c++ - How can I intercept dlsym calls using LD_PRELOAD?

Question

I want to intercept application's calls to dlsym. I have tried declaring inside the .so that I am preloading dlsym , and using dlsym itself to get it's real address, but that for quite obvious reasons didn't work.

Is there a way easier than taking process' memory maps, and using libelf to find the real location of dlsym inside loaded libdl.so?

Answer 1

I stumbled across the same problem with hdante's answer as the commenter: calling __libc_dlsym() directly crashes with a segfault. After reading some glibc sources, I came up with the following hack as a workaround:

extern void *_dl_sym(void *, const char *, void *);
extern void *dlsym(void *handle, const char *name)
{
    /* my target binary is even asking for dlsym() via dlsym()... */
    if (!strcmp(name,"dlsym")) 
        return (void*)dlsym;
    return _dl_sym(handle, name, dlsym);
}

NOTE two things with this "solution":

1. This code bypasses the locking which is done internally by (__libc_)dlsym(), so to make this threadsafe, you should add some locking.
2. The thrid argument of _dl_sym() is the address of the caller, glibc seems to reconstruct this value by stack unwinding, but I just use the address of the function itself. The caller address is used internally to find the link map the caller is in to get things like RTLD_NEXT right (and, using NULL as thrid argument will make the call fail with an error when using RTLD_NEXT). However, I have not looked at glibc's unwindind functionality, so I'm not 100% sure that the above code will do the right thing, and it may happen to work just by chance alone...

The solution presented so far has some significant drawbacks: _dl_sym() acts quite differently than the intended dlsym() in some situations. For example, trying to resolve a symbol which does not exist does exit the program instead of just returning NULL. To work around that, one can use _dl_sym() to just get the pointer to the original dlsym() and use that for everything else (like in the "standard" LD_PRELOAD hook approch without hooking dlsym at all):

extern void *_dl_sym(void *, const char *, void *);
extern void *dlsym(void *handle, const char *name)
{
    static void * (*real_dlsym)(void *, const char *)=NULL;
    if (real_dlsym == NULL)
        real_dlsym=_dl_sym(RTLD_NEXT, "dlsym", dlsym);
    /* my target binary is even asking for dlsym() via dlsym()... */
    if (!strcmp(name,"dlsym")) 
        return (void*)dlsym;
    return real_dlsym(handle,name);
}