dvanhorn · January 26, 2021 02:23
diff --git a/call_f.s b/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
diff --git a/dynamic.c b/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;
 }
diff --git a/f.c b/f.c
 int f() {
  return 42;
 }

 int (*ptr_to_f)() = f;
diff --git a/static.c b/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;
 }
diff --git a/transcript.md b/transcript.md
	#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;
	}