[PATCHv3] arm64: initialize per-cpu offsets earlier

[Mark Rutland]

On Thu, Mar 04, 2021 at 12:45:11PM +0000, Will Deacon wrote:
> On Tue, Mar 02, 2021 at 11:53:35AM +0000, Mark Rutland wrote:
> > The current initialization of the per-cpu offset register is difficult
> > to follow and this initialization is not always early enough for
> > upcoming instrumentation with KCSAN, where the instrumentation callbacks
> > use the per-cpu offset.
> > 
> > To make it possible to support KCSAN, and to simplify reasoning about
> > early bringup code, let's initialize the per-cpu offset earlier, before
> > we run any C code that may consume it. To do so, this patch adds a new
> > init_this_cpu_offset() helper that's called before the usual
> > primary/secondary start functions. For consistency, this is also used to
> > re-initialize the per-cpu offset after the runtime per-cpu areas have
> > been allocated (which can change CPU0's offset).
> 
> Is this still early enough now that we have the idreg overrides on the
> command-line, which are parsed from C code?

Hmm... no, it's not, given the override code can be instrumented and
calls potentially instrumented library code too, so we can't just
prevent kcsan instrumentation of the file.

I'll go give this a more thorough audit, since (while I had previously
convinced myself otherwise), the early KASLR bits look potentially
problematic too.

Thanks,
Mark.