Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls

[Ingo Molnar <mingo@kernel.org>]

* Dave Hansen <dave@sr71.net> wrote:

> On 07/11/2016 12:35 AM, Ingo Molnar wrote:
> > * Andy Lutomirski <luto@amacapital.net> wrote:
> > mprotect_pkey()'s effects are per MM, but the system calls related to managing the 
> > keys (alloc/free/get/set) are fundamentally per CPU.
> > 
> > Here's an example of how this could matter to applications:
> > 
> >  - 'writer thread' gets a RW- key into index 1 to a specific data area
> >  - a pool of 'reader threads' may get the same pkey index 1 R-- to read the data 
> >    area.
> > 
> > Same page tables, same index, two protections and two purposes.
> > 
> > With a global, per MM allocation of keys we'd have to use two indices: index 1 and 2.
> 
> I'm not sure how this would work.  A piece of data mapped at only one virtual 
> address can have only one key associated with it.

Yeah, indeed, got myself confused there - but the actual protection bits are per 
CPU (per task).

> Remember, PKRU is just a *bitmap*.  The only place keys are stored is in the 
> page tables.

A pkey is an index *and* a protection mask. So by representing it as a bitmask we 
lose per thread information. This is what I meant by 'incomplete shadowing' - for 
example the debug code couldn't work: if we cleared a pkey in a task we wouldn't 
know what to restore it to with the current data structures, right?

Thanks,

	Ingo

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