[PATCH v5sub2 1/8] arm64: add support for module PLTs

[Catalin Marinas]

On Fri, Feb 05, 2016 at 04:53:10PM +0100, Ard Biesheuvel wrote:
> On 5 February 2016 at 16:42, Catalin Marinas <catalin.marinas at arm.com> wrote:
> > On Thu, Feb 04, 2016 at 04:31:59PM +0100, Ard Biesheuvel wrote:
> >> On 4 February 2016 at 16:13, Catalin Marinas <catalin.marinas at arm.com> wrote:
> >> > On Mon, Feb 01, 2016 at 02:09:31PM +0100, Ard Biesheuvel wrote:
> >> >> This adds support for emitting PLTs at module load time for relative
> >> >> branches that are out of range. This is a prerequisite for KASLR, which
> >> >> may place the kernel and the modules anywhere in the vmalloc area,
> >> >> making it more likely that branch target offsets exceed the maximum
> >> >> range of +/- 128 MB.
> >> >
> >> > Any downside to trying to keep the kernel+modules coupled together so
> >> > that we avoid the PLT?
> >>
> >> First of all, note that it is unlikely that the PLTs are ever required
> >> in practice, considering that either
> >> a) the kernel is loaded at the default location right at the base of
> >> the vmalloc range, and in this case, the module space is reserved for
> >> modules only, or
> >> b) the kernel is loaded at some random offset in the 240+ GB vmalloc
> >> space, and it is unlikely that all VMA space around the kernel happens
> >> to be given out to non-randomized vmalloc/ioremap allocations
> >
> > My worry is that we merge some code that's rarely tested.
> 
> I understand. But unfortunately, having corner cases that are unlikely
> but not impossible comes with the territory of randomization.
> 
> Alternatively, we could take the performance hit if KASLR is in effect
> and allocate each module completely randomly as well. This way, the
> code is always exercised (for now), and we can always backpedal later
> if the performance is measurably worse.

I'm fine with this. You can post it as a separate patch that we can
easily revert/modify later (like turning it into a config option).

-- 
Catalin