Re: [PATCH v6 08/13] arm64: expose user PAC bit positions via ptrace

[Will Deacon]

On Mon, Dec 10, 2018 at 08:22:06AM -0600, Richard Henderson wrote:
> On 12/10/18 6:03 AM, Catalin Marinas wrote:
> >> However, it won't be too long before someone implements support for
> >> ARMv8.2-LVA, at which point, without changes to mandatory pointer tagging, we
> >> will only have 3 authentication bits: [54:52].  This seems useless and easily
> >> brute-force-able.
> > 
> > Such support is already here (about to be queued):
> > 
> > https://lore.kernel.org/linux-arm-kernel/20181206225042.11548-1-steve.capper@xxxxxxx/
> 
> Thanks for the pointer.
> 
> >> Unfortunately, there is no obvious path to making this optional that does not
> >> break compatibility with Documentation/arm64/tagged-pointers.txt.
> > 
> > There is also the ARMv8.5 MTE (memory tagging) which relies on tagged
> > pointers.
> 
> So it does.  I hadn't read through that extension completely before.
> 
> > An alternative would be to allow the opt-in to 52-bit VA, leaving it at
> > 48-bit by default. However, it has the problem of changing the PAC size
> > and not being able to return.
> 
> Perhaps the opt-in should be at exec time, with ELF flags (or equivalent) on
> the application.  Because, as you say, changing the shape of the PAC in the
> middle of execution is in general not possible.

I think we'd still have a potential performance problem with that approach,
since we'd end up having to context-switch TCR.T0SZ, which is permitted to
be cached in a TLB and would therefore force us to introduce TLB
invalidation when context-switching between tasks using 52-bit VAs and tasks
using 48-bit VAs.

There's a chance we could get the architecture tightened here, but it's
not something we've pushed for so far and it depends on what's already been
built.

Will