On Tue, Aug 15, 2017 at 10:11:27AM -0700, Dan Williams wrote:

quoted

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

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