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[linux] Avoid starting currticks() from zero every time
iPXE uses currticks() (along with the MAC address(es) of any network devices) to seed the (non-cryptographic) random number generator. The current implementation of linux_currticks() ensures that the first call to currticks() will always return zero; this results in identical random number sequences on each run of iPXE on a given machine. This can cause odd-looking behaviour due to e.g. the reuse of local TCP port numbers. Fix by effectively rounding down the start time recorded by linux_currticks() to the nearest whole second; this makes it unlikely that consecutive runs of iPXE will use the exact same RNG sequence. (Note that none of this affects the cryptographic RNG, which uses /dev/random as a source of entropy.) Signed-off-by: Michael Brown <mcb30@ipxe.org>
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