugger by pausing all the threads in the debuggee. Now it's
    up to debugger to handle it according to its will.

    So far so good, But one question that should bother you at this point is The
    programmer never inserted 0xCC instruction in his program when he is writing
    the code. Then who puts it in? It's the debugger that puts this 0xCC
    instruction in the debuggee processing using WriteProcessMemory Win32 Api in
    Windows. Let's see more closely how it is done. Generally, at any given
    point in time, a debuggee will be in running or debugging mode. In running
    mode, debugger has no control of the debuggee until debuggee hits the above
    said breakpoint instruction. In debug mode, it has full control of the
    debuggee execution, ie.., it can single step each line of the code, examine
    call stack/registers etc. This is exactly the mode in which you can also set
    additional breakpoints by hitting F9 or 'bp' or equivalent. So when you ask
    the debugger to put a breakpoint on the current line/instruction, the
    debugger makes  a note of the location/address of the instruction and it
    simply gives us below visual feedback but no change is made to debuggee

    
    Breakpoints in debuggers

    
    Breakpoints in debuggers

    After this, when you jump from debug mode to running mode by hitting the go
    command either via F5 in Visual Studio or 'g' command in Windbg/gdb, the
    debugger patches the first byte of all addresses at which we asked the
    debugger to set the breakpoints with 0xCC and let the debuggee run. This is
    in a way, the preparation done by debugger to make sure it gets notified
    back when CPU control reaches the user set breakpoint. Neat hack! In a
    similar way, once the breakpoint is hit, just before putting the debuggee in
    debug mode, It will update all the addresses back with their original
    content/byte. That is the reason why we don't see this 0xCC instruction when
    in debug mode, also this is exactly what we expect because we don't want our
    instructions garbled by the debugger with random 0xCC instructions when we
    want to examine. Now fasten your seat belts and let's see all of this in
    action!

3. Breakpoints Practice
=======================

    Let's consider the above program and see what happens when we set the
    breakpoint on the main and second printf lines in WinDbg. Once we set the
    breakpoints WinDbg only makes a note of these line addresses. Now when we
    hit 'g' WinDbg moves the debuggee to running mode. Let's verify this whether
    debugger has patched the printf line with 0xcc before moving to running
    mode. Unfortunately, we cannot dump the main function because the program is
    already in run mode! i.e., we cannot use commands like 'uf .'  to see
    whether the bytes are patched by Windbg. But fortunately, Process Hacker can
    help us in peering in the binary at the printf address when the program is
    running.  The below screenshot shows the contents of main function at printf
    line before hitting go command. So as expected no change at the push
    mnemonic before call instruction (just windbg kept a note of the
    breakpoints).

    #include <stdio.h>
    int main ()
    {
        // This loop takes some time so that we
        // get a chance to examine the address of
        // the breakpoint at the second printf :)
        for (int i = 1; i < 100000000; i++)
            printf("Hello World!");

        for (int i = 1; i < 10000000; i++)
            printf("Hello World!");

         return 0;
    }

    
    Breakpoints in debuggers

    
    Breakpoints in debuggers

    After the initial for loop is done executing  and just before hitting the
    second printf the CPU executes the patched 0xCC instead of original
    68600e2900 so this causes the breakpoint interrupt from CPU to OS and OS
    notifies this to debugger to put the debuggee in debug mode. Now debugger
    does the opposite of restoring the mnemonics from CC600e2900 to 68600e2900
    so that the programmer can examine the content of the function as if nothing
    has happened before! Pretty neat HACK!

    
    Breakpoints in debuggers

    I am stopping here and let this content sink in your brain. There's more to
    it, for example, when the program is running and we want to forcefully break
    in by hitting toolbar break button or ctrl+break etc then debuggers in
    Windows uses
    CreateRemoteThread|https://msdn.microsoft.com/en-us/library/windows/desktop/ms682437(v=vs.85).aspx
    Win32 API to create a new remote thread in the debuggee and calls the
    ntdll!DbgBreakPoint.