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 4:10 AM, Christoph Hellwig [off-list ref] wrote:

On Mon, Sep 11, 2017 at 11:47:14AM +0200, Jan Kara wrote:

quoted

On Fri 08-09-17 12:35:13, Dan Williams wrote:

quoted

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.

I thought you agreed to MAP_SHARED_VALIDATE here:

    https://marc.info/?l=linux-mm&m=150425124907931&w=2

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