On Wed, Oct 31, 2018 at 2:10 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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
