Re: [RFC PATCH 06/11] x86: make sure _etext includes function sections

Peter Zijlstra
2020-02-07T09:24:37Z

On Thu, Feb 06, 2020 at 12:02:36PM -0800, Andy Lutomirski wrote:
> Also, in the shiny new era of
> Intel-CPUs-can’t-handle-Jcc-spanning-a-cacheline, function alignment
> may actually matter.

*groan*, indeed. I just went and looked that up. I missed this one in
all the other fuss :/

So per:

https://www.intel.com/content/dam/support/us/en/documents/processors/mitigations-jump-conditional-code-erratum.pdf

the toolchain mitigations only work if the offset in the ifetch window
(32 bytes) is preserved. Which seems to suggest we ought to align all
functions to 32byte before randomizing it, otherwise we're almost
guaranteed to change this offset by the act of randomizing.