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, 9 insertions, 0 deletions

diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 52e8709c6df0..b0672756d056 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -10,6 +10,7 @@
 
 #include <linux/bio.h>
 #include <linux/blkdev.h>
+#include <linux/dm-ioctl.h>
 #include <linux/math64.h>
 #include <linux/ratelimit.h>
 
@@ -425,6 +426,14 @@ void dm_remap_zone_report(struct dm_target *ti, sector_t start,
 			  struct blk_zone *zones, unsigned int *nr_zones);
 union map_info *dm_get_rq_mapinfo(struct request *rq);
 
+/*
+ * Device mapper functions to parse and create devices specified by the
+ * parameter "dm-mod.create="
+ */
+int __init dm_early_create(struct dm_ioctl *dmi,
+			   struct dm_target_spec **spec_array,
+			   char **target_params_array);
+
 struct queue_limits *dm_get_queue_limits(struct mapped_device *md);
 
 /*