m_dup oddity -- creates mbuf chains with cluster containing 208 bytes of data

[Ed Maste]

A colleague discovered an oddity with m_dup(9), in that duping an
mbuf with 209 or more bytes of data creates a chain with 208 bytes
of data in a cluster attached to the first mbuf, with the rest of
the data chained on the end.

It seemed this behaviour appeared in v 1.130 of uipc_mbuf.c.  The
patch below restores the behaviour of putting MCLBYTES into the
initial mbuf cluster.

Comments?

Index: uipc_mbuf.c
===================================================================
RCS file: /usr/cvs/src/sys/kern/uipc_mbuf.c,v
retrieving revision 1.159
diff -u -u -3 -r1.159 uipc_mbuf.c
--- uipc_mbuf.c 8 Dec 2005 13:13:05 -0000       1.159
+++ uipc_mbuf.c 14 Dec 2005 15:54:06 -0000
@@ -849,7 +849,8 @@
                                m_free(n);
                                goto nospace;
                        }
-                       nsize = MHLEN;
+                       if ((n->m_flags & M_EXT) == 0)
+                               nsize = MHLEN;
                }
                n->m_len = 0;

--
Ed Maste, Sandvine Incorporated