The Macgyver of Dlopening

You can dlopen self. Yes, you heard that right. This repo is showing how this works with regard to cgo and compiling binaries with symbols exported dynamically so that they can dlopen themselves.

Let's try it out

Okay so ls src/ and you can see we have 2 C files, sqrt.c and helloworld.c. These are the functions we will call the symbols for in our main.go.

Running make in the directory creates our object files for the C code that we will link into our go binary.

In main.go the most important thing to note is:

int *Run (const char *fun, const char *arg)
{
    // Passing NULL to dlopen will dlopen self.
    void *hndl = dlopen (NULL, RTLD_LAZY);
    if (!hndl)
    {
        fprintf(stderr, "dlopen failed: %s\n", dlerror());
        return (void *)EXIT_FAILURE;
    }
...
}

This is really the heart of the Macgyver technique ;)

Ok so enough about the code, it's a really small amount of code so I'm sure you can tell right away what is happening.

Let's compile it

$ make

# OR to compile in a docker container
$ make docker

Now we have a binary in our current directory named macgyver.

Check that our symbols from our src/*.c files have been exported.

We pass -D to nm for dynamic exports, and -C for name de-mangling.

$  nm macgyver -DC | grep hello
0000000000454100 T hello

$ nm macgyver -DC | grep squareroot
0000000000454125 T squareroot

This works because we added -Wl,--export-dynamic to our -extldflags for go build. The -Wl,--whole-archive flag makes sure that all our symbols get exported into the final binary, because typically gcc (or any compiler) will only add in the things we need, but since they aren't getting called directly from our code, the compiler does not know we need them.

Let's run it

$ ./macgyver
Hello world, jessie!
Square root of 16 is 4.000000

Yay it worked so in our main.go we ran:

func run(fun, arg string) {
    f := C.CString(fun)
    a := C.CString(arg)
    defer C.free(unsafe.Pointer(f))
    defer C.free(unsafe.Pointer(a))
    C.Run(f, a)
}

func main() {
    run("hello", "jessie")
    run("squareroot", "16")
}

which passed hello and jessie to our helloworld.c module and squareroot and 16 to our sqrt.cmodule. Both modules had their symbols added to our final binary.

If you are more curious checkout the dlopen man page, linux.die.net/man/3/dlopen and gcc link options gcc.gnu.org/onlinedocs/gcc/Link-Options.html.