Re: dlopening the C runtime library

[Sam Varshavchik]

Ralf Wildenhues writes:

> * Sam Varshavchik wrote on Sun, Dec 06, 2009 at 02:52:52PM CET:
> > My use case is intercepting a main application's calls to library
> > functions. I set LD_PRELOAD to preload my own shared library which
> > provides replacement symbols for socket(), connect(), etc…
>
> What level of portability do you need?  GNU/Linux only?  ELF?

Additional portability is always a plus. ELF portability would certainly be 
nice.

> > My wrapper needs to find the real socket() and connect(). I haven't
> > checked this yet, but I suspect that since my shared library was
> > loaded ahead of libc, after lt_dlopen(NULL), using lt_dlsym() will
> > simply return a pointer to my own symbols, rather than libc's.
>
> Yes, that sounds like it, although I must admit that I haven't fully
> understood the above description.  Uses of -Bsymbolic may give you
> trouble.

You want to intercept an application's calls to a selected set of the 
standard library's function, such as open() and close(). The classical way 
is to put together a shared library, and use LD_PRELOAD to force the 
application to resolve its external references to your own replacements, 
instead of the standard library's. But then, you still need to invoke the 
default open() and close(), from the standard library.

With a platform-specific solution, you know the exact name of the standard 
library, which you can dlopen yourself and invoke the real open() and 
close(), as part of intercepting the application's function calls. I'm 
trying to figure out something that would have some portability to it.

As I said, I can have a prepared list of library names where standard 
library functions may be found, such as "libc", "c", "libnsl", and just 
dlopen each one until I find the real open() and close(). Or, I can probe 
these names in my configure script, and find the exact library name.

The problem is that on ELF, linking against "libc" grabs the SONAME from the 
ELF object, and that's what you end up loading at runtime. So, even though 
on a development server dlopen of "libc" works, it won't work on a 
runtime-only server.

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