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

[Kirill A. Shutemov]

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