[*] Support hop-counting in request header, protocol version 3

We steal 8 bits from the request offset to count hops when requests
get relayed by proxies. This still leaves plenty of bits for the
offset (56 bits, supporting images of up to 72 petabytes).
This is used to detect proxy cycles. The algorithm is not perfect
but should prevent endless relays of the same request.
This is backwards compatible to old clients and servers, as the server
only ever sets the hopcount in relayed requests if the upstream server
is using protocol version 3 or newer, and clients are automatically
upwards compatible as there is practically no image larger than 74PB,
so the newly introduced hop count field is always 0 even in requests
from old clients.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/shared/protocol.h b/src/shared/protocol.h
index eb1178d..c3ccbc1 100644
--- a/src/shared/protocol.h
+++ b/src/shared/protocol.h
@@ -13,6 +13,9 @@
 
 #define FLAGS8_SERVER (1)
 
+// 2017-10-16: We now support hop-counting, macro to pass hop count conditinally to a function
+#define COND_HOPCOUNT(vers,hopcount) ( (vers) >= 3 ? (hopcount) : 0 )
+
 #define REPLY_OK (0)
 #define REPLY_ERRNO (-1)
 #define REPLY_AGAIN (-2)
@@ -76,13 +79,16 @@ static inline bool dnbd3_select_image(int sock, const char *name, uint16_t rid,
 	return ret == len + (ssize_t)sizeof(request);
 }
 
-static inline bool dnbd3_get_block(int sock, uint64_t offset, uint32_t size, uint64_t handle)
+static inline bool dnbd3_get_block(int sock, uint64_t offset, uint32_t size, uint64_t handle, uint8_t hopCount)
 {
 	dnbd3_request_t request;
 	request.magic = dnbd3_packet_magic;
 	request.handle = handle;
 	request.cmd = CMD_GET_BLOCK;
+	// When writing before "fixup", we can get away with assigning to offset instead of offset_small if we
+	// do it before assigning to .hops. Faster on 64bit machines (so, on everything)
 	request.offset = offset;
+	request.hops = hopCount;
 	request.size = size;
 	fixup_request( request );
 	return sock_sendAll( sock, &request, sizeof(request), 2 ) == (ssize_t)sizeof(request);