LD_PRELOAD + RTLD_LOCAL

intermediate.c

#include <stdio.h>

void gnutls_handshake(void);

__attribute__((__constructor__))
static void ctor(void) {
	printf("%p from intermediate constructor\n", gnutls_handshake);
}

void intermediate_fn(void) {
	printf("%p from intermediate function\n", gnutls_handshake);
	gnutls_handshake();
}

main.c

#define _GNU_SOURCE

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

int main(int argc, char *argv[])
{
	void *handle;
	void (*fp)(void);
	int rtld_flags = RTLD_NOW;

	if (argc > 1 && argv[1][0] == 'g')
		rtld_flags |= RTLD_GLOBAL;
	else
		rtld_flags |= RTLD_LOCAL;

	handle = dlopen("./intermediate.so", rtld_flags);
	fp = dlsym(handle, "gnutls_handshake");
	printf("%p from main through dlsym intermediate.so\n", fp);

	fp = dlsym(handle, "intermediate_fn");
	fp();

	handle = dlopen("libgnutls.so", RTLD_LOCAL | RTLD_NOW);
	fp = dlsym(handle, "gnutls_handshake");
	printf("%p from main through dlsym libgnutls.so\n", fp);

	return 0;
}

Makefile

all: intermediate.so preload.so main
intermediate.so: intermediate.c
	cc -o $@ -fPIC -shared $< -lgnutls
preload.so: preload.c
	cc -o $@ -fPIC -shared $< -ldl
main: main.c
	cc -o $@ $< -ldl

preload.c

#define _GNU_SOURCE

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

void gnutls_handshake(void) {
	printf("%p from RTLD_NEXT inside preload\n", dlsym(RTLD_NEXT, "gnutls_handshake"));
}

__attribute__((__constructor__))
static void ctor(void) {
	printf("%p from preload constructor\n", gnutls_handshake);
}