dm: add support to directly boot to a mapped device

Add a "create" module parameter, which allows device-mapper targets to
be configured at boot time. This enables early use of DM targets in the
boot process (as the root device or otherwise) without the need of an
initramfs.

The syntax used in the boot param is based on the concise format from
the dmsetup tool to follow the rule of least surprise:

	dmsetup table --concise /dev/mapper/lroot

Which is:
	dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]

Where,
	<name>		::= The device name.
	<uuid>		::= xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx | ""
	<minor>		::= The device minor number | ""
	<flags>		::= "ro" | "rw"
	<table>		::= <start_sector> <num_sectors> <target_type> <target_args>
	<target_type>	::= "verity" | "linear" | ...

For example, the following could be added in the boot parameters:
dm-mod.create="lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" root=/dev/dm-0

Only the targets that were tested are allowed and the ones that don't
change any block device when the device is create as read-only. For
example, mirror and cache targets are not allowed. The rationale behind
this is that if the user makes a mistake, choosing the wrong device to
be the mirror or the cache can corrupt data.

The only targets initially allowed are:
* crypt
* delay
* linear
* snapshot-origin
* striped
* verity

Co-developed-by: Will Drewry <wad@chromium.org>
Co-developed-by: Kees Cook <keescook@chromium.org>
Co-developed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 3db222509e44..2557f198e175 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -436,6 +436,18 @@ config DM_DELAY
 
 	If unsure, say N.
 
+config DM_INIT
+	bool "DM \"dm-mod.create=\" parameter support"
+	depends on BLK_DEV_DM=y
+	---help---
+	Enable "dm-mod.create=" parameter to create mapped devices at init time.
+	This option is useful to allow mounting rootfs without requiring an
+	initramfs.
+	See Documentation/device-mapper/dm-init.txt for dm-mod.create="..."
+	format.
+
+	If unsure, say N.
+
 config DM_UEVENT
 	bool "DM uevents"
 	depends on BLK_DEV_DM