[ARM] switch ptrace to use an undefined instruction

This avoids a problem with the original ptrace code using a system
call (SWI) to implement single stepping; programs such as ltrace
do not expect to receive system call trace traps when breakpoints
are hit.

arch/arm/kernel/ptrace.c

@@ -18,10 +18,12 @@
 #include <linux/ptrace.h>
 #include <linux/user.h>
 #include <linux/security.h>
+#include <linux/init.h>
 
 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
 #include <asm/system.h>
+#include <asm/traps.h>
 
 #include "ptrace.h"
 
@@ -32,7 +34,7 @@
  * in exit.c or in signal.c.
  */
 
-#if 1
+#if 0
 /*
  * Breakpoint SWI instruction: SWI &9F0001
  */
@@ -479,25 +481,47 @@ void ptrace_break(struct task_struct *tsk, struct pt_regs *regs)
 {
 	siginfo_t info;
 
-	/*
-	 * The PC is always left pointing at the next instruction.  Fix this.
-	 */
-	regs->ARM_pc -= 4;
-
-	if (tsk->thread.debug.nsaved == 0)
-		printk(KERN_ERR "ptrace: bogus breakpoint trap\n");
-
 	ptrace_cancel_bpt(tsk);
 
 	info.si_signo = SIGTRAP;
 	info.si_errno = 0;
 	info.si_code  = TRAP_BRKPT;
-	info.si_addr  = (void *)instruction_pointer(regs) -
-			 (thumb_mode(regs) ? 2 : 4);
+	info.si_addr  = (void *)instruction_pointer(regs);
 
 	force_sig_info(SIGTRAP, &info, tsk);
 }
 
+static int break_trap(struct pt_regs *regs, unsigned int instr)
+{
+	ptrace_break(current, regs);
+	return 0;
+}
+
+static struct undef_hook arm_break_hook = {
+	.instr_mask	= 0x0fffffff,
+	.instr_val	= 0x07f001f0,
+	.cpsr_mask	= PSR_T_BIT,
+	.cpsr_val	= 0,
+	.fn		= break_trap,
+};
+
+static struct undef_hook thumb_break_hook = {
+	.instr_mask	= 0xffff,
+	.instr_val	= 0xde01,
+	.cpsr_mask	= PSR_T_BIT,
+	.cpsr_val	= PSR_T_BIT,
+	.fn		= break_trap,
+};
+
+static int __init ptrace_break_init(void)
+{
+	register_undef_hook(&arm_break_hook);
+	register_undef_hook(&thumb_break_hook);
+	return 0;
+}
+
+core_initcall(ptrace_break_init);
+
 /*
  * Read the word at offset "off" into the "struct user".  We
  * actually access the pt_regs stored on the kernel stack.

arch/arm/kernel/traps.c

@@ -34,7 +34,7 @@
 #include <asm/system.h>
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
-#include <asm/semaphore.h>
+#include <asm/traps.h>
 
 #include "ptrace.h"
 
@@ -240,17 +240,56 @@ void die_if_kernel(const char *str, struct pt_regs *regs, int err)
     	die(str, regs, err);
 }
 
+static LIST_HEAD(undef_hook);
+static spinlock_t undef_lock = SPIN_LOCK_UNLOCKED;
+
+void register_undef_hook(struct undef_hook *hook)
+{
+	spin_lock_irq(&undef_lock);
+	list_add(&hook->node, &undef_hook);
+	spin_unlock_irq(&undef_lock);
+}
+
+void unregister_undef_hook(struct undef_hook *hook)
+{
+	spin_lock_irq(&undef_lock);
+	list_del(&hook->node);
+	spin_unlock_irq(&undef_lock);
+}
+
 asmlinkage void do_undefinstr(struct pt_regs *regs)
 {
-	unsigned long *pc;
+	unsigned int correction = thumb_mode(regs) ? 2 : 4;
+	unsigned int instr;
+	struct undef_hook *hook;
 	siginfo_t info;
+	void *pc;
 
 	/*
-	 * According to the ARM ARM, PC is 2 or 4 bytes ahead, depending
-	 * whether we're in Thumb mode or not.
+	 * According to the ARM ARM, PC is 2 or 4 bytes ahead,
+	 * depending whether we're in Thumb mode or not.
+	 * Correct this offset.
 	 */
-	regs->ARM_pc -= thumb_mode(regs) ? 2 : 4;
-	pc = (unsigned long *)instruction_pointer(regs);
+	regs->ARM_pc -= correction;
+
+	pc = (void *)instruction_pointer(regs);
+	if (thumb_mode(regs)) {
+		get_user(instr, (u16 *)pc);
+	} else {
+		get_user(instr, (u32 *)pc);
+	}
+
+	spin_lock_irq(&undef_lock);
+	list_for_each_entry(hook, &undef_hook, node) {
+		if ((instr & hook->instr_mask) == hook->instr_val &&
+		    (regs->ARM_cpsr & hook->cpsr_mask) == hook->cpsr_val) {
+			if (hook->fn(regs, instr) == 0) {
+				spin_unlock_irq(&undef_lock);
+				return;
+			}
+		}
+	}
+	spin_unlock_irq(&undef_lock);
 
 #ifdef CONFIG_DEBUG_USER
 	printk(KERN_INFO "%s (%d): undefined instruction: pc=%p\n",
@@ -377,6 +416,7 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
 		return 0;
 
 	case NR(breakpoint): /* SWI BREAK_POINT */
+		regs->ARM_pc -= thumb_mode(regs) ? 2 : 4;
 		ptrace_break(current, regs);
 		return regs->ARM_r0;
 

include/asm-arm/traps.h

+#ifndef _ASMARM_TRAP_H
+#define _ASMARM_TRAP_H
+
+#include <linux/list.h>
+
+struct undef_hook {
+	struct list_head node;
+	u32 instr_mask;
+	u32 instr_val;
+	u32 cpsr_mask;
+	u32 cpsr_val;
+	int (*fn)(struct pt_regs *regs, unsigned int instr);
+};
+
+void register_undef_hook(struct undef_hook *hook);
+void unregister_undef_hook(struct undef_hook *hook);
+
+#endif