src: Fix previous my mistake and odd unaligned case. Pointed out...

[Repository Conversion <srcmastr%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/3d4973956f7a
branches:  trunk
changeset: 318022:3d4973956f7a
user:      knakahara <knakahara%NetBSD.org@localhost>
date:      Tue Apr 10 11:44:13 2018 +0000
description:
Fix previous my mistake and odd unaligned case. Pointed out by maxv@n.o, thanks.

It must be rare case to be required this copy routine...

diffstat:

 sys/net/if_l2tp.c |  22 ++++++++++++++++++----
 1 files changed, 18 insertions(+), 4 deletions(-)

diffs (53 lines):

diff -r 6ae9368dce39 -r 3d4973956f7a sys/net/if_l2tp.c
--- a/sys/net/if_l2tp.c Tue Apr 10 08:41:14 2018 +0000
+++ b/sys/net/if_l2tp.c Tue Apr 10 11:44:13 2018 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: if_l2tp.c,v 1.22 2018/04/09 10:32:32 knakahara Exp $   */
+/*     $NetBSD: if_l2tp.c,v 1.23 2018/04/10 11:44:13 knakahara Exp $   */
 
 /*
  * Copyright (c) 2017 Internet Initiative Japan Inc.
@@ -31,7 +31,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: if_l2tp.c,v 1.22 2018/04/09 10:32:32 knakahara Exp $");
+__KERNEL_RCSID(0, "$NetBSD: if_l2tp.c,v 1.23 2018/04/10 11:44:13 knakahara Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_inet.h"
@@ -481,10 +481,12 @@
         * If the head of the payload is not aligned, align it.
         */
        addr = mtod(m, vaddr_t);
-       if ((addr & 0x03) == 0) {
+       if ((addr & 0x03) != 0x2) {
                /* copy and align head of payload */
                struct mbuf *m_head;
                int copy_length;
+               u_int pad = roundup(sizeof(struct ether_header), 4)
+                       - sizeof(struct ether_header);
 
 #define L2TP_COPY_LENGTH               60
 
@@ -507,7 +509,19 @@
                }
                M_COPY_PKTHDR(m_head, m);
 
-               MH_ALIGN(m_head, L2TP_COPY_LENGTH);
+               /*
+                * m_head should be:
+                *                             L2TP_COPY_LENGTH
+                *                          <-  + roundup(pad, 4) - pad ->
+                *   +-------+--------+-----+--------------+-------------+
+                *   | m_hdr | pkthdr | ... | ether header |   payload   |
+                *   +-------+--------+-----+--------------+-------------+
+                *                          ^              ^
+                *                          m_data         4 byte aligned
+                */
+               MH_ALIGN(m_head, L2TP_COPY_LENGTH + roundup(pad, 4));
+               m_head->m_data += pad;
+
                memcpy(mtod(m_head, void *), mtod(m, void *), copy_length);
                m_head->m_len = copy_length;
                m->m_data += copy_length;