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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