[patch 1/3] vdso: print fatal signals

[patch 1/3] vdso: print fatal signals

[Ingo Molnar]

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

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

Index: linux-vdso-rand.q/Documentation/kernel-parameters.txt
===================================================================
--- linux-vdso-rand.q.orig/Documentation/kernel-parameters.txt
+++ linux-vdso-rand.q/Documentation/kernel-parameters.txt
@@ -1261,6 +1261,12 @@ running once the system is up.
 			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.
Index: linux-vdso-rand.q/include/linux/sysctl.h
===================================================================
--- linux-vdso-rand.q.orig/include/linux/sysctl.h
+++ linux-vdso-rand.q/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 */
Index: linux-vdso-rand.q/kernel/signal.c
===================================================================
--- linux-vdso-rand.q.orig/kernel/signal.c
+++ linux-vdso-rand.q/kernel/signal.c
@@ -763,6 +763,37 @@ 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)
@@ -1843,6 +1874,8 @@ 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
Index: linux-vdso-rand.q/kernel/sysctl.c
===================================================================
--- linux-vdso-rand.q.orig/kernel/sysctl.c
+++ linux-vdso-rand.q/kernel/sysctl.c
@@ -72,6 +72,7 @@ extern int printk_ratelimit_burst;
 extern int pid_max_min, pid_max_max;
 extern int sysctl_drop_caches;
 extern int percpu_pagelist_fraction;
+extern int print_fatal_signals;
 
 #if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86)
 int unknown_nmi_panic;
@@ -326,6 +327,14 @@ 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,

Re: [patch 1/3] vdso: print fatal signals

[Zachary Amsden]

Ingo Molnar wrote:
> Index: linux-vdso-rand.q/kernel/signal.c
> ===================================================================
> --- linux-vdso-rand.q.orig/kernel/signal.c
> +++ linux-vdso-rand.q/kernel/signal.c
> @@ -763,6 +763,37 @@ 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
>   


This looks ok for debugging boot problems.  Perhaps you could print the 
registers too?  The instruction dump won't help much for indirect access.

The get_user of eip+i is ok, but doesn't account for segment offsets.  
Not that I think it needs to here.  But it is one of a many growing 
number of places that now try to inspect or modify a potentially 
segmented area of memory (page fault handler must inspect for prefetch 
instructions, kprobes reads and patches code, FPU emulation).  Perhaps a 
common interface would be a nice thing at some point in time.

Zach