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[dm-devel] [PATCH v11] dm: add support to directly boot to a mapped device



Add a dm-mod.create= kernel module parameter.
It allows device-mapper targets to be configured at boot time for use early
in the boot process (as the root device or otherwise).

Signed-off-by: Will Drewry <wad chromium org>
Signed-off-by: Kees Cook <keescook chromium org>
[rework to use dm_ioctl calls]
Signed-off-by: Enric Balletbo i Serra <enric balletbo collabora com>
[refactored for upstream]
Signed-off-by: Helen Koike <helen koike collabora com>

---
Hello,

As mentioned in previous discussions, the need to create an initramfs adds
another layer of complexity when performing tests or preparing a BSP for
a board.
Taking care of initramfs is always a bit painful, it also occupies space and is
another level of complexity in the stack.
A practical example as mentioned by Kees is that Chrome OS has a limited amount
of storage available for the boot image as it is covered by the static root of
trust signature.
This feature is already used by Android and Chrome OS in devices already shipped
to the market/end-users.

So, as mentioned in the previous version, this patchset allows dm
devices to be configured in the kernel command line parameter for
use early in the boot process without an initramfs.

One of the main difference in this version is that instead of using "dm=",
I'm using a parameter in the module "dm-mod.create=" (thanks Ezequiel for
the idea).

The syntax used in the boot param is based on the concise format from the dmsetup
tool as described in its man page http://man7.org/linux/man-pages/man8/dmsetup.8.html#CONCISE_FORMAT

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" | ...

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

Please check the patch with the documentation on the format.

The idea to make it compatible with the dmsetup concise format is to make it
easier for users, allowing just copy & paste from the output of the command:

        sudo dmsetup table --concise /dev/mapper/lroot

I refactored the code for this version, instead of pretending to be
userspace and performing ioctls, the code just parse the cmd line
argument and create the device directly, which simplified and reduced a
lot the code (and it is much easier to read).

Also, not all dm targets are allowed. Only the ones that I tested were
allowed and the ones that doesn't change any block device when the dm is
create as read-only, i.e. mirror and cache are not allowed, because if
the user makes a mistake and chose the wrong device to be the mirror or
choose the wrong device to be the cache, it can corrupt data.

So the only targets allowed are:
* crypt
* delay
* linear
* snapshot-origin
* striped
* verity

I wrote a script to perform several tests using qemu that can be found at
https://gitlab.collabora.com/koike/dm-cmdline-test

And you can see the result of the tests at:
https://people.collabora.com/~koike/dm-test-module-param-logs.txt
At the end of this file:
"Total successes: 52. Total failures: 0"

And the output of qemu for each run:
https://people.collabora.com/~koike/dm-test-module-param-logs.tar.gz

I think it would nice to integrate these tests in some CI (maybe kernel
CI?)

Please let me know your comments.

Changes in v11:
 - Configure the device directly instead of performing in IOCTL (this
 removed a lot of parsing code)
 - Just enable the targets that were tested.
 - Simplify/refactor parsing, as a consequence, escaping characters is not
 allowed (but it wans't properly used in the previous version anyway)
 - don't use sscanf, the size wans't being limited and we could have a
 buffer overflow.
 - change constrained targets list
 - remove code from init/
 - use a module parameter instead of a kernel comand line parameter in
 init/
 - rename dm-boot to dm-init

Changes in v10:
 - https://lore.kernel.org/patchwork/project/lkml/list/?series=371523
 - new file: drivers/md/dm-boot.c
 - most of the parsing code was moved from init/do_mounts_dm.c to drivers/md/dm-boot.c
 - parsing code was in essence replaced by the concise parser from dmsetup
 _create_concise function:
https://sourceware.org/git/?p=lvm2.git;a=blob;f=libdm/dm-tools/dmsetup.c;h=835fdcdc75e8f0f0f7c4ed46cc9788a6616f58b8;hb=7498f8383397a93db95655ca227257836cbcac82#l1265
 the main reason is that this code is already being used/tested by dmsetup, so
 we can have some level of confidence that it works as expected. Besides this,
 it also looks more efficient.
 - Not all targets are allowed to be used by dm=, as pointed previously, there
 are some risks in creating a mapped device without some validation from
 userspace (see documentation from the patch listing which targets are allowed).
 - Instead of using a simple singly linked list (for devices and tables), use
 the struct list_head. This occupies unnecessary space in the code, but it makes
 the code cleaner and easier to read and less prone to silly errors.
 - Documentation and comments were reviewed and refactored, e.g.:
        * "is to possible" was removed
        * s/specified as a simple string/specified as a string/
 - Added docs above __align function, make it clear that the second parameter @a
 must be a power of two.
 - Clean ups: removal of unnecessary includes, macros, variables, some redundant
 checks and warnings.
 - when calling ioctls, the code was allocating and freeing the same structure
 a couple of times. So instead of executing kzalloc/kfree 3 times, execute
 kmalloc once and reuse the structure after a memset, then finally kfree it once.
 - update commit message

Changes in v9:
 - https://www.redhat.com/archives/linux-lvm/2018-September/msg00016.html
 - Add minor number to make it compatible with dmsetup concise format

Changes in v8:
 - https://www.redhat.com/archives/linux-lvm/2017-May/msg00055.html
 - Fix build error due commit
    e516db4f67 (dm ioctl: add a new DM_DEV_ARM_POLL ioctl)

Changes in v7:
 - http://lkml.iu.edu/hypermail/linux/kernel/1705.2/02657.html
 - Add a new function to issue the equivalent of a DM ioctl programatically.
 - Use the new ioctl interface to create the devices.
 - Use a comma-delimited and semi-colon delimited dmsetup-like commands.

Changes in v6:
 - https://www.redhat.com/archives/dm-devel/2017-April/msg00316.html

Changes in v5:
 - https://www.redhat.com/archives/dm-devel/2016-February/msg00112.html

 Documentation/device-mapper/dm-init.txt | 114 +++++++++
 drivers/md/Kconfig                      |  10 +
 drivers/md/Makefile                     |   4 +
 drivers/md/dm-init.c                    | 308 ++++++++++++++++++++++++
 drivers/md/dm-ioctl.c                   | 103 ++++++++
 include/linux/device-mapper.h           |   9 +
 6 files changed, 548 insertions(+)
 create mode 100644 Documentation/device-mapper/dm-init.txt
 create mode 100644 drivers/md/dm-init.c

diff --git a/Documentation/device-mapper/dm-init.txt b/Documentation/device-mapper/dm-init.txt
new file mode 100644
index 000000000000..8464ee7c01b8
--- /dev/null
+++ b/Documentation/device-mapper/dm-init.txt
@@ -0,0 +1,114 @@
+Early creation of mapped devices
+====================================
+
+It is possible to configure a device-mapper device to act as the root device for
+your system in two ways.
+
+The first is to build an initial ramdisk which boots to a minimal userspace
+which configures the device, then pivot_root(8) in to it.
+
+The second is to create one or more device-mappers using the module parameter
+"dm-mod.create=" through the kernel boot command line argument.
+
+The format is specified as a string of data separated by commas and optionally
+semi-colons, where:
+ - a comma is used to separate fields like name, uuid, flags and table
+   (specifies one device)
+ - a semi-colon is used to separate devices.
+
+So the format will look like this:
+
+ 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" | ... (see list below)
+
+The dm line should be equivalent to the one used by the dmsetup tool with the
+--concise argument.
+
+Target types
+============
+
+Not all target types are available as there are serious risks in allowing
+activation of certain DM targets without first using userspace tools to check
+the validity of associated metadata.
+
+	"cache":		constrained, userspace should verify cache device
+	"crypt":		allowed
+	"delay":		allowed
+	"era":			constrained, userspace should verify metadata device
+	"flakey":		constrained, meant for test
+	"linear":		allowed
+	"log-writes":		constrained, userspace should verify metadata device
+	"mirror":		constrained, userspace should verify main/mirror device
+	"raid":			constrained, userspace should verify metadata device
+	"snapshot":		constrained, userspace should verify src/dst device
+	"snapshot-origin":	allowed
+	"snapshot-merge":	constrained, userspace should verify src/dst device
+	"striped":		allowed
+	"switch":		constrained, userspace should verify dev path
+	"thin":			constrained, requires dm target message from userspace
+	"thin-pool":		constrained, requires dm target message from userspace
+	"verity":		allowed
+	"writecache":		constrained, userspace should verify cache device
+	"zero":			constrained, not meant for rootfs
+
+If the target is not listed above, it is constrained by default (not tested).
+
+Examples
+========
+An example of booting to a linear array made up of user-mode linux block
+devices:
+
+  dm-mod.create="lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" root=/dev/dm-0
+
+This will boot to a rw dm-linear target of 8192 sectors split across two block
+devices identified by their major:minor numbers.  After boot, udev will rename
+this target to /dev/mapper/lroot (depending on the rules). No uuid was assigned.
+
+An example of multiple device-mappers, with the dm-mod.create="..." contents is shown here
+split on multiple lines for readability:
+
+  vroot,,,ro,
+    0 1740800 verity 254:0 254:0 1740800 sha1
+      76e9be054b15884a9fa85973e9cb274c93afadb6
+      5b3549d54d6c7a3837b9b81ed72e49463a64c03680c47835bef94d768e5646fe;
+  vram,,,rw,
+    0 32768 linear 1:0 0,
+    32768 32768 linear 1:1 0
+
+Other examples (per target):
+
+"crypt":
+  dm-crypt,,8,ro,
+    0 1048576 crypt aes-xts-plain64
+    babebabebabebabebabebabebabebabebabebabebabebabebabebabebabebabe 0
+    /dev/sda 0 1 allow_discards
+
+"delay":
+  dm-delay,,4,ro,0 409600 delay /dev/sda1 0 500
+
+"linear":
+  dm-linear,,,rw,
+    0 32768 linear /dev/sda1 0,
+    32768 1024000 linear /dev/sda2 0,
+    1056768 204800 linear /dev/sda3 0,
+    1261568 512000 linear /dev/sda4 0
+
+"snapshot-origin":
+  dm-snap-orig,,4,ro,0 409600 snapshot-origin 8:2
+
+"striped":
+  dm-striped,,4,ro,0 1638400 striped 4 4096
+  /dev/sda1 0 /dev/sda2 0 /dev/sda3 0 /dev/sda4 0
+
+"verity":
+  dm-verity,,4,ro,
+    0 1638400 verity 1 8:1 8:2 4096 4096 204800 1 sha256
+    fb1a5a0f00deb908d8b53cb270858975e76cf64105d412ce764225d53b8f3cfd
+    51934789604d1b92399c52e7cb149d1b3a1b74bbbcb103b2a0aaacbed5c08584
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 3db222509e44..644e868cc2eb 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -436,6 +436,16 @@ 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
+
 config DM_UEVENT
 	bool "DM uevents"
 	depends on BLK_DEV_DM
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index 822f4e8753bc..a52b703e588e 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -69,6 +69,10 @@ obj-$(CONFIG_DM_INTEGRITY)	+= dm-integrity.o
 obj-$(CONFIG_DM_ZONED)		+= dm-zoned.o
 obj-$(CONFIG_DM_WRITECACHE)	+= dm-writecache.o
 
+ifeq ($(CONFIG_DM_INIT),y)
+dm-mod-objs			+= dm-init.o
+endif
+
 ifeq ($(CONFIG_DM_UEVENT),y)
 dm-mod-objs			+= dm-uevent.o
 endif
diff --git a/drivers/md/dm-init.c b/drivers/md/dm-init.c
new file mode 100644
index 000000000000..24ea75911c7c
--- /dev/null
+++ b/drivers/md/dm-init.c
@@ -0,0 +1,308 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * dm-init.c
+ * Copyright (C) 2017 The Chromium OS Authors <chromium-os-dev chromium org>
+ *
+ * This file is released under the GPLv2.
+ */
+
+#include <linux/ctype.h>
+#include <linux/device.h>
+#include <linux/device-mapper.h>
+#include <linux/init.h>
+#include <linux/list.h>
+#include <linux/moduleparam.h>
+
+#define DM_MSG_PREFIX "dm"
+#define DM_MAX_DEVICES 256
+#define DM_MAX_TARGETS 256
+#define DM_MAX_STR_SIZE 4096
+
+static char *create;
+
+/*
+ * Format: dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]
+ * Table format: <start_sector> <num_sectors> <target_type> <target_args>
+ *
+ * See Documentation/device-mapper/dm-init.txt for dm-mod.create="..." format
+ * details.
+ */
+
+struct dm_device {
+	struct dm_ioctl dmi;
+	struct dm_target_spec *table[DM_MAX_TARGETS];
+	char *target_args_array[DM_MAX_TARGETS];
+	struct list_head list;
+};
+
+const char *dm_allowed_targets[] __initconst = {
+	"crypt",
+	"delay",
+	"linear",
+	"snapshot-origin",
+	"striped",
+	"verity",
+};
+
+static int __init dm_verify_target_type(const char *target)
+{
+	unsigned int i;
+
+	for (i = 0; i < ARRAY_SIZE(dm_allowed_targets); i++) {
+		if (!strcmp(dm_allowed_targets[i], target))
+			return 0;
+	}
+	return -EINVAL;
+}
+
+static void __init dm_setup_cleanup(struct list_head *devices)
+{
+	struct dm_device *dev, *tmp;
+	unsigned int i;
+
+	list_for_each_entry_safe(dev, tmp, devices, list) {
+		list_del(&dev->list);
+		for (i = 0; i < dev->dmi.target_count; i++) {
+			kfree(dev->table[i]);
+			kfree(dev->target_args_array[i]);
+		}
+		kfree(dev);
+	}
+}
+
+/**
+ * str_field_delimit - delimit a string based on a separator char.
+ * @str: the pointer to the string to delimit.
+ * @separator: char that delimits the field
+ *
+ * Find a @separator and replace it by '\0'.
+ * Remove leading and trailing spaces.
+ * Return the remainder string after the @separator.
+ */
+static char __init *str_field_delimit(char **str, char separator)
+{
+	char *s;
+
+	/* TODO: add support for escaped characters */
+	*str = skip_spaces(*str);
+	s = strchr(*str, separator);
+	/* Delimit the field and remove trailing spaces */
+	if (s)
+		*s = '\0';
+	*str = strim(*str);
+	return s ? ++s : NULL;
+}
+
+/**
+ * dm_parse_table_entry - parse a table entry
+ * @dev: device to store the parsed information.
+ * @str: the pointer to a string with the format:
+ *	<start_sector> <num_sectors> <target_type> <target_args>[, ...]
+ *
+ * Return the remainder string after the table entry, i.e, after the comma which
+ * delimits the entry or NULL if reached the end of the string.
+ */
+static char __init *dm_parse_table_entry(struct dm_device *dev, char *str)
+{
+	const unsigned int n = dev->dmi.target_count - 1;
+	struct dm_target_spec *sp;
+	unsigned int i;
+	/* fields:  */
+	char *field[4];
+	char *next;
+
+	field[0] = str;
+	/* Delimit first 3 fields that are separated by space */
+	for (i = 0; i < ARRAY_SIZE(field) - 1; i++) {
+		field[i + 1] = str_field_delimit(&field[i], ' ');
+		if (!field[i + 1])
+			return ERR_PTR(-EINVAL);
+	}
+	/* Delimit last field that can be terminated by comma */
+	next = str_field_delimit(&field[i], ',');
+
+	sp = kzalloc(sizeof(*sp), GFP_KERNEL);
+	if (!sp)
+		return ERR_PTR(-ENOMEM);
+	dev->table[n] = sp;
+
+	/* start_sector */
+	if (kstrtoull(field[0], 0, &sp->sector_start))
+		return ERR_PTR(-EINVAL);
+	/* num_sector */
+	if (kstrtoull(field[1], 0, &sp->length))
+		return ERR_PTR(-EINVAL);
+	/* target_type */
+	strscpy(sp->target_type, field[2], sizeof(sp->target_type));
+	if (dm_verify_target_type(sp->target_type)) {
+		DMERR("invalid type \"%s\"", sp->target_type);
+		return ERR_PTR(-EINVAL);
+	}
+	/* target_args */
+	dev->target_args_array[n] = kstrndup(field[3], GFP_KERNEL,
+					     DM_MAX_STR_SIZE);
+	if (!dev->target_args_array[n])
+		return ERR_PTR(-ENOMEM);
+
+	return next;
+}
+
+/**
+ * dm_parse_table - parse "dm-mod.create=" table field
+ * @dev: device to store the parsed information.
+ * @str: the pointer to a string with the format:
+ *	<table>[,<table>+]
+ */
+static int __init dm_parse_table(struct dm_device *dev, char *str)
+{
+	char *table_entry = str;
+
+	while (table_entry) {
+		DMDEBUG("parsing table \"%s\"", str);
+		if (++dev->dmi.target_count >= DM_MAX_TARGETS) {
+			DMERR("too many targets %u > %d",
+			      dev->dmi.target_count, DM_MAX_TARGETS);
+			return -EINVAL;
+		}
+		table_entry = dm_parse_table_entry(dev, table_entry);
+		if (IS_ERR(table_entry)) {
+			DMERR("couldn't parse table");
+			return PTR_ERR(table_entry);
+		}
+	}
+
+	return 0;
+}
+
+/**
+ * dm_parse_device_entry - parse a device entry
+ * @dev: device to store the parsed information.
+ * @str: the pointer to a string with the format:
+ *	name,uuid,minor,flags,table[; ...]
+ *
+ * Return the remainder string after the table entry, i.e, after the semi-colon
+ * which delimits the entry or NULL if reached the end of the string.
+ */
+static char __init *dm_parse_device_entry(struct dm_device *dev, char *str)
+{
+	/* There are 5 fields: name,uuid,minor,flags,table; */
+	char *field[5];
+	unsigned int i;
+	char *next;
+
+	field[0] = str;
+	/* Delimit first 4 fields that are separated by comma */
+	for (i = 0; i < ARRAY_SIZE(field) - 1; i++) {
+		field[i+1] = str_field_delimit(&field[i], ',');
+		if (!field[i+1])
+			return ERR_PTR(-EINVAL);
+	}
+	/* Delimit last field that can be delimited by semi-colon */
+	next = str_field_delimit(&field[i], ';');
+
+	/* name */
+	strscpy(dev->dmi.name, field[0], sizeof(dev->dmi.name));
+	/* uuid */
+	strscpy(dev->dmi.uuid, field[1], sizeof(dev->dmi.uuid));
+	/* minor */
+	if (strlen(field[2])) {
+		if (kstrtoull(field[2], 0, &dev->dmi.dev))
+			return ERR_PTR(-EINVAL);
+		dev->dmi.flags |= DM_PERSISTENT_DEV_FLAG;
+	}
+	/* flags */
+	if (!strcmp(field[3], "ro"))
+		dev->dmi.flags |= DM_READONLY_FLAG;
+	else if (strcmp(field[3], "rw"))
+		return ERR_PTR(-EINVAL);
+	/* table */
+	if (dm_parse_table(dev, field[4]))
+		return ERR_PTR(-EINVAL);
+
+	return next;
+}
+
+/**
+ * dm_parse_devices - parse "dm-mod.create=" argument
+ * @devices: list of struct dm_device to store the parsed information.
+ * @str: the pointer to a string with the format:
+ *	<device>[;<device>+]
+ */
+static int __init dm_parse_devices(struct list_head *devices, char *str)
+{
+	unsigned long ndev = 0;
+	struct dm_device *dev;
+	char *device = str;
+
+	DMDEBUG("parsing \"%s\"", str);
+	while (device) {
+		dev = kzalloc(sizeof(*dev), GFP_KERNEL);
+		if (!dev)
+			return -ENOMEM;
+		list_add_tail(&dev->list, devices);
+
+		if (++ndev >= DM_MAX_DEVICES) {
+			DMERR("too many targets %u > %d",
+			      dev->dmi.target_count, DM_MAX_TARGETS);
+			return -EINVAL;
+		}
+
+		device = dm_parse_device_entry(dev, device);
+		if (IS_ERR(device)) {
+			DMERR("couldn't parse device");
+			return PTR_ERR(device);
+		}
+	}
+
+	return 0;
+}
+
+/**
+ * dm_init_init - parse "dm-mod.create=" argument and configure drivers
+ */
+static int __init dm_init_init(void)
+{
+	struct dm_device *dev;
+	LIST_HEAD(devices);
+	char *str;
+	int r;
+
+	if (!create)
+		return 0;
+
+	if (strlen(create) >= DM_MAX_STR_SIZE) {
+		DMERR("Argument is too big. Limit is %d\n", DM_MAX_STR_SIZE);
+		return -EINVAL;
+	}
+	str = kstrndup(create, GFP_KERNEL, DM_MAX_STR_SIZE);
+	if (!str)
+		return -ENOMEM;
+
+	r = dm_parse_devices(&devices, str);
+	if (r)
+		goto out;
+
+	DMINFO("waiting for all devices to be available before creating mapped devices\n");
+	wait_for_device_probe();
+
+	list_for_each_entry(dev, &devices, list) {
+		if (dm_early_create(&dev->dmi, dev->table,
+				    dev->target_args_array))
+			break;
+	}
+out:
+	kfree(str);
+	dm_setup_cleanup(&devices);
+	return r;
+}
+
+late_initcall(dm_init_init);
+
+module_param(create, charp, 0);
+MODULE_PARM_DESC(create, "Create a mapped device when the module is loaded using this argument\n"
+			 "This is mostly useful in early boot to allow mounting rootfs without requiring an initramfs\n"
+			 "Format: <name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]\n"
+			 "Table format: <start_sector> <num_sectors> <target_type> <target_args>\n"
+			 "Examples:\n"
+			 "dm-mod.create=\"lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0\"\n");
diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index f666778ad237..c740153b4e52 100644
--- a/drivers/md/dm-ioctl.c
+++ b/drivers/md/dm-ioctl.c
@@ -2018,3 +2018,106 @@ int dm_copy_name_and_uuid(struct mapped_device *md, char *name, char *uuid)
 
 	return r;
 }
+
+
+/**
+ * dm_early_create - create a mapped device in early boot.
+ *
+ * @dmi: Contains main information of the device mapping to be created.
+ * @spec_array: array of pointers to struct dm_target_spec. Describes the
+ * mapping table of the device.
+ * @target_params_array: array of strings with the parameters to a specific
+ * target.
+ *
+ * Instead of having the struct dm_target_spec and the parameters for every
+ * target embedded at the end of struct dm_ioctl (as performed in a normal
+ * ioctl), pass them as arguments, so the caller doesn't need to serialize them.
+ * The size of the spec_array and target_params_array is given by
+ * @dmi->target_count.
+ * This function is supposed to be called in early boot, so locking mechanisms
+ * to protect against concurrent loads are not required.
+ */
+int __init dm_early_create(struct dm_ioctl *dmi,
+			   struct dm_target_spec **spec_array,
+			   char **target_params_array)
+{
+	int r, m = DM_ANY_MINOR;
+	struct dm_table *t, *old_map;
+	struct mapped_device *md;
+	unsigned int i;
+
+	if (!dmi->target_count)
+		return -EINVAL;
+
+	r = check_name(dmi->name);
+	if (r)
+		return r;
+
+	if (dmi->flags & DM_PERSISTENT_DEV_FLAG)
+		m = MINOR(huge_decode_dev(dmi->dev));
+
+	/* alloc dm device */
+	r = dm_create(m, &md);
+	if (r)
+		return r;
+
+	/* hash insert */
+	r = dm_hash_insert(dmi->name, *dmi->uuid ? dmi->uuid : NULL, md);
+	if (r)
+		goto err_destroy_dm;
+
+	/* alloc table */
+	r = dm_table_create(&t, get_mode(dmi), dmi->target_count, md);
+	if (r)
+		goto err_destroy_dm;
+
+	/* add targets */
+	for (i = 0; i < dmi->target_count; i++) {
+		r = dm_table_add_target(t, spec_array[i]->target_type,
+					(sector_t) spec_array[i]->sector_start,
+					(sector_t) spec_array[i]->length,
+					target_params_array[i]);
+		if (r) {
+			DMWARN("error adding target to table");
+			goto err_destroy_table;
+		}
+	}
+
+	/* finish table */
+	r = dm_table_complete(t);
+	if (r)
+		goto err_destroy_table;
+
+	md->type = dm_table_get_type(t);
+	/* setup md->queue to reflect md's type (may block) */
+	r = dm_setup_md_queue(md, t);
+	if (r) {
+		DMWARN("unable to set up device queue for new table.");
+		goto err_destroy_table;
+	}
+
+	/* Set new map */
+	dm_suspend(md, 0);
+	old_map = dm_swap_table(md, t);
+	if (IS_ERR(old_map)) {
+		r = PTR_ERR(old_map);
+		goto err_destroy_table;
+	}
+	set_disk_ro(dm_disk(md), !!(dmi->flags & DM_READONLY_FLAG));
+
+	/* resume device */
+	r = dm_resume(md);
+	if (r)
+		goto err_destroy_table;
+
+	DMINFO("%s (%s) is ready", md->disk->disk_name, dmi->name);
+	dm_put(md);
+	return 0;
+
+err_destroy_table:
+	dm_table_destroy(t);
+err_destroy_dm:
+	dm_put(md);
+	dm_destroy(md);
+	return r;
+}
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index e528baebad69..8fa618381e78 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>
 
@@ -431,6 +432,14 @@ 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);
 
 /*
-- 
2.20.1


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