x86: add the "print code before the trapping instruction" feature to 64 bit

The 32 bit x86 tree has a very useful feature that prints the Code: line
for the code even before the trapping instrution (and the start of the
trapping instruction is then denoted with a <>). Unfortunately, the 64 bit
x86 tree does not yet have this feature, making diagnosing backtraces harder
than needed.

This patch adds this feature in the same was as the 32 bit tree has
(including the same kernel boot parameter), and including a bugfix
to make the code use probe_kernel_address() rarther than a buggy (deadlocking)
__get_user.
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/kernel-parameters.txt

@@ -416,8 +416,8 @@ and is between 256 and 4096 characters. It is defined in the file
 			[SPARC64] tick
 			[X86-64] hpet,tsc
 
-	code_bytes	[IA32] How many bytes of object code to print in an
-			oops report.
+	code_bytes	[IA32/X86_64] How many bytes of object code to print
+			in an oops report.
 			Range: 0 - 8192
 			Default: 64
 

arch/x86/kernel/traps_64.c

@@ -74,6 +74,8 @@ asmlinkage void alignment_check(void);
 asmlinkage void machine_check(void);
 asmlinkage void spurious_interrupt_bug(void);
 
+static unsigned int code_bytes = 64;
+
 static inline void conditional_sti(struct pt_regs *regs)
 {
 	if (regs->flags & X86_EFLAGS_IF)
@@ -459,12 +461,15 @@ EXPORT_SYMBOL(dump_stack);
 void show_registers(struct pt_regs *regs)
 {
 	int i;
-	int in_kernel = !user_mode(regs);
 	unsigned long sp;
 	const int cpu = smp_processor_id();
 	struct task_struct *cur = cpu_pda(cpu)->pcurrent;
+	u8 *ip;
+	unsigned int code_prologue = code_bytes * 43 / 64;
+	unsigned int code_len = code_bytes;
 
 	sp = regs->sp;
+	ip = (u8 *) regs->ip - code_prologue;
 	printk("CPU %d ", cpu);
 	__show_regs(regs);
 	printk("Process %s (pid: %d, threadinfo %p, task %p)\n",
@@ -474,21 +479,27 @@ void show_registers(struct pt_regs *regs)
 	 * When in-kernel, we also print out the stack and code at the
 	 * time of the fault..
 	 */
-	if (in_kernel) {
+	if (!user_mode(regs)) {
+		unsigned char c;
 		printk("Stack: ");
 		_show_stack(NULL, regs, (unsigned long *)sp, regs->bp);
+		printk("\n");
 
-		printk("\nCode: ");
-		if (regs->ip < PAGE_OFFSET)
-			goto bad;
-
-		for (i=0; i<20; i++) {
-			unsigned char c;
-			if (__get_user(c, &((unsigned char*)regs->ip)[i])) {
-bad:
+		printk(KERN_EMERG "Code: ");
+		if (ip < (u8 *)PAGE_OFFSET || probe_kernel_address(ip, c)) {
+			/* try starting at RIP */
+			ip = (u8 *) regs->ip;
+			code_len = code_len - code_prologue + 1;
+		}
+		for (i = 0; i < code_len; i++, ip++) {
+			if (ip < (u8 *)PAGE_OFFSET ||
+					probe_kernel_address(ip, c)) {
 				printk(" Bad RIP value.");
 				break;
 			}
+			if (ip == (u8 *)regs->ip)
+				printk("<%02x> ", c);
+			else
 				printk("%02x ", c);
 		}
 	}
@@ -1164,3 +1175,14 @@ static int __init kstack_setup(char *s)
 	return 0;
 }
 early_param("kstack", kstack_setup);
+
+
+static int __init code_bytes_setup(char *s)
+{
+	code_bytes = simple_strtoul(s, NULL, 0);
+	if (code_bytes > 8192)
+		code_bytes = 8192;
+
+	return 1;
+}
+__setup("code_bytes=", code_bytes_setup);