Preserve alignment of first mbuf in m_copypacket.

This is useful when doing copies of packet where some leading
space has been preallocated to insert protocol headers.
Note that there are in fact almost no users of m_copypacket.

MFC candidate.

1 files changed, 4 insertions, 0 deletions

diff --git a/sys/kern/uipc_mbuf.c b/sys/kern/uipc_mbuf.c
index c8cf25c..84d68ef 100644
--- a/sys/kern/uipc_mbuf.c
+++ b/sys/kern/uipc_mbuf.c
@@ -697,6 +697,9 @@ nospace:
  * An optimization of the common case `m_copym(m, 0, M_COPYALL, how)'.
  * Note that the copy is read-only, because clusters are not copied,
  * only their reference counts are incremented.
+ * Preserve alignment of the first mbuf so if the creator has left
+ * some room at the beginning (e.g. for inserting protocol headers)
+ * the copies still have the room available.
  */
 struct mbuf *
 m_copypacket(struct mbuf *m, int how)
@@ -716,6 +719,7 @@ m_copypacket(struct mbuf *m, int how)
 		n->m_flags |= M_EXT;
 		MEXT_ADD_REF(m);
 	} else {
+		n->m_data = n->m_pktdat + (m->m_data - m->m_pktdat );
 		bcopy(mtod(m, char *), mtod(n, char *), n->m_len);
 	}