Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Dec 18, 2015 at 1:12 PM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
>
> But, if we are picking out an execute-only pkey more dynamically, we've
> got to keep the default value for the entire process somewhere.

How dynamic do we want to make this, though?

I haven't looked at the details, and perhaps more importantly, I don't
know what exactly are the requirements you've gotten from the people
who are expected to actually use this.

I think we might want to hardcode a couple of keys as "kernel
reserved". And I'd rather reserve them up-front than have some user
program be unhappy later when we want to use them.

I guess we want to leave key #0 for "normal page", so my suggesting to
use that for the execute-only was probably misguided.

But I do think we might want to have that "no read access" as a real
fixed key too, because I think the kernel itself would want to use it:

 (a) to make sure that it gets the right fault when user space passes
in a execute-only address to a system call.

 (b) for much more efficient PAGEALLOC_DEBUG for kernel mappings.

so I do think that we'd want to reserve two of the 16 keys up front.

Would it be ok for the expected users to have those keys simply be
fixed? With key 0 being used for all default pages, and key 1 being
used for all execute-only pages? And then defaulting PKRU to 4,
disallowing access to that key #1?

I could imagine that some kernel person would want to use even more
keys, but I think two fixed keys are kind of the minimal we'd want to
use.

           Linus