[PATCH] kprobes: Temporary disarming of reentrant probe for x86_64

This patch includes x86_64 architecture specific changes to support temporary
disarming on reentrancy of probes.
Signed-of-by: Prasanna S Panchamukhi <prasanna@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/kprobes.c

@@ -45,12 +45,10 @@
 
 static DECLARE_MUTEX(kprobe_mutex);
 
-/* kprobe_status settings */
-#define KPROBE_HIT_ACTIVE	0x00000001
-#define KPROBE_HIT_SS		0x00000002
-
 static struct kprobe *current_kprobe;
 static unsigned long kprobe_status, kprobe_old_rflags, kprobe_saved_rflags;
+static struct kprobe *kprobe_prev;
+static unsigned long kprobe_status_prev, kprobe_old_rflags_prev, kprobe_saved_rflags_prev;
 static struct pt_regs jprobe_saved_regs;
 static long *jprobe_saved_rsp;
 static kprobe_opcode_t *get_insn_slot(void);
@@ -240,6 +238,31 @@ void arch_remove_kprobe(struct kprobe *p)
 	down(&kprobe_mutex);
 }
 
+static inline void save_previous_kprobe(void)
+{
+	kprobe_prev = current_kprobe;
+	kprobe_status_prev = kprobe_status;
+	kprobe_old_rflags_prev = kprobe_old_rflags;
+	kprobe_saved_rflags_prev = kprobe_saved_rflags;
+}
+
+static inline void restore_previous_kprobe(void)
+{
+	current_kprobe = kprobe_prev;
+	kprobe_status = kprobe_status_prev;
+	kprobe_old_rflags = kprobe_old_rflags_prev;
+	kprobe_saved_rflags = kprobe_saved_rflags_prev;
+}
+
+static inline void set_current_kprobe(struct kprobe *p, struct pt_regs *regs)
+{
+	current_kprobe = p;
+	kprobe_saved_rflags = kprobe_old_rflags
+		= (regs->eflags & (TF_MASK | IF_MASK));
+	if (is_IF_modifier(p->ainsn.insn))
+		kprobe_saved_rflags &= ~IF_MASK;
+}
+
 static void prepare_singlestep(struct kprobe *p, struct pt_regs *regs)
 {
 	regs->eflags |= TF_MASK;
@@ -319,10 +342,30 @@ int kprobe_handler(struct pt_regs *regs)
 				regs->eflags |= kprobe_saved_rflags;
 				unlock_kprobes();
 				goto no_kprobe;
-			}
+			} else if (kprobe_status == KPROBE_HIT_SSDONE) {
+				/* TODO: Provide re-entrancy from
+				 * post_kprobes_handler() and avoid exception
+				 * stack corruption while single-stepping on
+				 * the instruction of the new probe.
+				 */
 				arch_disarm_kprobe(p);
 				regs->rip = (unsigned long)p->addr;
 				ret = 1;
+			} else {
+				/* We have reentered the kprobe_handler(), since
+				 * another probe was hit while within the
+				 * handler. We here save the original kprobe
+				 * variables and just single step on instruction
+				 * of the new probe without calling any user
+				 * handlers.
+				 */
+				save_previous_kprobe();
+				set_current_kprobe(p, regs);
+				p->nmissed++;
+				prepare_singlestep(p, regs);
+				kprobe_status = KPROBE_REENTER;
+				return 1;
+			}
 		} else {
 			p = current_kprobe;
 			if (p->break_handler && p->break_handler(p, regs)) {
@@ -352,11 +395,7 @@ int kprobe_handler(struct pt_regs *regs)
 	}
 
 	kprobe_status = KPROBE_HIT_ACTIVE;
-	current_kprobe = p;
-	kprobe_saved_rflags = kprobe_old_rflags
-	    = (regs->eflags & (TF_MASK | IF_MASK));
-	if (is_IF_modifier(p->ainsn.insn))
-		kprobe_saved_rflags &= ~IF_MASK;
+	set_current_kprobe(p, regs);
 
 	if (p->pre_handler && p->pre_handler(p, regs))
 		/* handler has already set things up, so skip ss setup */
@@ -506,14 +545,23 @@ int post_kprobe_handler(struct pt_regs *regs)
 	if (!kprobe_running())
 		return 0;
 
-	if (current_kprobe->post_handler)
+	if ((kprobe_status != KPROBE_REENTER) && current_kprobe->post_handler) {
+		kprobe_status = KPROBE_HIT_SSDONE;
 		current_kprobe->post_handler(current_kprobe, regs, 0);
+	}
 
 	if (current_kprobe->post_handler != trampoline_post_handler)
 		resume_execution(current_kprobe, regs);
 	regs->eflags |= kprobe_saved_rflags;
 
+	/* Restore the original saved kprobes variables and continue. */
+	if (kprobe_status == KPROBE_REENTER) {
+		restore_previous_kprobe();
+		goto out;
+	} else {
 		unlock_kprobes();
+	}
+out:
 	preempt_enable_no_resched();
 
 	/*