Can a C program modify its executable file?

Question

I had a little too much time on my hands and started wondering if I could write a self-modifying program. To that end, I wrote a "Hello World" in C, then used a hex editor to find the location of the "Hello World" string in the compiled executable. Is it possible to modify this program to open itself and overwrite the "Hello World" string?

char* str = "Hello World\n";

int main(int argc, char* argv) {

  printf(str);

  FILE * file = fopen(argv, "r+");

  fseek(file, 0x1000, SEEK_SET);
  fputs("Goodbyewrld\n", file);      
  fclose(file);    

  return 0;
}

This doesn't work, I'm assuming there's something preventing it from opening itself since I can split this into two separate programs (A "Hello World" and something to modify it) and it works fine.

EDIT: My understanding is that when the program is run, it's loaded completely into ram. So the executable on the hard drive is, for all intents and purposes a copy. Why would it be a problem for it to modify itself?

Is there a workaround?

Thanks

Answer 1

On Windows, when a program is run the entire *.exe file is mapped into memory using the memory-mapped-file functions in Windows. This means that the file isn't necessarily all loaded at once, but instead the pages of the file are loaded on-demand as they are accessed.

When the file is mapped in this way, another application (including itself) can't write to the same file to change it while it's running. (Also, on Windows the running executable can't be renamed either, but it can on Linux and other Unix systems with inode-based filesystems).

It is possible to change the bits mapped into memory, but if you do this the OS does it using "copy-on-write" semantics, which means that the underlying file isn't changed on disk, but a copy of the page(s) in memory is made with your modifications. Before being allowed to do this though, you usually have to fiddle with protection bits on the memory in question (e.g. VirtualProtect).

At one time, it used to be common for low-level assembly programs that were in very constrained memory environments to use self-modifying code. However, nobody does this anymore because we're not running in the same constrained environments, and modern processors have long pipelines that get very upset if you start changing code from underneath them.

Answer 2

If you are using Windows, you can do the following:

Step-by-Step Example:

1. Call VirtualProtect() on the code pages you want to modify, with the PAGE_WRITECOPY protection.
2. Modify the code pages.
3. Call VirtualProtect() on the modified code pages, with the PAGE_EXECUTE protection.
4. Call FlushInstructionCache().

For more information, see How to Modify Executable Code in Memory (Archived: Aug. 2010)

Answer 3

It is very operating system dependent. Some operating systems lock the file, so you could try to cheat by making a new copy of it somewhere, but the you're just running another compy of the program.

Other operating systems do security checks on the file, e.g. iPhone, so writing it will be a lot of work, plus it resides as a readonly file.

With other systems you might not even know where the file is.

Answer 4

All present answers more or less revolve around the fact that today you cannot easily do self-modifying machine code anymore. I agree that that is basically true for today's PCs.

However, if you really want to see own self-modifying code in action, you have some possibilities available:

• Try out microcontrollers, the simpler ones do not have advanced pipelining. The cheapest and quickest choice I found is an MSP430 USB-Stick

• If an emulation is ok for you, you can run an emulator for an older non-pipelined platform.

• If you wanted self-modifying code just for the fun of it, you can have even more fun with self-destroying code (more exactly enemy-destroying) at Corewars.

• If you are willing to move from C to say a Lisp dialect, code that writes code is very natural there. I would suggest Scheme which is intentionally kept small.

Answer 5

If we're talking about doing this in an x86 environment it shouldn't be impossible. It should be used with caution though because x86 instructions are variable-length. A long instruction may overwrite the following instruction(s) and a shorter one will leave residual data from the overwritten instruction which should be noped (NOP instruction).

When the x86 first became protected the intel reference manuals recommended the following method for debugging access to XO (execute only) areas:

1. create a new, empty selector ("high" part of far pointers)
2. set its attributes to that of the XO area
3. the new selector's access properties must be set RO DATA if you only want to look at what's in it
4. if you want to modify the data the access properties must be set to RW DATA

So the answer to the problem is in the last step. The RW is necessary if you want to be able to insert the breakpoint instruction which is what debuggers do. More modern processors than the 80286 have internal debug registers to enable non-intrusive monitoring functionality which could result in a breakpoint being issued.

Windows made available the building blocks for doing this starting with Win16. They are probably still in place. I think Microsoft calls this class of pointer manipulation "thunking."

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I once wrote a very fast 16-bit database engine in PL/M-86 for DOS. When Windows 3.1 arrived (running on 80386s) I ported it to the Win16 environment. I wanted to make use of the 32-bit memory available but there was no PL/M-32 available (or Win32 for that matter).

to solve the problem my program used thunking in the following way

1. defined 32-bit far pointers (sel_16:offs_32) using structures
2. allocated 32-bit data areas (<=> >64KB size) using global memory and received them in 16-bit far pointer (sel_16:offs_16) format
3. filled in the data in the structures by copying the selector, then calculating the offset using 16-bit multiplication with 32-bit results.
4. loaded the pointer/structure into es:ebx using the instruction size override prefix
5. accessed the data using a combination of the instruction size and operand size prefixes

Once the mechanism was bug free it worked without a hitch. The largest memory areas my program used were 2304*2304 double precision which comes out to around 40MB. Even today, I would call this a "large" block of memory. In 1995 it was 30% of a typical SDRAM stick (128 MB PC100).

Answer 6

There are non-portable ways to do this on many platforms. In Windows you can do this with WriteProcessMemory(), for example. However, in 2010 it's usually a very bad idea to do this. This isn't the days of DOS where you code in assembly and do this to save space. It's very hard to get right, and you're basically asking for stability and security problems. Unless you are doing something very low-level like a debugger I would say don't bother with this, the problems you will introduce are not worth whatever gain you might have.

Answer 7

Self-modifying code is used for modifications in memory, not in file (like run-time unpackers as UPX do). Also, the file representation of a program is more difficult to operate because of relative virtual addresses, possible relocations and modifications to the headers needed for most updates (eg. by changing the Hello world! to longer Hello World you'll need to extend the data segment in file).

I'll suggest that you first learn to do it in memory. For file updates the simplest and more generic approach would be running a copy of the program so that it would modify the original.

EDIT: And don't forget about the main reasons the self-modifying code is used:

1) Obfuscation, so that the code that is actually executed isn't the code you'll see with simple statical analysis of the file.

2) Performance, something like JIT.

None of them benefits from modifying the executable.

Answer 8

If you operating on Windows, I believe it locks the file to prevent it from being modified while its being run. Thats why you often needs to exit a program in order to install an update. The same is not true on a linux system.

Answer 9

On newer versions of Windows CE (atleast 5.x an newer) where apps run in user space, (compared to earlier versions where all apps ran in supervisor mode), apps cannot even read it's own executable file.