Re: [PATCH 16/17] prmem: pratomic-long

[Kees Cook <keescook@chromium.org>]

On Wed, Oct 31, 2018 at 2:10 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Oct 30, 2018 at 04:28:16PM +0000, Will Deacon wrote:
>> On Tue, Oct 30, 2018 at 04:58:41PM +0100, Peter Zijlstra wrote:
>> > Like mentioned elsewhere; if you do write_enable() + write_disable()
>> > thingies, it all becomes:
>> >
>> >     write_enable();
>> >     atomic_foo(&bar);
>> >     write_disable();
>> >
>> > No magic gunk infested duplication at all. Of course, ideally you'd then
>> > teach objtool about this (or a GCC plugin I suppose) to ensure any
>> > enable reached a disable.
>>
>> Isn't the issue here that we don't want to change the page tables for the
>> mapping of &bar, but instead want to create a temporary writable alias
>> at a random virtual address?
>>
>> So you'd want:
>>
>>       wbar = write_enable(&bar);
>>       atomic_foo(wbar);
>>       write_disable(wbar);
>>
>> which is probably better expressed as a map/unmap API. I suspect this
>> would also be the only way to do things for cmpxchg() loops, where you
>> want to create the mapping outside of the loop to minimise your time in
>> the critical section.
>
> Ah, so I was thikning that the altnerative mm would have stuff in the
> same location, just RW instead of RO.

I was hoping for the same location too. That allows use to use a gcc
plugin to mark, say, function pointer tables, as read-only, and
annotate their rare updates with write_rare() without any
recalculation.

-Kees

-- 
Kees Cook