libblkid: reduce probing area for crazy CDROMs

Linux kernel reports devices greater than area readable by read(2).
The readable area is usually 2-3 CD blocks smaller (CD block is
2048-bytes) than size returned by BLKGETSIZE. This patch checks for
this issues to avoid I/O errors in probing functions.

Reported-by: Thomas Schmitt <scdbackup@gmx.net>
Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/libblkid/src/probe.c b/libblkid/src/probe.c
index 0c8625c57aab801e867903834b70cf79daadefee..0455c37975876857768d916e71e5e36977397d41 100644 (file)
--- a/libblkid/src/probe.c
+++ b/libblkid/src/probe.c
@@ -752,6 +752,51 @@ int blkid_probe_is_cdrom(blkid_probe pr)
        return pr && (pr->flags & BLKID_FL_CDROM_DEV);
 }
 
+static int is_sector_readable(int fd, uint64_t sector)
+{
+       char buf[512];
+       ssize_t sz;
+
+       if (blkid_llseek(fd, sector * 512, SEEK_SET) < 0)
+               goto failed;
+
+       sz = read(fd, buf, sizeof(buf));
+       if (sz != (ssize_t) sizeof(buf))
+               goto failed;
+
+       return 1;
+failed:
+       DBG(LOWPROBE, ul_debug("CDROM: read sector %ju failed %m", sector));
+       errno = 0;
+       return 0;
+}
+
+/*
+ * 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;
+
+       for (n = nsectors - 12; n < nsectors; n++) {
+               if (!is_sector_readable(pr->fd, n))
+                       goto failed;
+       }
+
+       DBG(LOWPROBE, ul_debug("CDROM: full size available"));
+       return;
+failed:
+       /* 'n' is the failed sector, reduce device size to n-1; */
+       DBG(LOWPROBE, ul_debug("CDROM: reduce size from %ju to %ju.",
+                               (uintmax_t) pr->size,
+                               (uintmax_t) (n - 1) << 9));
+       pr->size = n << 9;
+}
+
 /**
  * blkid_probe_set_device:
  * @pr: probe
@@ -844,8 +889,11 @@ int blkid_probe_set_device(blkid_probe pr, int fd,
        else if (S_ISBLK(sb.st_mode) &&
            !blkid_probe_is_tiny(pr) &&
            blkid_probe_is_wholedisk(pr) &&
-           ioctl(fd, CDROM_GET_CAPABILITY, NULL) >= 0)
+           ioctl(fd, CDROM_GET_CAPABILITY, NULL) >= 0) {
+
                pr->flags |= BLKID_FL_CDROM_DEV;
+               cdrom_size_correction(pr);
+       }
 #endif
 
        DBG(LOWPROBE, ul_debug("ready for low-probing, offset=%"PRIu64", size=%"PRIu64"",