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

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Date:   Mon, 11 Sep 2017 13:10:30 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jan Kara <jack@...e.cz>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        torvalds@...ux-foundation.org, Arnd Bergmann <arnd@...db.de>,
        linux-nvdimm@...ts.01.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>, hch@....de
Subject: Re: [RFC PATCH v8 2/2] mm: introduce MAP_SHARED_VALIDATE, a
        mechanism to safely define new mmap flags

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.