From 226fbf47146cb040a886724dee6ba4250518539e Mon Sep 17 00:00:00 2001
From: julian <julian@FreeBSD.org>
Date: Tue, 26 Oct 1999 11:40:23 +0000
Subject: Minor hack in the netgraph interface to ethernets.

---
 sys/net/if_ethersubr.c | 71 ++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 60 insertions(+), 11 deletions(-)

(limited to 'sys/net')

diff --git a/sys/net/if_ethersubr.c b/sys/net/if_ethersubr.c
index 04e3ea8..79d8b62 100644
--- a/sys/net/if_ethersubr.c
+++ b/sys/net/if_ethersubr.c
@@ -1133,29 +1133,78 @@ bad:
 
 /*
  * pass an mbuf out to the connected hook
- */
+ * More complicated than just an m_prepend, as it tries to save later nodes
+ * from needing to do lots of m_pullups.
+ */	     
 static void
 ngether_send(struct arpcom *ac, struct ether_header *eh, struct mbuf *m)
-{
+{       
+	int room;
 	node_p node = AC2NG(ac);
 	struct ether_header *eh2;
-
+		
 	if (node && LIST_FIRST(&(node->hooks))) {
-		M_PREPEND(m, sizeof(*eh), M_DONTWAIT);
-		if (m == 0)
-			return;
-		eh2 = mtod(m, struct ether_header *);
 		/*
-		 * Possibly 'eh' already points
-		 * to the right place.
+		 * Possibly the header is already on the front,
 		 */
-		if (eh2 != eh)
-	 		bcopy(eh, eh2, sizeof(*eh));
+		eh2 = mtod(m, struct ether_header *) - 1;
+		if ( eh == eh2) {
+			/*
+			 * This is the case so just move the markers back to 
+			 * re-include it. We lucked out.
+			 * This allows us to avoid a yucky m_pullup
+			 * in later nodes if it works.
+			 */ 
+			m->m_len += sizeof(*eh); 
+			m->m_data -= sizeof(*eh);
+			m->m_pkthdr.len += sizeof(*eh);
+		} else { 
+			/*
+			 * Alternatively there may be room even though
+			 * it is stored somewhere else. If so, copy it in.
+			 * This only safe because we KNOW that this packet has
+			 * just been generated by an ethernet card, so there
+			 * are no aliases to the buffer. (unlike in outgoing
+			 * packets).
+			 * Nearly all ethernet cards will end up producing mbufs
+			 * that fall into these cases. So we are not optimising
+			 * contorted cases.
+			 */
+	      
+			if (m->m_flags & M_EXT) {
+				room = (mtod(m, caddr_t) - m->m_ext.ext_buf);
+				if (room > m->m_ext.ext_size) /* garbage */
+					room = 0; /* fail immediatly */
+			} else {
+				room = (mtod(m, caddr_t) - m->m_pktdat);
+			}
+			if (room > sizeof (*eh)) {  
+				/* we have room, just copy it and adjust */
+				m->m_len += sizeof(*eh);
+				m->m_data -= sizeof(*eh);
+				m->m_pkthdr.len += sizeof(*eh);
+				bcopy ((caddr_t)eh, (caddr_t)eh2, sizeof(*eh));
+			} else {
+				/*
+				 * Doing anything more is likely to get more 
+				 * expensive than it's worth..
+				 * it's probable that everything else is in one
+				 * big lump. The next node will do an m_pullup()
+				 * for exactly the amount of data it needs and
+				 * hopefully everything after that will not
+				 * need one. So let's just use m_prepend.
+				 */
+				m = m_prepend(m, MHLEN, M_DONTWAIT);
+				if (m == NULL)
+					return;
+			}
+		}
 		ng_queue_data(LIST_FIRST(&(node->hooks)), m, NULL);
 	} else {
 		m_freem(m);
 	}
 }
+
 /*
  * do local shutdown processing..
  * This node will refuse to go away, unless the hardware says to..
-- 
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