[stackghost]

The call to zlib basically overwrites a minimal ELF into a portion of the `su` binary, which exceve's /bin/sh.

[Sophira]

To be specific, the zlib'd binary basically does this (except that it directly uses Linux syscalls to do so rather then C wrappers):

    setuid(0);
    execve("/bin/sh", NULL, NULL);
    exit(0);