[PATCH v2 4/4] dm ioctl: inform caller about already-existing device

[Demi Marie Obenour <demi@invisiblethingslab.com>]

Not only is this helpful for debugging, it also saves the caller an
ioctl in the case where a device should be used if it exists or created
otherwise.  To ensure existing userspace is not broken, this feature is
only enabled in strict mode.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
 drivers/md/dm-ioctl.c         | 31 +++++++++++++++++++++++++------
 include/uapi/linux/dm-ioctl.h |  4 ++--
 2 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index 7eab9a8c77ff3286346a1c44581d3b6370a731eb..4eb3eda2fe169f64259458dd678833f444d76ce0 100644
--- a/drivers/md/dm-ioctl.c
+++ b/drivers/md/dm-ioctl.c
@@ -259,11 +259,14 @@ static void free_cell(struct hash_cell *hc)
 	}
 }
 
+static void __dev_status(struct mapped_device *md, struct dm_ioctl *param);
+
 /*
  * The kdev_t and uuid of a device can never change once it is
  * initially inserted.
  */
-static int dm_hash_insert(const char *name, const char *uuid, struct mapped_device *md)
+static int dm_hash_insert(const char *name, const char *uuid, struct mapped_device *md,
+			  struct dm_ioctl *param)
 {
 	struct hash_cell *cell, *hc;
 
@@ -280,6 +283,8 @@ static int dm_hash_insert(const char *name, const char *uuid, struct mapped_devi
 	down_write(&_hash_lock);
 	hc = __get_name_cell(name);
 	if (hc) {
+		if (param)
+			__dev_status(hc->md, param);
 		dm_put(hc->md);
 		goto bad;
 	}
@@ -290,6 +295,8 @@ static int dm_hash_insert(const char *name, const char *uuid, struct mapped_devi
 		hc = __get_uuid_cell(uuid);
 		if (hc) {
 			__unlink_name(cell);
+			if (param)
+				__dev_status(hc->md, param);
 			dm_put(hc->md);
 			goto bad;
 		}
@@ -901,10 +908,12 @@ static int dev_create(struct file *filp, struct dm_ioctl *param, size_t param_si
 		m = MINOR(huge_decode_dev(param->dev));
 
 	r = dm_create(m, &md, param->flags & DM_DISABLE_UEVENTS_FLAG);
-	if (r)
+	if (r) {
+		DMERR("Could not create device-mapper device");
 		return r;
+	}
 
-	r = dm_hash_insert(param->name, *param->uuid ? param->uuid : NULL, md);
+	r = dm_hash_insert(param->name, *param->uuid ? param->uuid : NULL, md, param);
 	if (r) {
 		dm_put(md);
 		dm_destroy(md);
@@ -2234,6 +2243,7 @@ static int ctl_ioctl(struct file *file, uint command, struct dm_ioctl __user *us
 	int ioctl_flags;
 	int param_flags;
 	unsigned int cmd;
+	bool do_copy;
 	struct dm_ioctl *param;
 	ioctl_fn fn = NULL;
 	size_t input_param_size;
@@ -2307,9 +2317,18 @@ static int ctl_ioctl(struct file *file, uint command, struct dm_ioctl __user *us
 	param->flags |= old_flags;
 
 	/*
-	 * Copy the results back to userland.
+	 * Copy the results back to userland if either:
+	 *
+	 * - The ioctl succeeded.
+	 * - The ioctl is DM_DEV_CREATE, the return value is -EBUSY,
+	 *   and strict parameter checking is enabled.
 	 */
-	if (!r && copy_to_user(user, param, param->data_size))
+	do_copy = r == 0;
+	if (r == -EBUSY && !sloppy_checks(param) && cmd == DM_DEV_CREATE_CMD) {
+		param->flags |= DM_EXISTS_FLAG;
+		do_copy = true;
+	}
+	if (do_copy && copy_to_user(user, param, param->data_size))
 		r = -EFAULT;
 
 out:
@@ -2488,7 +2507,7 @@ int __init dm_early_create(struct dm_ioctl *dmi,
 		return r;
 
 	/* hash insert */
-	r = dm_hash_insert(dmi->name, *dmi->uuid ? dmi->uuid : NULL, md);
+	r = dm_hash_insert(dmi->name, *dmi->uuid ? dmi->uuid : NULL, md, NULL);
 	if (r)
 		goto err_destroy_dm;
 
diff --git a/include/uapi/linux/dm-ioctl.h b/include/uapi/linux/dm-ioctl.h
index b338a4f9a299acda9430995d63fbb490e70c8cd8..195d60be4aba4f20b7220b900c87d6d3a86014a2 100644
--- a/include/uapi/linux/dm-ioctl.h
+++ b/include/uapi/linux/dm-ioctl.h
@@ -344,8 +344,8 @@ enum {
 /* Status bits */
 #define DM_READONLY_FLAG	(1 << 0) /* In/Out */
 #define DM_SUSPEND_FLAG		(1 << 1) /* In/Out */
-#define DM_EXISTS_FLAG		(1 << 2) /* Not used by kernel, reserved for
-					  * libdevmapper in userland
+#define DM_EXISTS_FLAG		(1 << 2) /* Set when trying to create a
+					  * device that already exists
 					  */
 #define DM_PERSISTENT_DEV_FLAG	(1 << 3) /* In */
 
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab