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

Dan Williams
2017-09-11T17:01:51Z

On Mon, Sep 11, 2017 at 4:10 AM, Christoph Hellwig <hch@lst.de> wrote:
> 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.
>

I thought you agreed to MAP_SHARED_VALIDATE here: