> Though, it really seems as if this can be done *without* a system
> call.  ld.aout_so has done this sort of check for a long time
> internally, as does ld.elf_so (I hope :-).

Do you mean the following line in rtld.c?
	careful = (uid != euid) || (gid != egid);

This does work for ld.so case, but doesn't work for library.

Because a caller of the library may do seteuid(getuid()) before
calling the library, and then it can do seteuid(0) to regain
it's setuid capability.

This is why issetugid(2) is needed.
--
soda