[PATCH #upstream-fixes] libata: fix off-by-one error in ata_tf_read_block()

[Tejun Heo <htejun@gmail.com>]

ata_tf_read_block() has off-by-one error when converting CHS address
to LBA.  The bug isn't very visible because ata_tf_read_block() is
used only when generating sense data for a failed RW command and CHS
addressing isn't used too often these days.

This problem was spotted by Atsushi Nemoto.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
---
> Well, I expect fix by you (or other libata hackers) since I think you
> can write better warning message and changelog than me ;)

In that case, sure.  Thanks a lot for the nice spotting.  :-)

 drivers/ata/libata-core.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index 072ba5e..e71149b 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -709,7 +709,13 @@ u64 ata_tf_read_block(struct ata_taskfile *tf, struct ata_device *dev)
 		head = tf->device & 0xf;
 		sect = tf->lbal;
 
-		block = (cyl * dev->heads + head) * dev->sectors + sect;
+		if (!sect) {
+			ata_dev_printk(dev, KERN_WARNING, "device reported "
+				       "invalid CHS sector 0\n");
+			sect = 1; /* oh well */
+		}
+
+		block = (cyl * dev->heads + head) * dev->sectors + sect - 1;
 	}
 
 	return block;