drbd: fix decoding of bitmap vli rle for device sizes > 64 TB

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@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 5 insertions, 1 deletions

diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
index 12c59eb3b127..6fa6673b36b3 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);