From: Ingo Molnar <mingo@elte.hu>

Add the print-fatal-signals=1 boot option and the
/proc/sys/kernel/print-fatal-signals runtime switch.

This feature prints some minimal information about userspace segfaults to
the kernel console.  This is useful to find early bootup bugs where
userspace debugging is very hard.

Defaults to off.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 Documentation/kernel-parameters.txt |    6 ++++
 include/linux/sysctl.h              |    1 
 kernel/signal.c                     |   33 ++++++++++++++++++++++++++
 kernel/sysctl.c                     |    9 +++++++
 4 files changed, 49 insertions(+)

diff -puN Documentation/kernel-parameters.txt~vdso-print-fatal-signals Documentation/kernel-parameters.txt
--- a/Documentation/kernel-parameters.txt~vdso-print-fatal-signals
+++ a/Documentation/kernel-parameters.txt
@@ -1268,6 +1268,12 @@ and is between 256 and 4096 characters. 
 			autoconfiguration.
 			Ranges are in pairs (memory base and size).
 
+	print-fatal-signals=
+			[KNL] debug: print fatal signals
+			print-fatal-signals=1: print segfault info to
+			the kernel console.
+			default: off.
+
 	profile=	[KNL] Enable kernel profiling via /proc/profile
 			Format: [schedule,]<number>
 			Param: "schedule" - profile schedule points.
diff -puN include/linux/sysctl.h~vdso-print-fatal-signals include/linux/sysctl.h
--- a/include/linux/sysctl.h~vdso-print-fatal-signals
+++ a/include/linux/sysctl.h
@@ -93,6 +93,7 @@ enum
 	KERN_CAP_BSET=14,	/* int: capability bounding set */
 	KERN_PANIC=15,		/* int: panic timeout */
 	KERN_REALROOTDEV=16,	/* real root device to mount after initrd */
+	KERN_PRINT_FATAL=17,	/* int: print fatal signals (0/1) */
 
 	KERN_SPARC_REBOOT=21,	/* reboot command on Sparc */
 	KERN_CTLALTDEL=22,	/* int: allow ctl-alt-del to reboot */
diff -puN kernel/signal.c~vdso-print-fatal-signals kernel/signal.c
--- a/kernel/signal.c~vdso-print-fatal-signals
+++ a/kernel/signal.c
@@ -760,6 +760,37 @@ out_set:
 #define LEGACY_QUEUE(sigptr, sig) \
 	(((sig) < SIGRTMIN) && sigismember(&(sigptr)->signal, (sig)))
 
+int print_fatal_signals = 0;
+
+static void print_fatal_signal(struct pt_regs *regs, int signr)
+{
+	printk("%s/%d: potentially unexpected fatal signal %d.\n",
+		current->comm, current->pid, signr);
+
+#ifdef __i386__
+	printk("code at %08lx: ", regs->eip);
+	{
+		int i;
+		for (i = 0; i < 16; i++) {
+			unsigned char insn;
+
+			__get_user(insn, (unsigned char *)(regs->eip + i));
+			printk("%02x ", insn);
+		}
+	}
+#endif
+	printk("\n");
+	show_regs(regs);
+}
+
+static int __init setup_print_fatal_signals(char *str)
+{
+	get_option (&str, &print_fatal_signals);
+
+	return 1;
+}
+
+__setup("print-fatal-signals=", setup_print_fatal_signals);
 
 static int
 specific_send_sig_info(int sig, struct siginfo *info, struct task_struct *t)
@@ -1918,6 +1949,8 @@ relock:
 		 * Anything else is fatal, maybe with a core dump.
 		 */
 		current->flags |= PF_SIGNALED;
+		if ((signr != SIGKILL) && print_fatal_signals)
+			print_fatal_signal(regs, signr);
 		if (sig_kernel_coredump(signr)) {
 			/*
 			 * If it was able to dump core, this kills all
diff -puN kernel/sysctl.c~vdso-print-fatal-signals kernel/sysctl.c
--- a/kernel/sysctl.c~vdso-print-fatal-signals
+++ a/kernel/sysctl.c
@@ -74,6 +74,7 @@ extern int pid_max_min, pid_max_max;
 extern int sysctl_drop_caches;
 extern int percpu_pagelist_fraction;
 extern int compat_log;
+extern int print_fatal_signals;
 
 #if defined(CONFIG_ADAPTIVE_READAHEAD)
 extern int readahead_ratio;
@@ -379,6 +380,14 @@ static ctl_table kern_table[] = {
 		.proc_handler	= &proc_dointvec,
 	},
 #endif
+	{
+		.ctl_name	= KERN_PRINT_FATAL,
+		.procname	= "print-fatal-signals",
+		.data		= &print_fatal_signals,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 #ifdef __sparc__
 	{
 		.ctl_name	= KERN_SPARC_REBOOT,
_