Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding new mmap flags

[Dan Williams]

On Tue, Aug 15, 2017 at 9:28 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Mon, Aug 14, 2017 at 11:12 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> The mmap 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 feature. Use the fact that specifying MAP_SHARED and
>> MAP_PRIVATE at the same time is invalid as a cute hack to allow a new
>> set of validated flags to be introduced.
>
> While this is cute, is it actually better than a new syscall?

After playing with MAP_DIRECT defined as (MAP_SHARED|MAP_PRIVATE|0x40)
I think a new syscall is better. It's very easy to make the mistake
that "MAP_DIRECT" defines a single flag vs representing a multi-bit
encoding.