How to detect symbol interposition?

[Florian Weimer <fweimer at redhat dot com>]

I need a way to detect symbol interposition from within libc.

It seems that this is not directly distinguishable from interposition 
based on addresses alone:

#include <stdio.h>
#include <stdlib.h>

int
main (void)
{
  printf ("main malloc: %p\n", malloc);
}

If I have a hidden alias of malloc within libc (say, __libc_malloc), so 
that I can get the original address, it will be not equal to malloc 
because malloc in the entire process will point to the PLT stub.  The 
PLT stub will call malloc, of course, but you cannot tell this from its 
address.

Any suggestions?

Thanks,
Florian