anti-ROP mechanism in libc

Theo (deraadt@) writes in to the tech@ mailing list, with a clever idea that we would like to try.

This change randomizes the order of symbols in libc.so at boot time.

This is done by saving all the independent .so sub-files into an ararchive, and then relinking them into a new libc.so in random order,at each boot. The cost is less than a second on the systems I amusing.

For now, this is only done for libc, because it is generally the mostgadget heavy library; spilled registers are more likely to pointwithin the libc segment; and also the gadgets are close to system callstubs. As a result of the change, gadgets are no longer found atfixed offsets from spilled registers.

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