mfftw

call_f.s

	global frog
	global duck
	default rel
	section .text
	extern ptr_to_f
	extern f
frog:
	sub rsp, 8
	lea rax, [rel f wrt ..plt]
	call rax
	add rsp, 8
	ret
duck:
	sub rsp, 8
	lea rax, [rel ptr_to_f wrt ..got]
	mov rax, [rax]
	call [rax]
	add rsp, 8
	ret

dynamic.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>

int main(int argc, char** argv) {
  void *handle;
  int (*func)();

  handle = dlopen("./f.so", RTLD_LAZY);
  if (!handle) {
    fprintf(stderr, "Fail to open");
    return EXIT_FAILURE;
  }

  func = dlsym(handle, argv[1]);
  if (!func) {
    fprintf(stderr, "Error: %s\n", dlerror());
    dlclose(handle);
    return EXIT_FAILURE;
  }
  printf("%d\n", func());

  return EXIT_SUCCESS;
}

f.c

int f() {
  return 42;
}

int (*ptr_to_f)() = f;

static.c

#include <stdlib.h>
#include <stdio.h>

extern int frog();
extern int duck();

int main(int argc, char** argv) {
  printf("%d\n", strcmp(argv[1], "duck") ?
	 duck() : frog());
  return EXIT_SUCCESS;
}

transcript.md

the solution is to be relative to got and add yet another level of indirection. Thanks to Matthew Flatt.

dvanhorn@starburst:small $ nasm -f elf64 call_f.s -o call_f.o
dvanhorn@starburst:small $ gcc static.c f.c call_f.o -o static
dvanhorn@starburst:small $ gcc -ldl dynamic.c -o dynamic
dvanhorn@starburst:small $ gcc -shared f.c call_f.o -o f.so
dvanhorn@starburst:small $ ./static frog
42
dvanhorn@starburst:small $ ./static duck
42
dvanhorn@starburst:small $ ./dynamic frog
42
dvanhorn@starburst:small $ ./dynamic duck
42