[PATCH 4.9 031/109] kprobes/x86: Disable preemption in ftrace-based jprobes

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Masami Hiramatsu <mhiramat@xxxxxxxxxx>


[ Upstream commit 5bb4fc2d8641219732eb2bb654206775a4219aca ]

Disable preemption in ftrace-based jprobe handlers as
described in Documentation/kprobes.txt:

  "Probe handlers are run with preemption disabled."

This will fix jprobes behavior when CONFIG_PREEMPT=y.

Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E . McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/150581530024.32348.9863783558598926771.stgit@devbox
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/kernel/kprobes/ftrace.c |   23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

--- a/arch/x86/kernel/kprobes/ftrace.c
+++ b/arch/x86/kernel/kprobes/ftrace.c
@@ -26,7 +26,7 @@
 #include "common.h"
 
 static nokprobe_inline
-int __skip_singlestep(struct kprobe *p, struct pt_regs *regs,
+void __skip_singlestep(struct kprobe *p, struct pt_regs *regs,
 		      struct kprobe_ctlblk *kcb, unsigned long orig_ip)
 {
 	/*
@@ -41,20 +41,21 @@ int __skip_singlestep(struct kprobe *p,
 	__this_cpu_write(current_kprobe, NULL);
 	if (orig_ip)
 		regs->ip = orig_ip;
-	return 1;
 }
 
 int skip_singlestep(struct kprobe *p, struct pt_regs *regs,
 		    struct kprobe_ctlblk *kcb)
 {
-	if (kprobe_ftrace(p))
-		return __skip_singlestep(p, regs, kcb, 0);
-	else
-		return 0;
+	if (kprobe_ftrace(p)) {
+		__skip_singlestep(p, regs, kcb, 0);
+		preempt_enable_no_resched();
+		return 1;
+	}
+	return 0;
 }
 NOKPROBE_SYMBOL(skip_singlestep);
 
-/* Ftrace callback handler for kprobes */
+/* Ftrace callback handler for kprobes -- called under preepmt disabed */
 void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
 			   struct ftrace_ops *ops, struct pt_regs *regs)
 {
@@ -77,13 +78,17 @@ void kprobe_ftrace_handler(unsigned long
 		/* Kprobe handler expects regs->ip = ip + 1 as breakpoint hit */
 		regs->ip = ip + sizeof(kprobe_opcode_t);
 
+		/* To emulate trap based kprobes, preempt_disable here */
+		preempt_disable();
 		__this_cpu_write(current_kprobe, p);
 		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
-		if (!p->pre_handler || !p->pre_handler(p, regs))
+		if (!p->pre_handler || !p->pre_handler(p, regs)) {
 			__skip_singlestep(p, regs, kcb, orig_ip);
+			preempt_enable_no_resched();
+		}
 		/*
 		 * If pre_handler returns !0, it sets regs->ip and
-		 * resets current kprobe.
+		 * resets current kprobe, and keep preempt count +1.
 		 */
 	}
 end: