How debugger works

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In this article, I’d like to tell you how real debugger works. What happens under the hood and why it happens. We’ll even write our own small debugger and see it in action.

I will talk about Linux, although same principles apply to other operating systems. Also, we’ll talk about x86 architecture. This is because it is the most common architecture today. On the other hand, even if you’re working with other architecture, you will find this article useful because, again, same principles work everywhere.

Kernel support

Actual debugging requires operating system kernel support and here’s why. Think about it. We’re living in a world where one process reading memory belonging to another process is a serious security vulnerability. Yet, when debugging a program, we would like to access a memory that is part of debugged process’s (debuggie) memory space, from debugger process. It is a bit of a problem, isn’t it? We could, of course, try somehow to use same memory space for both debugger and debuggie, but then what if debuggie itself creates processes. This really complicates things.

Debugger support has to be part of the operating system kernel. Kernel able to read and write memory that belongs to each and every process in the system. Furthermore, as long as process is not running, kernel can see value of its registers and debugger have to be able to know values of the debuggie registers. Otherwise it won’t be able to tell you where the debuggie has stopped (when we pressed CTRL-C in gdb for instance).

As we spoke about where debugger support starts we already mentioned several of the features that we need in order to have debugging support in operating system. We don’t want just any process to be able to debug other processes. Someone has to monitor debuggers and debuggies. Hence the debugger has to tell the kernel that it is going to debug certain process and kernel has to either permit or deny this request. Therefore, we need an ability to tell the kernel that certain process is a debugger and it is about to debug other process. Also we need an ability to query and set values from debuggie’s memory space. And we need an ability to query and set values of the debuggie’s registers, when it stops.

And operating system lets us to do all this. Each operating system does it in it’s manner of course. Linux provides single system call named ptrace() (defined in sys/ptrace.h), which allows to do all these operations and much more.


ptrace() accepts four arguments. First is one of the values from enum __ptrace_request that defined in sys/ptrace.h. This argument specifies what operation we would like to do, whether it is reading debuggie registers or altering values in its memory. Second argument specifies pid of the debuggie process. It’s not very obvious, but single process can debug several other processes. Thus we have to tell exactly what process we’re referring. Last two arguments are optional arguments for the call.

Starting to debug

One of the first things debuggers do to start debugging certain process is attaching to it or running it. There is a ptrace() operation for each one of these cases.

First called PTRACE_TRACEME, tells the kernel that calling process wants its parent to debug itself. I.e. me calling ptrace( PTRACE_TRACEME ) means I want my dad to debug me. This comes handy when you want debugger process to spawn the debuggie. In this case you do fork() creating a new process, then ptrace( PTRACE_TRACEME ) and then you call exec() or execve().

Second operation called PTRACE_ATTACH. It tells the kernel that calling process should become debugging parent of the process being called. Debugging parent means debugger and a parent process.

Debugger-debuggie synchronization

Alright. Now we told operating system that we are going to debug certain process. Operating system made it our child process. Good. This is a great time for us to have the debuggie stopped and us doing preparations before we actually start to debug. We may want to, for instance, analyze executable that we run and place a breakpoints before we actually start debugging. So, how do we stop the debuggie and let debugger do its thing?

Operating system does that for us using signals. Actually, operating system notifies us, the debugger, about all kinds of events that occur in debuggie and it does all that with signals. This includes the “debuggie is ready to shoot” signal. In particular, if we attach to existing process it receives SIGSTOP and we receive SIGCHLD once it actually stops. If we spawn a new process and it did ptrace( PTRACE_TRACEME ) it will receive SIGTRAP signal once it attempts to exec() or execve(). We will be notified with SIGCHLD about this, of course.

A new debugger was born

Now lets see code that actually demonstrates that. Complete listing can be found here.

The debuggie does the following…

if (ptrace( PTRACE_TRACEME, 0, NULL, NULL )) {
    perror( "ptrace" );

execve( “/bin/ls”, argv, envp );
Note the ptrace( PTRACE_TRACEME ) followed by execve(). This is what real debuggers do to spawn the process that going to be debugged. As you know, execve() replaces current executable image and memory of the current process with the executable and memory space belonging to program that being execve()‘d. Once kernel finishes this operation, it sends SIGTRAP to calling process and SIGCHLD to the debugger. The debugger receives appropriate notifications via signals and via wait() that returns. Here is the debugger’s code.

do {
    child = wait( &status );
    printf( "Debugger exited wait()\n" );
    if (WIFSTOPPED( status )) {
        printf( "Child has stopped due to signal %d\n",
        WSTOPSIG( status ) );
    if (WIFSIGNALED( status )) {
        printf( "Child %ld received signal %d\n", (long)child, WTERMSIG(status) );
} while (!WIFEXITED( status ));

Compiling and running listing1.c produces following output:

In debuggie process 14095
In debugger process 14094
Process 14094 received signal 17
Debugger exited wait()
Child has stopped due to signal 5

Here we can clearly see that debugger indeed receives a signal and gets notified via wait(). If we want to place a breakpoint before we start to debug the process, this is our chance. Lets talk about how we can do something like that.

The magic behind INT 3

It is time to dig a bit into subject that is not adored by most of the programmers and that is assembler language. I am afraid we don’t have much choice because breakpoints work on assembler level.

We have to understand that each our compiled program is actually a set of instructions that tells CPU what to do. Some of our C expressions translated into single instruction, while others may be translated into hundreds and even thousands of instructions. Instruction may be bigger or smaller. From 1 byte up to 15 bytes long for modern CPUs (Intel x86_64).

Debuggers mostly operate on CPU instruction level. The matter of fact that gdb understands C/C++ code and allows you to place breakpoints at certain C/C++ line is only an enhancement over gdb‘s basic ability to place breakpoints on certain instruction.

There are several ways to place breakpoints. The most widely used is the INT 3 instruction. It is a single byte operation code instruction that once reached by CPU, tells it to call special breakpoint interrupt handler, provided by operating system during its initialization. Since INT 3 instruction operation code is so small, we can safely substitute any instruction with it. Once operating system’s interrupt handler called, it figures what process reached a breakpoint and notifies it and its debugging process via signals.

Breakpoints hands on

Lets return to our debuggie/debugger friends. As we mentioned debugger does have a chance to place a breakpoint before letting the debuggie process to run. Lets see how this can be done.

Breakpoints placed with INT 3 instruction. Before writing the actual 0xcc (INT 3 operation code), we should figure where to place the instruction. For purpose of this article we will do it manually. On the contrary, real debuggers include complex logic that calculates where and when to place the breakpoints. gdb places several breakpoints by itself, without you even knowing about it. And obviously it has functionality that places breakpoints once you ask it to do so.

In our previous example we had our debuggie process executing ls. It is not suitable for our next demonstration. We will need a sample program that would let us easily demonstrate breakpoints in action. Here it is.


int main()
    printf( "~~~~~~~~~~~~> Before breakpoint\n" );
    // The breakpoint
    printf( "~~~~~~~~~~~~> After breakpoint\n" );
    return 0;

And here is the disassembler output of the main() routine.

0000000000400508 :
400508: 55 push %rbp
400509: 48 89 e5 mov %rsp,%rbp
40050c: bf 18 06 40 00 mov $0x400618,%edi
400511: e8 12 ff ff ff callq 400428
400516: bf 2a 06 40 00 mov $0x40062a,%edi
40051b: e8 08 ff ff ff callq 400428
400520: b8 00 00 00 00 mov $0x0,%eax
400525: c9 leaveq
400526: c3 retq

We can see that if we will place a breakpoint at address 0×400516, we will see a printout before reaching the breakpoint and right after reaching it. For the sake of our demonstration, we will place a breakpoint at this address. Once we will reach the breakpoint, we will sleep and then let the debuggie running. We should see debuggie producing first printout, then sleeping for a few seconds and then producing second printout.

We’ll achieve our goal in several steps.

First of all, we should fork() off the debuggie. We already did something similar. Next step is to intercept the execve() call in debuggie. Been there, done that. Here’s something new. We should modify a byte at address 0×400516 from 0xbf to 0xcc, saving original value (0xbf). This is how we place the breakpoint. Next, we’re going to wait() for the process. Once it will reach the breakpoint, we’ll be notified. Once the debuggie reaches the breakpoint we want to restore the code we broke with our 0xcc to its original state.

In addition, we want to fix value of RIP register. This register tells CPU what is the location in memory of next meaningful instruction for it to execute. It’s value will be 0×400517, one byte after 0xcc that we placed. We want to set the RIP register to 0×400516 value because we don’t want the CPU to skip over that MOV instruction that we broke with our 0xcc.

Finally, we want to wait five seconds for the sake of demonstration and let the debuggie continue running.
First things first. Lets see how we do step 3.

addr = 0x400516;

data = ptrace( PTRACE_PEEKTEXT, child, (void *)addr, NULL );
orig_data = data;
data = (data & ~0xff) | 0xcc;
ptrace( PTRACE_POKETEXT, child, (void *)addr, data );

Again, we can see how ptrace() does the job for us. First we peek 8 (sizeof( long )) bytes from address 0×400516. On some architectures this could cause lots of headache because of unaligned memory access. Luckily, we’re on x86_64 and unaligned memory accesses are permitted. Next we set the lowest byte to be 0xcc – INT 3 instruction. Finally, we place 8 bytes back to their place.

We’ve seen how we can wait for certain event in debuggie. Also, we now know how to restore the original value at address 0×400516. So we can skip over steps 4-5 and jump right into step 6. This is something that we haven’t done so far.

What we have to do is to read debuggie registers, change them and write them back. Again ptrace() does all the job for us.

struct user_regs_struct regs;
ptrace( PTRACE_GETREGS, child, NULL, &regs ); = addr;
ptrace( PTRACE_SETREGS, child, NULL, &regs );

Things are not too well documented here. For instance ptrace() documentation never mentions struct user_regs_struct, however this is what ptrace() system call expects to receive in kernel. Once we know what we should use as ptrace() arguments, it is easy. We use PTRACE_GETREGS operation to obtain values of debuggie’s registers, we modify the RIP register and write them back with PTRACE_SETREGS operation. Clear and simple.

Lets see how things actually work. You can find complete listing of debugger process here. Compiling and running listing2.c, produces following output.

In debuggie process 29843
In debugger process 29842
Process 29842 received signal 17
~~~~~~~~~~~~> Before breakpoint
Process 29842 received signal 17
RIP before resuming child is 400517
Time before debugger falling asleep: 1206346035
Time after debugger falling asleep: 1206346040. Resuming debuggie...
~~~~~~~~~~~~> After breakpoint
Process 29842 received signal 17
Debuggie exited...
Debugger exiting...

You can see that “Before breakpoint” printout appears 5 seconds before “After breakpoint” printout. The “RIP before resuming child is 400517″ clearly indicates that the debuggie has stopped on address 0×400517, as we expected.

Single steps

After seeing how easy to place a breakpoint, you can guess that stepping over one line of C/C++ code is simply a matter of placing a breakpoint on the next line of code. This is exactly what gdb does when you want it to single step over some expression.


Debuggers and how they work often associated with some kind of magic.

Debuggers, and gdb as an example, are exceptionally complicated piece of software. Placing breakpoints and single stepping is only a small fraction of what it is able to do. gdb in particular works on dozens of hardware architectures. It supports remote debugging. It is perhaps the most advanced and complicated executable analyzer. It knows when a program loads dynamic library and analyzes the code of that library automatically. It supports bunch of programming languages – from C/C++ to ADA. And these are just few out of its features.

On the contrary, we’ve seen how easy to start debugging certain process, place a breakpoint, etc. The basic functionality that allows debugging is in the operating system and in the CPU, waiting for us to use it.


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