ARM kprobes: special hook for the kprobes breakpoint handler

The kprobes code is already able to cope with reentrant probes, so its
handler must be called outside of the region protected by undef_lock.

If ever this lock is released when handlers are called then this commit
could be reverted.

Signed-off-by: Nicolas Pitre <nico@marvell.com>

arch/arm/kernel/kprobes.c

@@ -26,12 +26,6 @@
 #include <asm/traps.h>
 #include <asm/cacheflush.h>
 
-/*
- * This undefined instruction must be unique and
- * reserved solely for kprobes' use.
- */
-#define KPROBE_BREAKPOINT_INSTRUCTION	0xe7f001f8
-
 #define MIN_STACK_SIZE(addr) 				\
 	min((unsigned long)MAX_STACK_SIZE,		\
 	    (unsigned long)current_thread_info() + THREAD_START_SP - (addr))
@@ -206,7 +200,7 @@ void __kprobes kprobe_handler(struct pt_regs *regs)
 	}
 }
 
-static int kprobe_trap_handler(struct pt_regs *regs, unsigned int instr)
+int kprobe_trap_handler(struct pt_regs *regs, unsigned int instr)
 {
 	kprobe_handler(regs);
 	return 0;

arch/arm/kernel/traps.c

@@ -19,6 +19,7 @@
 #include <linux/kallsyms.h>
 #include <linux/delay.h>
 #include <linux/init.h>
+#include <linux/kprobes.h>
 
 #include <asm/atomic.h>
 #include <asm/cacheflush.h>
@@ -313,6 +314,17 @@ asmlinkage void __exception do_undefinstr(struct pt_regs *regs)
 		get_user(instr, (u32 __user *)pc);
 	}
 
+#ifdef CONFIG_KPROBES
+	/*
+	 * It is possible to have recursive kprobes, so we can't call
+	 * the kprobe trap handler with the undef_lock held.
+	 */
+	if (instr == KPROBE_BREAKPOINT_INSTRUCTION && !user_mode(regs)) {
+		kprobe_trap_handler(regs, instr);
+		return;
+	}
+#endif
+
 	spin_lock_irqsave(&undef_lock, flags);
 	list_for_each_entry(hook, &undef_hook, node) {
 		if ((instr & hook->instr_mask) == hook->instr_val &&