[DCCP]: Add socket option to query the current MPS

This enables applications to query the current value of the Maximum
Packet Size via a socket option, suggested as a SHOULD in (RFC 4340,
p. 102).

This socket option is useful to avoid the annoying bail-out via
`-EMSGSIZE'.  In particular, as fragmentation is not currently
supported (and its use is partly discouraged in RFC 4340).

With this option, it is possible to size buffers accordingly, e.g.

	int buflen = dccp_get_cur_mps(sockfd);

	/* or */
	if (msgsize > dccp_get_cur_mps(sockfd))
		die("message is too large for this path");

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 0 deletions

diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index 7e4f54a4ecaf..c0b685efe245 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -587,6 +587,10 @@ static int do_dccp_getsockopt(struct sock *sk, int level, int optname,
         case DCCP_SOCKOPT_SERVICE:
                 return dccp_getsockopt_service(sk, len,
                                                (__be32 __user *)optval, optlen);
+        case DCCP_SOCKOPT_GET_CUR_MPS:
+                val = dp->dccps_mss_cache;
+                len = sizeof(val);
+                break;
         case DCCP_SOCKOPT_SEND_CSCOV:
                 val = dp->dccps_pcslen;
                 len = sizeof(val);