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

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Thu, Aug 17, 2017 at 1:09 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>
> > > > -model=small/medium assume you are on the low 32-bit. It generates
> > > > instructions where the virtual addresses have the high 32-bit to be 
> > > > zero.
> > >
> > > How are these assumptions hardcoded by GCC? Most of the instructions 
> > > should be
> > > relocatable straight away, as most call/jump/branch instructions are
> > > RIP-relative.
> >
> > I think PIE is capable to use relative instructions well. mcmodel=large 
> > assumes
> > symbols can be anywhere.
>
> So if the numbers in your changelog and Kconfig text cannot be trusted, 
> there's
> this description of the size impact which I suspect is less susceptible to
> measurement error:
>
> +         The kernel and modules will generate slightly more assembly (1 to 2%
> +         increase on the .text sections). The vmlinux binary will be
> +         significantly smaller due to less relocations.
>
> ... but describing a 1-2% kernel text size increase as "slightly more 
> assembly"
> shows a gratituous disregard to kernel code generation quality! In reality 
> that's
> a huge size increase that in most cases will almost directly transfer to a 
> 1-2%
> slowdown for kernel intense workloads.
>
>
> Where does that size increase come from, if PIE is capable of using relative
> instructins well? Does it come from the loss of a generic register and the
> resulting increase in register pressure, stack spills, etc.?

I will try to gather more information on the size increase. The size
increase might be smaller with gcc 4.9 given performance was much
better.

>
> So I'm still unhappy about this all, and about the attitude surrounding it.
>
> Thanks,
>
>         Ingo




-- 
Thomas

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