ipv4: teach ipconfig about the MTU option in DHCP

The DHCP spec allows the server to specify the MTU.  This can be useful
for netbooting with UDP-based NFS-root on a network using jumbo frames.
This patch allows the kernel IP autoconfiguration to handle this option
correctly.

It would be possible to use initramfs and add a script to set the MTU,
but that seems like a complicated solution if no initramfs is otherwise
necessary, and would bloat the kernel image more than this code would.

This patch was originally submitted to LKML in 2003 by Hans-Peter Jansen.

Signed-off-by: Chris Friesen <cfriesen@nortel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 37 insertions, 4 deletions

diff --git a/net/ipv4/ipconfig.c b/net/ipv4/ipconfig.c
index 88bf051d0cb..f8d04c25645 100644
--- a/net/ipv4/ipconfig.c
+++ b/net/ipv4/ipconfig.c
@@ -160,6 +160,9 @@ static char user_dev_name[IFNAMSIZ] __initdata = { 0, };
 /* Protocols supported by available interfaces */
 static int ic_proto_have_if __initdata = 0;
 
+/* MTU for boot device */
+static int ic_dev_mtu __initdata = 0;
+
 #ifdef IPCONFIG_DYNAMIC
 static DEFINE_SPINLOCK(ic_recv_lock);
 static volatile int ic_got_reply __initdata = 0;    /* Proto(s) that replied */
@@ -286,7 +289,7 @@ set_sockaddr(struct sockaddr_in *sin, __be32 addr, __be16 port)
         sin->sin_port = port;
 }
 
-static int __init ic_dev_ioctl(unsigned int cmd, struct ifreq *arg)
+static int __init ic_devinet_ioctl(unsigned int cmd, struct ifreq *arg)
 {
         int res;
 
@@ -297,6 +300,17 @@ static int __init ic_dev_ioctl(unsigned int cmd, struct ifreq *arg)
         return res;
 }
 
+static int __init ic_dev_ioctl(unsigned int cmd, struct ifreq *arg)
+{
+        int res;
+
+        mm_segment_t oldfs = get_fs();
+        set_fs(get_ds());
+        res = dev_ioctl(&init_net, cmd, (struct ifreq __user *) arg);
+        set_fs(oldfs);
+        return res;
+}
+
 static int __init ic_route_ioctl(unsigned int cmd, struct rtentry *arg)
 {
         int res;
@@ -321,20 +335,31 @@ static int __init ic_setup_if(void)
         memset(&ir, 0, sizeof(ir));
         strcpy(ir.ifr_ifrn.ifrn_name, ic_dev->name);
         set_sockaddr(sin, ic_myaddr, 0);
-        if ((err = ic_dev_ioctl(SIOCSIFADDR, &ir)) < 0) {
+        if ((err = ic_devinet_ioctl(SIOCSIFADDR, &ir)) < 0) {
                 printk(KERN_ERR "IP-Config: Unable to set interface address (%d).\n", err);
                 return -1;
         }
         set_sockaddr(sin, ic_netmask, 0);
-        if ((err = ic_dev_ioctl(SIOCSIFNETMASK, &ir)) < 0) {
+        if ((err = ic_devinet_ioctl(SIOCSIFNETMASK, &ir)) < 0) {
                 printk(KERN_ERR "IP-Config: Unable to set interface netmask (%d).\n", err);
                 return -1;
         }
         set_sockaddr(sin, ic_myaddr | ~ic_netmask, 0);
-        if ((err = ic_dev_ioctl(SIOCSIFBRDADDR, &ir)) < 0) {
+        if ((err = ic_devinet_ioctl(SIOCSIFBRDADDR, &ir)) < 0) {
                 printk(KERN_ERR "IP-Config: Unable to set interface broadcast address (%d).\n", err);
                 return -1;
         }
+        /* Handle the case where we need non-standard MTU on the boot link (a network
+         * using jumbo frames, for instance).  If we can't set the mtu, don't error
+         * out, we'll try to muddle along.
+         */
+        if (ic_dev_mtu != 0) {
+                strcpy(ir.ifr_name, ic_dev->name);
+                ir.ifr_mtu = ic_dev_mtu;
+                if ((err = ic_dev_ioctl(SIOCSIFMTU, &ir)) < 0)
+                        printk(KERN_ERR "IP-Config: Unable to set interface mtu to %d (%d).\n",
+                                         ic_dev_mtu, err);
+        }
         return 0;
 }
 
@@ -623,6 +648,7 @@ ic_dhcp_init_options(u8 *options)
                         12,     /* Host name */
                         15,     /* Domain name */
                         17,     /* Boot path */
+                        26,     /* MTU */
                         40,     /* NIS domain name */
                 };
 
@@ -798,6 +824,7 @@ static void __init ic_do_bootp_ext(u8 *ext)
 {
        u8 servers;
        int i;
+        u16 mtu;
 
 #ifdef IPCONFIG_DEBUG
         u8 *c;
@@ -837,6 +864,10 @@ static void __init ic_do_bootp_ext(u8 *ext)
                         if (!root_server_path[0])
                                 ic_bootp_string(root_server_path, ext+1, *ext, sizeof(root_server_path));
                         break;
+                case 26:        /* Interface MTU */
+                        memcpy(&mtu, ext+1, sizeof(mtu));
+                        ic_dev_mtu = ntohs(mtu);
+                        break;
                 case 40:        /* NIS Domain name (_not_ DNS) */
                         ic_bootp_string(utsname()->domainname, ext+1, *ext, __NEW_UTS_LEN);
                         break;
@@ -1403,6 +1434,8 @@ static int __init ip_auto_config(void)
         printk(",\n     bootserver=%pI4", &ic_servaddr);
         printk(", rootserver=%pI4", &root_server_addr);
         printk(", rootpath=%s", root_server_path);
+        if (ic_dev_mtu)
+                printk(", mtu=%d", ic_dev_mtu);
         printk("\n");
 #endif /* !SILENT */