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

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Fri, Sep 22, 2017 at 9:32 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * 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.

That make sense.

>
> Ok, I suspect we can try it, but my preconditions for merging it would be:
>
>   1) Linus doesn't NAK it (obviously)

Of course.

>   2) we first implement the additional entropy bits that Linus suggested.
>
> does this work for you?

Sure, I can look at how feasible that is. If it is, can I send
everything as part of the same patch set? The additional entropy would
be enabled for all KASLR but PIE will be off-by-default of course.

>
> Thanks,
>
>         Ingo



-- 
Thomas

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