net: change maximum number of UDP segments to 128

The commit fc8b2a61
("net: more strict VIRTIO_NET_HDR_GSO_UDP_L4 validation")
adds check of potential number of UDP segments vs
UDP_MAX_SEGMENTS in linux/virtio_net.h.
After this change certification test of USO guest-to-guest
transmit on Windows driver for virtio-net device fails,
for example with packet size of ~64K and mss of 536 bytes.
In general the USO should not be more restrictive than TSO.
Indeed, in case of unreasonably small mss a lot of segments
can cause queue overflow and packet loss on the destination.
Limit of 128 segments is good for any practical purpose,
with minimal meaningful mss of 536 the maximal UDP packet will
be divided to ~120 segments.
The number of segments for UDP packets is validated vs
UDP_MAX_SEGMENTS also in udp.c (v4,v6), this does not affect
quest-to-guest path but does affect packets sent to host, for
example.
It is important to mention that UDP_MAX_SEGMENTS is kernel-only
define and not available to user mode socket applications.
In order to request MSS smaller than MTU the applications
just uses setsockopt with SOL_UDP and UDP_SEGMENT and there is
no limitations on socket API level.

Fixes: fc8b2a61 ("net: more strict VIRTIO_NET_HDR_GSO_UDP_L4 validation")
Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/udp.h

@@ -108,7 +108,7 @@ struct udp_sock {
 #define udp_assign_bit(nr, sk, val)		\
 	assign_bit(UDP_FLAGS_##nr, &udp_sk(sk)->udp_flags, val)
 
-#define UDP_MAX_SEGMENTS	(1 << 6UL)
+#define UDP_MAX_SEGMENTS	(1 << 7UL)
 
 #define udp_sk(ptr) container_of_const(ptr, struct udp_sock, inet.sk)
 

tools/testing/selftests/net/udpgso.c

@@ -34,7 +34,7 @@
 #endif
 
 #ifndef UDP_MAX_SEGMENTS
-#define UDP_MAX_SEGMENTS	(1 << 6UL)
+#define UDP_MAX_SEGMENTS	(1 << 7UL)
 #endif
 
 #define CONST_MTU_TEST	1500