linux-kernel - Re: [PATCH v4 3/3] fs, xfs: introduce MAP_DIRECT for creating block-map-sealed file ranges

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Date:   Wed, 16 Aug 2017 13:25:26 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jan Kara <jack@...e.cz>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Linux API <linux-api@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 3/3] fs, xfs: introduce MAP_DIRECT for creating
 block-map-sealed file ranges

On Tue, Aug 15, 2017 at 10:11:27AM -0700, Dan Williams wrote:
> > We had issues before with user-imposed ETXTBSY. See MAP_DENYWRITE.
> >
> > Are we sure it won't a source of denial-of-service attacks?
> 
> I believe MAP_DENYWRITE allowed any application with read access to be
> able to deny writes which is obviously problematic. MAP_DIRECT is
> different. You need write access to the file so you can already
> destroy data that another application might depend on, and this only
> blocks allocation and reflink.
> 
> However, I'm not opposed to adding more safety around this. I think we
> can address this concern with an fcntl seal as Dave suggests, but the
> seal only applies to the 'struct file' instance and only gates whether
> MAP_DIRECT is allowed on that file. The act of setting
> F_MAY_SEAL_IOMAP requires CAP_IMMUTABLE, but MAP_DIRECT does not. This
> allows the 'permission to mmap(MAP_DIRECT)' to be passed around with
> an open file descriptor.

Sounds like a good approach to me.

-- 
 Kirill A. Shutemov