c++ ~ shared object -> get host application offsets - linux

Question

Im writing a shared library for a FreeBSD application.
This library gets loaded by LD_PRELOAD.
This application has multiple compile-versions, so some function offsets might change and my library wont work there.
Now i want to read the offsets at loading the library.
The offsets are changing, so i think my only way is to read the offsets of specific function names.
The offsets are simply the offsets of functions or labels.
Now the problem - how to do it?
Example
In the first version, i call the main version like that:
int(*main)(int argc, char *argv[])=(int(*)(int,char*[]))0x081F3XXX;
but in the second, the offset has changed:
int(*main)(int argc, char *argv[])=(int(*)(int,char*[]))0x08233XXX;
Programmers (me) are lazy and don't want to compile their libs for every version.. I want to create a lib, that is for every version!
I simply need the offsets of the functions via function name, the rest is no problem..
Thats how i call the library:
LD_PRELOAD="/path/to/library.so" ./executable
or
env LD_PRELOAD="/path/to/library.so" ./executable
Edit with test code
Here my testcode regarding to the comments:
Main.cpp:
#include <stdio.h>
void test() {
printf("Test done.\n");
}
int main(int argc, char * argv[]) {
printf("Program started\n");
test();
}
lib.cpp
#include <stdio.h>
#include <dlfcn.h>
void __attribute__ ((constructor)) my_load(void);
void my_load(void) {
printf("Library loaded\n");
printf("test - offset: 0x%x\n",dlsym(NULL,"test"));
}
test.sh
g++ main.cpp -o program
g++ -shared lib.cpp -o lib.so
env LD_PRELOAD="lib.so" ./program
-> Result:
Library loaded
test - offset: 0x0
Program started
Test done.
Does not seem as would it work :s
Edit 15:45
printf("test - offset: 0x%x\n",dlsym(dlopen("/home/test/test_proc/program",RTLD_GLOBAL),"test"));
This also does not work.. Maybe dlsym is the wrong way?

Answer 1

I reproduced your program on Mac OS X using Clang, and found a solution. First, the boring parts:
To make it compile cleanly I had to change your %x format specifier to %p for the pointer.
Then, on Mac OS X I had to pass RTLD_MAIN_ONLY as the first argument to dlsym(). I guess this is platform-dependent; on Linux it does seem to be NULL as you have.
Now, the meat of the fix!
You're searching with dlsym() for a symbol called test. But there is no such symbol in your application. Why? Because you're using C++, and C++ does "name mangling." You could use any number of tools to figure out the mangled name and try to load that with dlsym(), but it could change with different compilers. So instead, just inhibit name mangling by enclosing your test() function in extern "C":
extern "C" {
void test() {
printf("Test done.\n");
}
}
This fixed it for me:
$ DYLD_INSERT_LIBRARIES=lib.so ./program
Library loaded
test - offset: 0x1027d1eb0
Program started
Test done.