[PATCH] drivers/cdrom: improved ioctl for media change detection

[Lukas Prediger]

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@xxxxxxxx>
---
This is my first patch sent to the kernel and I followed the recommended
process as closely as I could. If I misstepped somewhere, please let me know.
I also tried to find a mailing list more specific to the problem but was unable,
please inform me if there is one where this should be sent instead.

Finally, I wasn't sure whether/how to add my name to the updated docs
and if I should update the date on them (there have been some recent commits
that did not result in credits given to the commit author or updating
the dates in the docs, but those did not add any new content either), so
info in the most appropriate way would be welcome.

Looking forward to your feedback.
---
 Documentation/cdrom/cdrom-standard.rst      | 14 +++++-
 Documentation/userspace-api/ioctl/cdrom.rst |  6 ++-
 drivers/cdrom/cdrom.c                       | 56 +++++++++++++++++++--
 include/linux/cdrom.h                       |  1 +
 include/uapi/linux/cdrom.h                  | 15 ++++++
 5 files changed, 86 insertions(+), 6 deletions(-)

diff --git a/Documentation/cdrom/cdrom-standard.rst b/Documentation/cdrom/cdrom-standard.rst
index 5845960ca382..8b219ba9b427 100644
--- a/Documentation/cdrom/cdrom-standard.rst
+++ b/Documentation/cdrom/cdrom-standard.rst
@@ -3,9 +3,10 @@ A Linux CD-ROM standard
 =======================
 
 :Author: David van Leeuwen <david@xxxxxxxxxxxxxxxxxxx>
-:Date: 12 March 1999
+:Date: 5 August 2021
 :Updated by: Erik Andersen (andersee@xxxxxxxxxx)
 :Updated by: Jens Axboe (axboe@xxxxxxxx)
+:Updated by: Lukas Prediger (lumip@xxxxxxxx)
 
 
 Introduction
@@ -907,6 +908,17 @@ 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
+	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_.
diff --git a/Documentation/userspace-api/ioctl/cdrom.rst b/Documentation/userspace-api/ioctl/cdrom.rst
index 3b4c0506de46..59305d04d034 100644
--- a/Documentation/userspace-api/ioctl/cdrom.rst
+++ b/Documentation/userspace-api/ioctl/cdrom.rst
@@ -3,8 +3,9 @@ Summary of CDROM ioctl calls
 ============================
 
 - Edward A. Falk <efalk@xxxxxxxxxx>
+- Lukas Prediger <lumip@xxxxxxxx>
 
-November, 2004
+August, 2021
 
 This document attempts to describe the ioctl(2) calls supported by
 the CDROM layer.  These are by-and-large implemented (as of Linux 2.6)
@@ -54,6 +55,9 @@ 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
diff --git a/drivers/cdrom/cdrom.c b/drivers/cdrom/cdrom.c
index feb827eefd1a..af0721c96e06 100644
--- a/drivers/cdrom/cdrom.c
+++ b/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_time = ktime_get_mono_fast_ns();
+}
+
 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_time = 0;
 
 	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;
 	}
@@ -2391,6 +2397,46 @@ 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 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.has_changed = (tmp_info.last_media_change < cdi->last_media_change_time);
+	tmp_info.last_media_change = cdi->last_media_change_time;
+
+	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)
 {
@@ -3375,6 +3421,8 @@ 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:
diff --git a/include/linux/cdrom.h b/include/linux/cdrom.h
index f48d0a31deae..20ac81e182fc 100644
--- a/include/linux/cdrom.h
+++ b/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;
+	ktime_t last_media_change_time;
 };
 
 struct cdrom_device_ops {
diff --git a/include/uapi/linux/cdrom.h b/include/uapi/linux/cdrom.h
index 6c34f6e2f1f7..9df481845b9e 100644
--- a/include/uapi/linux/cdrom.h
+++ b/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 +297,19 @@ struct cdrom_generic_command
 	};
 };
 
+/* This struct is used by CDROM_TIMED_MEDIA_CHANGE */
+struct cdrom_timed_media_change_info
+{
+	__u64   last_media_change;	/* Timestamp of the last detected media
+					 * change. May be set by caller, updated
+					 * upon successful return of ioctl.
+					 */
+	__u8    has_changed;		/* Set to 1 by ioctl if last detected media
+					 * change was more recent than
+					 * last_media_change set by caller.
+					 */
+};
+
 /*
  * A CD-ROM physical sector size is 2048, 2052, 2056, 2324, 2332, 2336, 
  * 2340, or 2352 bytes long.  
-- 
2.25.1