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 | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index 17ece816d490b6c40d019da131ade44c9a201dab..44425093d3b908abf80e05e1fc99a26b17e18a42 100644
--- a/drivers/md/dm-ioctl.c
+++ b/drivers/md/dm-ioctl.c
@@ -256,11 +256,13 @@ 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;
 
@@ -277,6 +279,8 @@ 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;
 	}
@@ -287,6 +291,8 @@ 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;
 		}
@@ -901,12 +907,14 @@ 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;
+	}
 
 	param->flags &= ~DM_INACTIVE_PRESENT_FLAG;
 
-	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);
@@ -2269,7 +2277,6 @@ static int ctl_ioctl(struct file *file, uint command, struct dm_ioctl __user *us
 		goto out;
 	/* This XOR keeps only the flags validate_params has changed. */
 	old_flags ^= param->flags;
-
 	param->data_size = sloppy_checks(param) ? offsetof(struct dm_ioctl, data) : sizeof(struct dm_ioctl);
 	r = fn(file, param, input_param_size);
 
@@ -2284,9 +2291,14 @@ 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))
+	if ((!r || (!sloppy_checks(param) && cmd == DM_DEV_CREATE_CMD && r == -EBUSY)) &&
+	    copy_to_user(user, param, param->data_size))
 		r = -EFAULT;
 
 out:
@@ -2465,7 +2477,7 @@ 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;
 
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab