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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Message-ID: <CAPcyv4iSzq=1XQEvabSmSCQ6LPU6U4QRDTSB46=SUuZGg9RAfA@mail.gmail.com>
Date:   Mon, 11 Sep 2017 10:01:49 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jan Kara <jack@...e.cz>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
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 4:10 AM, Christoph Hellwig <hch@....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:

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