Re: [RFC PATCH v8 2/2] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

From: Christoph Hellwig
Date: Mon Sep 11 2017 - 07:10:40 EST

On Mon, Sep 11, 2017 at 11:47:14AM +0200, Jan Kara wrote:
> On Fri 08-09-17 12:35:13, Dan Williams wrote:
> > The mmap(2) syscall suffers from the ABI anti-pattern of not validating
> > unknown flags. However, proposals like MAP_SYNC and MAP_DIRECT need a
> > mechanism to define new behavior that is known to fail on older kernels
> > without the support. Define a new MAP_SHARED_VALIDATE flag pattern that
> > is guaranteed to fail on all legacy mmap implementations.
> >
> > With this in place new flags can be defined as:
> >
> > #define MAP_new (MAP_SHARED_VALIDATE | val)
> Is this changelog stale? Given MAP_SHARED_VALIDATE will be new mapping
> type, I'd expect we define new flags just as any other mapping flags...
> I see no reason why MAP_SHARED_VALIDATE should be or'ed to that.

Btw, I still think it should be a new hidden flag and not a new mapping
type. I brought this up last time, so maybe I missed the answer
to my concern.