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From bfebe74e3babe8c188a51269acc2673f1aea283f Mon Sep 17 00:00:00 2001
From: Karel Zak <kzak@redhat.com>
Date: Tue, 19 Apr 2016 12:39:05 +0200
Subject: [PATCH] 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>
---
libblkid/src/probe.c | 50 +++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 49 insertions(+), 1 deletion(-)
diff --git a/libblkid/src/probe.c b/libblkid/src/probe.c
index 0c8625c..0455c37 100644
--- 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"",
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