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

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On 08/21/17 07:28, Peter Zijlstra wrote:
> 
> Ah, I see, this is large mode and that needs to use MOVABS to load 64bit
> immediates. Still, small RIP relative should be able to live at any
> point as long as everything lives inside the same 2G relative range, so
> would still allow the goal of increasing the KASLR range.
> 
> So I'm not seeing how we need large mode for that. That said, after
> reading up on all this, RIP relative will not be too pretty either,
> while CALL is naturally RIP relative, data still needs an explicit %rip
> offset, still loads better than the large model.
> 

The large model makes no sense whatsoever.  I think what we're actually
looking for is the small-PIC model.

Ingo asked:
> I.e. is there no GCC code generation mode where code can be placed anywhere 
> in the 
> canonical address space, yet call and jump distance is within 31 bits so that 
> the 
> generated code is fast?

That's the small-PIC model.  I think if all symbols are forced to hidden
then it won't even need a GOT/PLT.

We do need to consider how we want modules to fit into whatever model we
choose, though.  They can be adjacent, or we could go with a more
traditional dynamic link model where the modules can be separate, and
chained together with the main kernel via the GOT.

        -hpa

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