Re: [Xen-devel] x86: PIE support and option to extend KASLR randomization

[Ingo Molnar <mingo@xxxxxxxxxx>]

* Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:

> On Thu, Sep 21, 2017 at 8:59 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > ( Sorry about the delay in answering this. I could blame the delay on the 
> > merge
> >   window, but in reality I've been procrastinating this is due to the 
> > permanent,
> >   non-trivial impact PIE has on generated C code. )
> >
> > * Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> >
> >> 1) PIE sometime needs two instructions to represent a single
> >> instruction on mcmodel=kernel.
> >
> > What again is the typical frequency of this occurring in an x86-64 defconfig
> > kernel, with the very latest GCC?
> 
> I am not sure what is the best way to measure that.

If this is the dominant factor then 'sizeof vmlinux' ought to be enough:

> With ORC: PIE .text is 0.814224% than baseline

I.e. the overhead is +0.81% in both size and (roughly) in number of 
instructions 
executed.

BTW., I think things improved with ORC because with ORC we have RBP as an extra 
register and with PIE we lose RBX - so register pressure in code generation is 
lower.

Ok, I suspect we can try it, but my preconditions for merging it would be:

  1) Linus doesn't NAK it (obviously)
  2) we first implement the additional entropy bits that Linus suggested.

does this work for you?

Thanks,

        Ingo

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