From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gabriel de Perthuis <g2p.code@gmail.com>
Subject: linear writes on read-only zero devices,
	any alternatives?
Date: Sun, 3 Mar 2013 11:36:23 +0000 (UTC)
Message-ID: <kgvcjn$l3e$3@ger.gmane.org>
Reply-To: device-mapper development <dm-devel@redhat.com>
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Hello,

I'm trying to build a block device with a small writable area (a loopback
device) and a large read-only area full of zeroes.

Stacking a read-only zero device doesn't work, because linear writes to it
anyway (tested in 3.8.1, steps below).

It seems to be a bug, but in case it isn't or can't be fixed, could one 
of the dm targets (linear, error, or zero) get a read-only flag that 
ensures that writes do fail?

Test case:
sudo dmsetup create --readonly --table '0 1024 zero' rozero
sudo dmsetup create --table '0 1024 linear /dev/mapper/rozero 0' synthetic
sudo dd if=/dev/urandom of=/dev/mapper/synthetic bs=512 count=1024