From 734d2725db879f3f6fcdc2b1d2a5deae105f5e95 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Sun, 18 Aug 2013 19:08:07 -0700 Subject: [PATCH] ipv4: raise IP_MAX_MTU to theoretical limit As discussed last year [1], there is no compelling reason to limit IPv4 MTU to 0xFFF0, while real limit is 0xFFFF [1] : http://marc.info/?l=linux-netdev&m=135607247609434&w=2 Willem raised this issue again because some of our internal regression tests broke after lo mtu being set to 65536. IP_MTU reports 0xFFF0, and the test attempts to send a RAW datagram of mtu + 1 bytes, expecting the send() to fail, but it does not. Alexey raised interesting points about TCP MSS, that should be addressed in follow-up patches in TCP stack if needed, as someone could also set an odd mtu anyway. Signed-off-by: Eric Dumazet Cc: Alexey Kuznetsov Cc: Willem de Bruijn Signed-off-by: David S. Miller --- net/ipv4/route.c | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-) diff --git a/net/ipv4/route.c b/net/ipv4/route.c index e805481eff72..727f4365bcdf 100644 --- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -112,7 +112,8 @@ #define RT_FL_TOS(oldflp4) \ ((oldflp4)->flowi4_tos & (IPTOS_RT_MASK | RTO_ONLINK)) -#define IP_MAX_MTU 0xFFF0 +/* IPv4 datagram length is stored into 16bit field (tot_len) */ +#define IP_MAX_MTU 0xFFFF #define RT_GC_TIMEOUT (300*HZ) @@ -1227,10 +1228,7 @@ static unsigned int ipv4_mtu(const struct dst_entry *dst) mtu = 576; } - if (mtu > IP_MAX_MTU) - mtu = IP_MAX_MTU; - - return mtu; + return min_t(unsigned int, mtu, IP_MAX_MTU); } static struct fib_nh_exception *find_exception(struct fib_nh *nh, __be32 daddr) -- 2.30.2