block nbd: use req.cookie instead of req.handle

The NBD spec was recently changed [1] to refer to the opaque client
identifier as a 'cookie' rather than a 'handle', but has for a much
longer time listed it as a 64-bit value, and declares that all values
in the NBD protocol are sent in network byte order (big-endian).

Because the value is opaque to the server, it doesn't usually matter
what endianness we send as the client - as long as we are consistent
that either we byte-swap on both write and read, or on neither, then
we can match server replies back to our requests.  That said, our
internal use of the cookie is as a 64-bit number (well, as two 32-bit
numbers concatenated together), rather than as 8 individual bytes; so
prior to this commit, we ARE leaking the native endianness of our
internals as a client out to the server.  We don't know of any server
that will actually inspect the opaque value and behave differently
depending on whether a little-endian or big-endian client is sending
requests, but since we DO log the cookie value, a wireshark capture of
the network traffic is easier to correlate back to the kernel traffic
of a big-endian host (where the u64 and char[8] representations are
the same) than of a little-endian host (where if wireshark honors the
NBD spec and displays a u64 in network byte order, it is byte-swapped
from what the kernel logged).

The fix in this patch is thus two-part: it now consistently uses
network byte order for the opaque value (no difference to a big-endian
machine, but an extra byteswap on a little-endian machine; probably in
the noise compared to the overhead of network traffic in general), and
now uses a 64-bit integer instead of char[8] as its preferred access
to the opaque value (direct assignment instead of memcpy()).

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20230410180611.1051618-4-eblake@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -609,7 +609,7 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
 		request.len = htonl(size);
 	}
 	handle = nbd_cmd_handle(cmd);
-	memcpy(request.handle, &handle, sizeof(handle));
+	request.cookie = cpu_to_be64(handle);
 
 	trace_nbd_send_request(&request, nbd->index, blk_mq_rq_from_pdu(cmd));
 
@@ -621,7 +621,7 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
 	trace_nbd_header_sent(req, handle);
 	if (result < 0) {
 		if (was_interrupted(result)) {
-			/* If we havne't sent anything we can just return BUSY,
+			/* If we haven't sent anything we can just return BUSY,
 			 * however if we have sent something we need to make
 			 * sure we only allow this req to be sent until we are
 			 * completely done.
@@ -735,7 +735,7 @@ static struct nbd_cmd *nbd_handle_reply(struct nbd_device *nbd, int index,
 	u32 tag;
 	int ret = 0;
 
-	memcpy(&handle, reply->handle, sizeof(handle));
+	handle = be64_to_cpu(reply->cookie);
 	tag = nbd_handle_to_tag(handle);
 	hwq = blk_mq_unique_tag_to_hwq(tag);
 	if (hwq < nbd->tag_set.nr_hw_queues)