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

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Message-ID: <CALCETrU811Ac+DpiUP8MdayA6cD3Jk+Dd0RXAqk5YM6Lj9YsDQ@mail.gmail.com>
Date:   Tue, 15 Aug 2017 09:28:21 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jan Kara <jack@...e.cz>, Arnd Bergmann <arnd@...db.de>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding
 new mmap flags

On Mon, Aug 14, 2017 at 11:12 PM, Dan Williams <dan.j.williams@...el.com> 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?