Subject: net: sysrq via icmp
From: Carsten Emde <C.Emde@osadl.org>
Date: Tue, 19 Jul 2011 13:51:17 +0100

There are (probably rare) situations when a system crashed and the system
console becomes unresponsive but the network icmp layer still is alive.
Wouldn't it be wonderful, if we then could submit a sysreq command via ping?

This patch provides this facility. Please consult the updated documentation
Documentation/sysrq.txt for details.

Signed-off-by: Carsten Emde <C.Emde@osadl.org>

---
 Documentation/admin-guide/sysrq.rst |   12 ++++++++++++
 include/net/netns/ipv4.h            |    1 +
 net/ipv4/icmp.c                     |   30 ++++++++++++++++++++++++++++++
 net/ipv4/sysctl_net_ipv4.c          |    7 +++++++
 4 files changed, 50 insertions(+)

Index: linux-4.16.12-rt5/Documentation/admin-guide/sysrq.rst
===================================================================
--- linux-4.16.12-rt5.orig/Documentation/admin-guide/sysrq.rst
+++ linux-4.16.12-rt5/Documentation/admin-guide/sysrq.rst
@@ -77,6 +77,18 @@ On all
 
                echo t > /proc/sysrq-trigger
 
+On all
+        Enable network SysRq by writing a cookie to icmp_echo_sysrq, e.g.::
+
+                echo 0x01020304 >/proc/sys/net/ipv4/icmp_echo_sysrq
+
+        Send an ICMP echo request with this pattern plus the particular
+        SysRq command key. Example::
+
+                ping -c1 -s57 -p0102030468
+
+        will trigger the SysRq-H (help) command.
+
 What are the 'command' keys?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
Index: linux-4.16.12-rt5/include/net/netns/ipv4.h
===================================================================
--- linux-4.16.12-rt5.orig/include/net/netns/ipv4.h
+++ linux-4.16.12-rt5/include/net/netns/ipv4.h
@@ -82,6 +82,7 @@ struct netns_ipv4 {
 
        int sysctl_icmp_echo_ignore_all;
        int sysctl_icmp_echo_ignore_broadcasts;
+       int sysctl_icmp_echo_sysrq;
        int sysctl_icmp_ignore_bogus_error_responses;
        int sysctl_icmp_ratelimit;
        int sysctl_icmp_ratemask;
Index: linux-4.16.12-rt5/net/ipv4/icmp.c
===================================================================
--- linux-4.16.12-rt5.orig/net/ipv4/icmp.c
+++ linux-4.16.12-rt5/net/ipv4/icmp.c
@@ -69,6 +69,7 @@
 #include <linux/jiffies.h>
 #include <linux/kernel.h>
 #include <linux/fcntl.h>
+#include <linux/sysrq.h>
 #include <linux/socket.h>
 #include <linux/in.h>
 #include <linux/inet.h>
@@ -924,6 +925,30 @@ static bool icmp_redirect(struct sk_buff
 }
 
 /*
+ * 32bit and 64bit have different timestamp length, so we check for
+ * the cookie at offset 20 and verify it is repeated at offset 50
+ */
+#define CO_POS0                20
+#define CO_POS1                50
+#define CO_SIZE                sizeof(int)
+#define ICMP_SYSRQ_SIZE        57
+
+/*
+ * We got a ICMP_SYSRQ_SIZE sized ping request. Check for the cookie
+ * pattern and if it matches send the next byte as a trigger to sysrq.
+ */
+static void icmp_check_sysrq(struct net *net, struct sk_buff *skb)
+{
+       int cookie = htonl(net->ipv4.sysctl_icmp_echo_sysrq);
+       char *p = skb->data;
+
+       if (!memcmp(&cookie, p + CO_POS0, CO_SIZE) &&
+           !memcmp(&cookie, p + CO_POS1, CO_SIZE) &&
+           p[CO_POS0 + CO_SIZE] == p[CO_POS1 + CO_SIZE])
+               handle_sysrq(p[CO_POS0 + CO_SIZE]);
+}
+
+/*
  *     Handle ICMP_ECHO ("ping") requests.
  *
  *     RFC 1122: 3.2.2.6 MUST have an echo server that answers ICMP echo
@@ -950,6 +975,11 @@ static bool icmp_echo(struct sk_buff *sk
                icmp_param.data_len        = skb->len;
                icmp_param.head_len        = sizeof(struct icmphdr);
                icmp_reply(&icmp_param, skb);
+
+               if (skb->len == ICMP_SYSRQ_SIZE &&
+                   net->ipv4.sysctl_icmp_echo_sysrq) {
+                       icmp_check_sysrq(net, skb);
+               }
        }
        /* should there be an ICMP stat for ignored echos? */
        return true;
Index: linux-4.16.12-rt5/net/ipv4/sysctl_net_ipv4.c
===================================================================
--- linux-4.16.12-rt5.orig/net/ipv4/sysctl_net_ipv4.c
+++ linux-4.16.12-rt5/net/ipv4/sysctl_net_ipv4.c
@@ -555,6 +555,13 @@ static struct ctl_table ipv4_net_table[]
                .proc_handler   = proc_dointvec
        },
        {
+               .procname       = "icmp_echo_sysrq",
+               .data           = &init_net.ipv4.sysctl_icmp_echo_sysrq,
+               .maxlen         = sizeof(int),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec
+       },
+       {
                .procname       = "icmp_ignore_bogus_error_responses",
                .data           = &init_net.ipv4.sysctl_icmp_ignore_bogus_error_responses,
                .maxlen         = sizeof(int),