[PATCH 4/5] drbd: fix decoding of bitmap vli rle for device sizes > 64 TB

[Philipp Reisner]

From: Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>

Symptoms: disconnect after bitmap exchange due to
bitmap overflow (e:49731075554) while decoding bm RLE packet

In the decoding step of the variable length integer run length encoding
there was potentially an uncatched bitshift by wordsize (variable >> 64).

The result of which is "undefined" :(
(only "sometimes" the result is the desired 0)

Fix: don't do any bit shift magic for shift == 64, just assign.

Signed-off-by: Philipp Reisner <philipp.reisner@xxxxxxxxxx>
Signed-off-by: Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>
---
 drivers/block/drbd/drbd_receiver.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
index 12c59eb..6fa6673 100644
--- a/drivers/block/drbd/drbd_receiver.c
+++ b/drivers/block/drbd/drbd_receiver.c
@@ -4125,7 +4125,11 @@ recv_bm_rle_bits(struct drbd_conf *mdev,
 				(unsigned int)bs.buf_len);
 			return -EIO;
 		}
-		look_ahead >>= bits;
+		/* if we consumed all 64 bits, assign 0; >> 64 is "undefined"; */
+		if (likely(bits < 64))
+			look_ahead >>= bits;
+		else
+			look_ahead = 0;
 		have -= bits;
 
 		bits = bitstream_get_bits(&bs, &tmp, 64 - have);
-- 
1.7.9.5

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