[PATCH v2 4/7] dm: linear: Add linear target

[Tobias Waldekranz <tobias@waldekranz.com>]

This target allows you to map in a region of another device as a
contiguous range of blocks of a dm device.

This is the basic building block used by LVM to stitch together
logical volumes from one or more ranges of blocks from one or more
physical disks.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
---

Notes:
    v1 -> v2:
    - Propagate errors from lower devices
    - Whitespace

 drivers/block/dm/Kconfig     |   6 ++
 drivers/block/dm/Makefile    |   1 +
 drivers/block/dm/dm-linear.c | 129 +++++++++++++++++++++++++++++++++++
 3 files changed, 136 insertions(+)
 create mode 100644 drivers/block/dm/dm-linear.c

diff --git a/drivers/block/dm/Kconfig b/drivers/block/dm/Kconfig
index 3316386d0c..03a0876eda 100644
--- a/drivers/block/dm/Kconfig
+++ b/drivers/block/dm/Kconfig
@@ -5,3 +5,9 @@ menuconfig DM_BLK
 	  other data sources. Modeled after, and intended to be
 	  compatible with, the Linux kernel's device mapper subsystem.
 
+config DM_BLK_LINEAR
+	bool "Linear target"
+	depends on DM_BLK
+	help
+	  Maps a contiguous region of an existing device into a dm
+	  device.
diff --git a/drivers/block/dm/Makefile b/drivers/block/dm/Makefile
index 9c045087c0..f52d19f9c4 100644
--- a/drivers/block/dm/Makefile
+++ b/drivers/block/dm/Makefile
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
 obj-$(CONFIG_DM_BLK) += dm-core.o
+obj-$(CONFIG_DM_BLK_LINEAR) += dm-linear.o
diff --git a/drivers/block/dm/dm-linear.c b/drivers/block/dm/dm-linear.c
new file mode 100644
index 0000000000..797d761f63
--- /dev/null
+++ b/drivers/block/dm/dm-linear.c
@@ -0,0 +1,129 @@
+// SPDX-License-Identifier: GPL-2.0-only
+// SPDX-FileCopyrightText: © 2025 Tobias Waldekranz <tobias@waldekranz.com>, Wires
+
+#include <block.h>
+#include <disks.h>
+#include <fcntl.h>
+#include <xfuncs.h>
+
+#include <linux/kstrtox.h>
+
+#include "dm-target.h"
+
+struct dm_linear {
+	struct cdev *cdev;
+	loff_t offset;
+};
+
+static int dm_linear_read(struct dm_target *ti, void *buf,
+			  sector_t block, blkcnt_t num_blocks)
+{
+	struct dm_linear *l = ti->private;
+	ssize_t ret;
+
+	block <<= SECTOR_SHIFT;
+	num_blocks <<= SECTOR_SHIFT;
+
+	ret = cdev_read(l->cdev, buf, num_blocks, l->offset + block, 0);
+	if (ret < num_blocks)
+		return (ret < 0) ? ret : -EIO;
+
+	return 0;
+}
+
+static int dm_linear_write(struct dm_target *ti, const void *buf,
+			   sector_t block, blkcnt_t num_blocks)
+{
+	struct dm_linear *l = ti->private;
+	ssize_t ret;
+
+	block <<= SECTOR_SHIFT;
+	num_blocks <<= SECTOR_SHIFT;
+
+	ret = cdev_write(l->cdev, buf, num_blocks, l->offset + block, 0);
+	if (ret < num_blocks)
+		return (ret < 0) ? ret : -EIO;
+
+	return 0;
+}
+
+static int dm_linear_create(struct dm_target *ti, unsigned int argc, char **argv)
+{
+	struct dm_linear *l;
+	int err;
+
+	if (argc != 2) {
+		dm_target_err(ti, "Expected 2 arguments (\"<dev> <offset>\"), got %u\n",
+			      argc);
+		err = -EINVAL;
+		goto err;
+	}
+
+	l = xzalloc(sizeof(*l));
+	ti->private = l;
+
+	if (kstrtoull(argv[1], 0, &l->offset)) {
+		dm_target_err(ti, "Invalid offset: \"%s\"\n", argv[1]);
+		err = -EINVAL;
+		goto err_free;
+	}
+	l->offset <<= SECTOR_SHIFT;
+
+	l->cdev = cdev_open_by_path_name(argv[0], O_RDWR);
+	if (!l->cdev) {
+		dm_target_err(ti, "Cannot open device %s: %m\n", argv[0]);
+		err = -ENODEV;
+		goto err_free;
+	}
+
+	l->cdev = cdev_readlink(l->cdev);
+
+	if ((ti->size << SECTOR_SHIFT) > (l->cdev->size - l->offset)) {
+		dm_target_err(ti, "%s is too small to map %llu blocks at %llu, %llu available\n",
+			      argv[0], ti->size, l->offset >> SECTOR_SHIFT,
+			      (l->cdev->size - l->offset) >> SECTOR_SHIFT);
+		err = -ENOSPC;
+		goto err_close;
+	}
+
+	return 0;
+
+err_close:
+	cdev_close(l->cdev);
+err_free:
+	free(l);
+err:
+	return err;
+}
+
+static int dm_linear_destroy(struct dm_target *ti)
+{
+	struct dm_linear *l = ti->private;
+
+	cdev_close(l->cdev);
+	free(l);
+	return 0;
+}
+
+static char *dm_linear_asprint(struct dm_target *ti)
+{
+	struct dm_linear *l = ti->private;
+
+	return xasprintf("dev:%s offset:%llu",
+			 cdev_name(l->cdev), l->offset >> SECTOR_SHIFT);
+}
+
+static struct dm_target_ops dm_linear_ops = {
+	.name = "linear",
+	.asprint = dm_linear_asprint,
+	.create = dm_linear_create,
+	.destroy = dm_linear_destroy,
+	.read = dm_linear_read,
+	.write = dm_linear_write,
+};
+
+static int __init dm_linear_init(void)
+{
+	return dm_target_register(&dm_linear_ops);
+}
+device_initcall(dm_linear_init);
-- 
2.43.0