Add doublefault handling with a task gate.

This potentially helps debugging, since otherwise a double fault
would generate a triple fault and then reboot the machine. Now
instead it can print out a note about where the problem happened,
unless all the kernel data structures are truly buggered.

arch/i386/kernel/Makefile

@@ -6,7 +6,8 @@ EXTRA_TARGETS := head.o init_task.o
 
 obj-y	:= process.o semaphore.o signal.o entry.o traps.o irq.o vm86.o \
 		ptrace.o i8259.o ioport.o ldt.o setup.o time.o sys_i386.o \
-		pci-dma.o i386_ksyms.o i387.o dmi_scan.o bootflag.o
+		pci-dma.o i386_ksyms.o i387.o dmi_scan.o bootflag.o \
+		doublefault.o
 
 obj-y				+= cpu/
 obj-y				+= timers/

arch/i386/kernel/cpu/common.c

@@ -490,6 +490,10 @@ void __init cpu_init (void)
 	load_TR_desc();
 	load_LDT(&init_mm.context);
 
+	/* Set up doublefault TSS pointer in the GDT */
+	__set_tss_desc(cpu, GDT_ENTRY_DOUBLEFAULT_TSS, &doublefault_tss);
+	cpu_gdt_table[cpu][GDT_ENTRY_DOUBLEFAULT_TSS].b &= 0xfffffdff;
+
 	/* Clear %fs and %gs. */
 	asm volatile ("xorl %eax, %eax; movl %eax, %fs; movl %eax, %gs");
 

arch/i386/kernel/doublefault.c

+#include <linux/mm.h>
+#include <linux/sched.h>
+#include <linux/init.h>
+#include <linux/init_task.h>
+#include <linux/fs.h>
+
+#include <asm/uaccess.h>
+#include <asm/pgtable.h>
+#include <asm/desc.h>
+
+#define DOUBLEFAULT_STACKSIZE (1024)
+static unsigned long doublefault_stack[DOUBLEFAULT_STACKSIZE];
+#define STACK_START (unsigned long)(doublefault_stack+DOUBLEFAULT_STACKSIZE)
+
+#define ptr_ok(x) ((x) > 0xc0000000 && (x) < 0xc1000000)
+
+static void doublefault_fn(void)
+{
+	struct Xgt_desc_struct gdt_desc = {0, 0};
+	unsigned long gdt, tss;
+
+	__asm__ __volatile__("sgdt %0": "=m" (gdt_desc): :"memory");
+	gdt = gdt_desc.address;
+
+	printk("double fault, gdt at %08lx [%d bytes]\n", gdt, gdt_desc.size);
+
+	if (ptr_ok(gdt)) {
+		gdt += GDT_ENTRY_TSS << 3;
+		tss = *(u16 *)(gdt+2);
+		tss += *(u8 *)(gdt+4) << 16;
+		tss += *(u8 *)(gdt+7) << 24;
+		printk("double fault, tss at %08lx\n", tss);
+
+		if (ptr_ok(tss)) {
+			struct tss_struct *t = (struct tss_struct *)tss;
+
+			printk("eip = %08lx, esp = %08lx\n", t->eip, t->esp);
+
+			printk("eax = %08lx, ebx = %08lx, ecx = %08lx, edx = %08lx\n",
+				t->eax, t->ebx, t->ecx, t->edx);
+			printk("esi = %08lx, edi = %08lx\n",
+				t->esi, t->edi);
+		}
+	}
+
+	for (;;) /* nothing */;
+}
+
+struct tss_struct doublefault_tss __cacheline_aligned = {
+	.esp0		= STACK_START,
+	.ss0		= __KERNEL_DS,
+	.ldt		= 0,
+	.bitmap		= INVALID_IO_BITMAP_OFFSET,
+	.io_bitmap	= { [0 ... IO_BITMAP_SIZE ] = ~0 },
+
+	.eip		= (unsigned long) doublefault_fn,
+	.eflags		= 0x00000082,
+	.esp		= STACK_START,
+	.es		= __USER_DS,
+	.cs		= __KERNEL_CS,
+	.ss		= __KERNEL_DS,
+	.ds		= __USER_DS,
+
+	.__cr3		= __pa(swapper_pg_dir)
+};

arch/i386/kernel/head.S

@@ -476,6 +476,13 @@ ENTRY(cpu_gdt_table)
 	.quad 0x00009a0000000000	/* 0xc0 APM CS 16 code (16 bit) */
 	.quad 0x0040920000000000	/* 0xc8 APM DS    data */
 
+	.quad 0x0000000000000000	/* 0xd0 - unused */
+	.quad 0x0000000000000000	/* 0xd8 - unused */
+	.quad 0x0000000000000000	/* 0xe0 - unused */
+	.quad 0x0000000000000000	/* 0xe8 - unused */
+	.quad 0x0000000000000000	/* 0xf0 - unused */
+	.quad 0x0000000000000000	/* 0xf8 - GDT entry 31: double-fault TSS */
+
 #if CONFIG_SMP
 	.fill (NR_CPUS-1)*GDT_ENTRIES,8,0 /* other CPU's GDT */
 #endif

arch/i386/kernel/traps.c

@@ -775,7 +775,7 @@ void __init trap_init_f00f_bug(void)
 }
 #endif
 
-#define _set_gate(gate_addr,type,dpl,addr) \
+#define _set_gate(gate_addr,type,dpl,addr,seg) \
 do { \
   int __d0, __d1; \
   __asm__ __volatile__ ("movw %%dx,%%ax\n\t" \
@@ -785,7 +785,7 @@ do { \
 	:"=m" (*((long *) (gate_addr))), \
 	 "=m" (*(1+(long *) (gate_addr))), "=&a" (__d0), "=&d" (__d1) \
 	:"i" ((short) (0x8000+(dpl<<13)+(type<<8))), \
-	 "3" ((char *) (addr)),"2" (__KERNEL_CS << 16)); \
+	 "3" ((char *) (addr)),"2" ((seg) << 16)); \
 } while (0)
 
 
@@ -797,22 +797,27 @@ do { \
  */
 void set_intr_gate(unsigned int n, void *addr)
 {
-	_set_gate(idt_table+n,14,0,addr);
+	_set_gate(idt_table+n,14,0,addr,__KERNEL_CS);
 }
 
 static void __init set_trap_gate(unsigned int n, void *addr)
 {
-	_set_gate(idt_table+n,15,0,addr);
+	_set_gate(idt_table+n,15,0,addr,__KERNEL_CS);
 }
 
 static void __init set_system_gate(unsigned int n, void *addr)
 {
-	_set_gate(idt_table+n,15,3,addr);
+	_set_gate(idt_table+n,15,3,addr,__KERNEL_CS);
 }
 
 static void __init set_call_gate(void *a, void *addr)
 {
-	_set_gate(a,12,3,addr);
+	_set_gate(a,12,3,addr,__KERNEL_CS);
+}
+
+static void __init set_task_gate(unsigned int n, unsigned int gdt_entry)
+{
+	_set_gate(idt_table+n,5,0,0,(gdt_entry<<3));
 }
 
 
@@ -843,7 +848,7 @@ void __init trap_init(void)
 	set_system_gate(5,&bounds);
 	set_trap_gate(6,&invalid_op);
 	set_trap_gate(7,&device_not_available);
-	set_trap_gate(8,&double_fault);
+	set_task_gate(8,GDT_ENTRY_DOUBLEFAULT_TSS);
 	set_trap_gate(9,&coprocessor_segment_overrun);
 	set_trap_gate(10,&invalid_TSS);
 	set_trap_gate(11,&segment_not_present);

include/asm-i386/desc.h

@@ -42,11 +42,13 @@ __asm__ __volatile__ ("movw %w3,0(%2)\n\t" \
 	"rorl $16,%%eax" \
 	: "=m"(*(n)) : "a" (addr), "r"(n), "ir"(limit), "i"(type))
 
-static inline void set_tss_desc(unsigned int cpu, void *addr)
+static inline void __set_tss_desc(unsigned int cpu, unsigned int entry, void *addr)
 {
-	_set_tssldt_desc(&cpu_gdt_table[cpu][GDT_ENTRY_TSS], (int)addr, 235, 0x89);
+	_set_tssldt_desc(&cpu_gdt_table[cpu][entry], (int)addr, 235, 0x89);
 }
 
+#define set_tss_desc(cpu,addr) __set_tss_desc(cpu, GDT_ENTRY_TSS, addr)
+
 static inline void set_ldt_desc(unsigned int cpu, void *addr, unsigned int size)
 {
 	_set_tssldt_desc(&cpu_gdt_table[cpu][GDT_ENTRY_LDT], (int)addr, ((size << 3)-1), 0x82);

include/asm-i386/processor.h

@@ -83,6 +83,7 @@ struct cpuinfo_x86 {
 extern struct cpuinfo_x86 boot_cpu_data;
 extern struct cpuinfo_x86 new_cpu_data;
 extern struct tss_struct init_tss[NR_CPUS];
+extern struct tss_struct doublefault_tss;
 
 #ifdef CONFIG_SMP
 extern struct cpuinfo_x86 cpu_data[];

include/asm-i386/segment.h

@@ -37,6 +37,13 @@
  *  23 - APM BIOS support
  *  24 - APM BIOS support
  *  25 - APM BIOS support 
+ *
+ *  26 - unused
+ *  27 - unused
+ *  28 - unused
+ *  29 - unused
+ *  30 - unused
+ *  31 - TSS for double fault handler
  */
 #define GDT_ENTRY_TLS_ENTRIES	3
 #define GDT_ENTRY_TLS_MIN	6
@@ -64,10 +71,12 @@
 #define GDT_ENTRY_PNPBIOS_BASE		(GDT_ENTRY_KERNEL_BASE + 6)
 #define GDT_ENTRY_APMBIOS_BASE		(GDT_ENTRY_KERNEL_BASE + 11)
 
+#define GDT_ENTRY_DOUBLEFAULT_TSS	31
+
 /*
- * The GDT has 25 entries but we pad it to cacheline boundary:
+ * The GDT has 32 entries
  */
-#define GDT_ENTRIES 28
+#define GDT_ENTRIES 32
 
 #define GDT_SIZE (GDT_ENTRIES * 8)