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@xxxxxxxxxxxxxx>
Tested-by: Alan Jenkins <alan-jenkins@xxxxxxxxxxxxxx>
diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 01cefbb..2415a1b 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -406,13 +406,23 @@ 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 cardreaders can't handle multisector accesses which touch
+ * the last one or two hardware sectors. There are likely to be even
+ * buggier devices, so apply a workaround to the last eight sectors.
*/
- 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;
+ if (sdp->last_sector_bug) {
+ unsigned threshold = get_capacity(disk) - 8 * (sdp->sector_size / 512);
+
+ if (block + this_count <= threshold) {
+ ; /* Okay as is */
+ } else 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/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
index f6a9fe0..0d8d9c1 100644
--- a/include/scsi/scsi_device.h
+++ b/include/scsi/scsi_device.h
@@ -139,7 +139,8 @@ struct scsi_device {
unsigned fix_capacity:1; /* READ_CAPACITY is too high by 1 */
unsigned guess_capacity:1; /* READ_CAPACITY might be too high by 1 */
unsigned retry_hwerror:1; /* Retry HARDWARE_ERROR */
- unsigned last_sector_bug:1; /* Always read last sector in a 1 sector read */
+ unsigned last_sector_bug:1; /* do not use multisector accesses on
+ the last 8 hardware sectors */
DECLARE_BITMAP(supported_events, SDEV_EVT_MAXBITS); /* supported events */
struct list_head event_list; /* asserted events */
--
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