libblkid: check for multi-session CDROMs

.. and read last session if probing offset is not specified.

udev uses cdrom_id to get last session offset, so people don't see a
problem with hybrid media (audio+data), but if you execute blkid on
command line (without -O <offset>) then you get I/O errors.

It seems that we can use the same way as kernel filesystem iso9960
driver when session= mount option is not specified ... just use
CDROMMULTISESSION ioctl to get last session offset and probe this last
session rather than all medium.

Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 45 insertions, 6 deletions

diff --git a/libblkid/src/probe.c b/libblkid/src/probe.c
index 63baed33e..e46615702 100644
--- a/libblkid/src/probe.c
+++ b/libblkid/src/probe.c
@@ -772,15 +772,54 @@ failed:
 }
 
 /*
- * Linux kernel reports (BLKGETSIZE) cdrom device size greater than area
- * readable by read(2). We have to reduce the probing area to avoid unwanted
- * I/O errors in probing functions. It seems that unreadable are always last 2
- * or 3 CD blocks (CD block size is 2048 bytes, it means 12 in 512-byte
- * sectors).
+ * Issue #1:
+ *
+ *  On hybrid mutil-session media (audio+data) we get I/O errors almost
+ *  everywhere. The solution is to specify probing offset to the wanted
+ *  session. Udev uses this way when it calls libblkid (the proper data session
+ *  offset is based on ID_CDROM_MEDIA_SESSION_LAST_OFFSET from by udev cdrom_id
+ *  tool).
+ *
+ *  Unfortunately, it means that blkid will be blind when executed without a
+ *  probing offset, to avoid this problem we use CDROMMULTISESSION ioctl to get
+ *  last session offset. This way also uses iso9660 fyslesystem kernel driver
+ *  when you mount mutil-session CD without a sesssion= mount option.
+ *
+ * Issue #2:
+ *
+ *  Linux kernel reports (BLKGETSIZE) cdrom device size greater than area
+ *  readable by read(2). We have to reduce the probing area to avoid unwanted
+ *  I/O errors in probing functions. It seems that unreadable are always last 2
+ *  or 3 CD blocks (CD block size is 2048 bytes, it means 12 in 512-byte
+ *  sectors).
  */
 static void cdrom_size_correction(blkid_probe pr)
 {
-	uint64_t n, nsectors = pr->size >> 9;
+	uint64_t size, n, nsectors;
+
+	/* move offset */
+#ifdef CDROMMULTISESSION
+	if (pr->off == 0) {
+		struct cdrom_multisession ms_info = { 0 };
+		int i;
+
+		ms_info.addr_format = CDROM_LBA;
+		i = ioctl(pr->fd, CDROMMULTISESSION, (unsigned long) &ms_info);
+		if (i == 0 && ms_info.xa_flag) {
+			DBG(LOWPROBE, ul_debug("CDROM: multisession last session start: %u (CD sectors)", ms_info.addr.lba));
+
+			pr->off = (uint64_t) ms_info.addr.lba * 2048;
+			DBG(LOWPROBE, ul_debug("CDROM:  moving probing offset from 0 to %ju", pr->off));
+
+			size = pr->size - pr->off;
+			DBG(LOWPROBE, ul_debug("CDROM:  reduce probing size from %ju to %ju", pr->size, size));
+			pr->size = size;
+		}
+	}
+#endif
+
+	/* reduce size */
+	nsectors = pr->size >> 9;
 
 	for (n = nsectors - 12; n < nsectors; n++) {
 		if (!is_sector_readable(pr->fd, n))