drivers/cdrom: improved ioctl for media change detection

The current implementation of the CDROM_MEDIA_CHANGED ioctl relies on
global state, meaning that only one process can detect a disc change
while the ioctl call will return 0 for other calling processes afterwards
(see bug 213267).

This introduces a new cdrom ioctl, CDROM_TIMED_MEDIA_CHANGE, that
works by maintaining a timestamp of the last detected disc change instead
of a boolean flag: Processes calling this ioctl command can provide
a timestamp of the last disc change known to them and receive
an indication whether the disc was changed since then and the updated
timestamp.

I considered fixing the buggy behavior in the original
CDROM_MEDIA_CHANGED ioctl but that would require maintaining state
for each calling process in the kernel, which seems like a worse
solution than introducing this new ioctl.

Signed-off-by: Lukas Prediger <lumip@lumip.de>
Link: https://lore.kernel.org/all/20210912191207.74449-1-lumip@lumip.de
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210913230942.1188-1-phil@philpotter.co.uk
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/cdrom/cdrom-standard.rst

@@ -907,6 +907,17 @@ commands can be identified by the underscores in their names.
 	specifies the slot for which the information is given. The special
 	value *CDSL_CURRENT* requests that information about the currently
 	selected slot be returned.
+`CDROM_TIMED_MEDIA_CHANGE`
+	Checks whether the disc has been changed since a user supplied time
+	and returns the time of the last disc change.
+
+	*arg* is a pointer to a *cdrom_timed_media_change_info* struct.
+	*arg->last_media_change* may be set by calling code to signal
+	the timestamp of the last known media change (by the caller).
+	Upon successful return, this ioctl call will set
+	*arg->last_media_change* to the latest media change timestamp (in ms)
+	known by the kernel/driver and set *arg->has_changed* to 1 if
+	that timestamp is more recent than the timestamp set by the caller.
 `CDROM_DRIVE_STATUS`
 	Returns the status of the drive by a call to
 	*drive_status()*. Return values are defined in cdrom_drive_status_.

Documentation/userspace-api/ioctl/cdrom.rst

@@ -54,6 +54,9 @@ are as follows:
 	CDROM_SELECT_SPEED	Set the CD-ROM speed
 	CDROM_SELECT_DISC	Select disc (for juke-boxes)
 	CDROM_MEDIA_CHANGED	Check is media changed
+	CDROM_TIMED_MEDIA_CHANGE	Check if media changed
+					since given time
+					(struct cdrom_timed_media_change_info)
 	CDROM_DRIVE_STATUS	Get tray position, etc.
 	CDROM_DISC_STATUS	Get disc type, etc.
 	CDROM_CHANGER_NSLOTS	Get number of slots

drivers/cdrom/cdrom.c

@@ -344,6 +344,12 @@ static void cdrom_sysctl_register(void);
 
 static LIST_HEAD(cdrom_list);
 
+static void signal_media_change(struct cdrom_device_info *cdi)
+{
+	cdi->mc_flags = 0x3; /* set media changed bits, on both queues */
+	cdi->last_media_change_ms = ktime_to_ms(ktime_get());
+}
+
 int cdrom_dummy_generic_packet(struct cdrom_device_info *cdi,
 			       struct packet_command *cgc)
 {
@@ -616,6 +622,7 @@ int register_cdrom(struct gendisk *disk, struct cdrom_device_info *cdi)
 	ENSURE(cdo, generic_packet, CDC_GENERIC_PACKET);
 	cdi->mc_flags = 0;
 	cdi->options = CDO_USE_FFLAGS;
+	cdi->last_media_change_ms = ktime_to_ms(ktime_get());
 
 	if (autoclose == 1 && CDROM_CAN(CDC_CLOSE_TRAY))
 		cdi->options |= (int) CDO_AUTO_CLOSE;
@@ -1421,8 +1428,7 @@ static int cdrom_select_disc(struct cdrom_device_info *cdi, int slot)
 		cdi->ops->check_events(cdi, 0, slot);
 
 	if (slot == CDSL_NONE) {
-		/* set media changed bits, on both queues */
-		cdi->mc_flags = 0x3;
+		signal_media_change(cdi);
 		return cdrom_load_unload(cdi, -1);
 	}
 
@@ -1455,7 +1461,7 @@ static int cdrom_select_disc(struct cdrom_device_info *cdi, int slot)
 		slot = curslot;
 
 	/* set media changed bits on both queues */
-	cdi->mc_flags = 0x3;
+	signal_media_change(cdi);
 	if ((ret = cdrom_load_unload(cdi, slot)))
 		return ret;
 
@@ -1521,7 +1527,7 @@ int media_changed(struct cdrom_device_info *cdi, int queue)
 	cdi->ioctl_events = 0;
 
 	if (changed) {
-		cdi->mc_flags = 0x3;    /* set bit on both queues */
+		signal_media_change(cdi);
 		ret |= 1;
 		cdi->media_written = 0;
 	}
@@ -2336,6 +2342,49 @@ static int cdrom_ioctl_media_changed(struct cdrom_device_info *cdi,
 	return ret;
 }
 
+/*
+ * Media change detection with timing information.
+ *
+ * arg is a pointer to a cdrom_timed_media_change_info struct.
+ * arg->last_media_change may be set by calling code to signal
+ * the timestamp (in ms) of the last known media change (by the caller).
+ * Upon successful return, ioctl call will set arg->last_media_change
+ * to the latest media change timestamp known by the kernel/driver
+ * and set arg->has_changed to 1 if that timestamp is more recent
+ * than the timestamp set by the caller.
+ */
+static int cdrom_ioctl_timed_media_change(struct cdrom_device_info *cdi,
+		unsigned long arg)
+{
+	int ret;
+	struct cdrom_timed_media_change_info __user *info;
+	struct cdrom_timed_media_change_info tmp_info;
+
+	if (!CDROM_CAN(CDC_MEDIA_CHANGED))
+		return -ENOSYS;
+
+	info = (struct cdrom_timed_media_change_info __user *)arg;
+	cd_dbg(CD_DO_IOCTL, "entering CDROM_TIMED_MEDIA_CHANGE\n");
+
+	ret = cdrom_ioctl_media_changed(cdi, CDSL_CURRENT);
+	if (ret < 0)
+		return ret;
+
+	if (copy_from_user(&tmp_info, info, sizeof(tmp_info)) != 0)
+		return -EFAULT;
+
+	tmp_info.media_flags = 0;
+	if (tmp_info.last_media_change - cdi->last_media_change_ms < 0)
+		tmp_info.media_flags |= MEDIA_CHANGED_FLAG;
+
+	tmp_info.last_media_change = cdi->last_media_change_ms;
+
+	if (copy_to_user(info, &tmp_info, sizeof(*info)) != 0)
+		return -EFAULT;
+
+	return 0;
+}
+
 static int cdrom_ioctl_set_options(struct cdrom_device_info *cdi,
 		unsigned long arg)
 {
@@ -3313,6 +3362,8 @@ int cdrom_ioctl(struct cdrom_device_info *cdi, struct block_device *bdev,
 		return cdrom_ioctl_eject_sw(cdi, arg);
 	case CDROM_MEDIA_CHANGED:
 		return cdrom_ioctl_media_changed(cdi, arg);
+	case CDROM_TIMED_MEDIA_CHANGE:
+		return cdrom_ioctl_timed_media_change(cdi, arg);
 	case CDROM_SET_OPTIONS:
 		return cdrom_ioctl_set_options(cdi, arg);
 	case CDROM_CLEAR_OPTIONS:

include/linux/cdrom.h

@@ -64,6 +64,7 @@ struct cdrom_device_info {
 	int for_data;
 	int (*exit)(struct cdrom_device_info *);
 	int mrw_mode_page;
+	__s64 last_media_change_ms;
 };
 
 struct cdrom_device_ops {

include/uapi/linux/cdrom.h

@@ -147,6 +147,8 @@
 #define CDROM_NEXT_WRITABLE	0x5394	/* get next writable block */
 #define CDROM_LAST_WRITTEN	0x5395	/* get last block written on disc */
 
+#define CDROM_TIMED_MEDIA_CHANGE   0x5396  /* get the timestamp of the last media change */
+
 /*******************************************************
  * CDROM IOCTL structures
  *******************************************************/
@@ -295,6 +297,23 @@ struct cdrom_generic_command
 	};
 };
 
+/* This struct is used by CDROM_TIMED_MEDIA_CHANGE */
+struct cdrom_timed_media_change_info {
+	__s64	last_media_change;	/* Timestamp of the last detected media
+					 * change in ms. May be set by caller,
+					 * updated upon successful return of
+					 * ioctl.
+					 */
+	__u64	media_flags;		/* Flags returned by ioctl to indicate
+					 * media status.
+					 */
+};
+#define MEDIA_CHANGED_FLAG	0x1	/* Last detected media change was more
+					 * recent than last_media_change set by
+					 * caller.
+					 */
+/* other bits of media_flags available for future use */
+
 /*
  * A CD-ROM physical sector size is 2048, 2052, 2056, 2324, 2332, 2336, 
  * 2340, or 2352 bytes long.