[SCSI] extend the last_sector_bug flag to cover more sectors

The last_sector_bug flag was added to work around a bug in certain usb
cardreaders, where they would crash if a multiple sector read included the
last sector. The original implementation avoids this by e.g. splitting an 8
sector read which includes the last sector into a 7 sector read, and a single
sector read for the last sector.  The flag is enabled for all USB devices.

This revealed a second bug in other usb cardreaders, which crash when they
get a multiple sector read which stops 1 sector short of the last sector.
Affected hardware includes the Kingston "MobileLite" external USB cardreader
and the internal USB cardreader on the Asus EeePC.

Extend the last_sector_bug workaround to ensure that any access which touches
the last 8 hardware sectors of the device is a single sector long.  Requests
are shrunk as necessary to meet this constraint.

This gives us a safety margin against potential unknown or future bugs
affecting multi-sector access to the end of the device.  The two known bugs
only affect the last 2 sectors.  However, they suggest that these devices
are prone to fencepost errors and that multi-sector access to the end of the
device is not well tested.  Popular OS's use multi-sector accesses, but they
rarely read the last few sectors.  Linux (with udev & vol_id) automatically
reads sectors from the end of the device on insertion.  It is assumed that
single sector accesses are more thoroughly tested during development.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Tested-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

2 files changed, 21 insertions, 6 deletions

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 8e08d51a0f05..e5e7d7856454 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -375,6 +375,7 @@ static int sd_prep_fn(struct request_queue *q, struct request *rq)
         struct gendisk *disk = rq->rq_disk;
         struct scsi_disk *sdkp;
         sector_t block = rq->sector;
+        sector_t threshold;
         unsigned int this_count = rq->nr_sectors;
         unsigned int timeout = sdp->timeout;
         int ret;
@@ -422,13 +423,21 @@ static int sd_prep_fn(struct request_queue *q, struct request *rq)
         }
 
         /*
-         * Some devices (some sdcards for one) don't like it if the
-         * last sector gets read in a larger then 1 sector read.
+         * Some SD card readers can't handle multi-sector accesses which touch
+         * the last one or two hardware sectors.  Split accesses as needed.
          */
-        if (unlikely(sdp->last_sector_bug &&
-            rq->nr_sectors > sdp->sector_size / 512 &&
-            block + this_count == get_capacity(disk)))
-                this_count -= sdp->sector_size / 512;
+        threshold = get_capacity(disk) - SD_LAST_BUGGY_SECTORS *
+                (sdp->sector_size / 512);
+
+        if (unlikely(sdp->last_sector_bug && block + this_count > threshold)) {
+                if (block < threshold) {
+                        /* Access up to the threshold but not beyond */
+                        this_count = threshold - block;
+                } else {
+                        /* Access only a single hardware sector */
+                        this_count = sdp->sector_size / 512;
+                }
+        }
 
         SCSI_LOG_HLQUEUE(2, scmd_printk(KERN_INFO, SCpnt, "block=%llu\n",
                                         (unsigned long long)block));
diff --git a/drivers/scsi/sd.h b/drivers/scsi/sd.h
index 550b2f70a1f8..95b9f06534d5 100644
--- a/drivers/scsi/sd.h
+++ b/drivers/scsi/sd.h
@@ -31,6 +31,12 @@
  */
 #define SD_BUF_SIZE             512
 
+/*
+ * Number of sectors at the end of the device to avoid multi-sector
+ * accesses to in the case of last_sector_bug
+ */
+#define SD_LAST_BUGGY_SECTORS   8
+
 struct scsi_disk {
         struct scsi_driver *driver;     /* always &sd_template */
         struct scsi_device *device;