[PATCH -tip v3 5/7] kprobes/x86: Disable preempt ftrace-based jprobe

Masami Hiramatsu
2017-09-19T10:02:16Z

Disable preempt in ftrace-based jprobe handler as
described in Documentation/kprobes.txt, there is

"Probe handlers are run with preemption disabled."

This will fix jprobes behavior when CONFIG_PREEMPT=y.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
arch/x86/kernel/kprobes/ftrace.c | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
index 041f7b6dfa0f..bcfee4f69b0e 100644
--- 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, struct pt_regs *regs,
__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 ip, unsigned long parent_ip,
/* 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.
*/
}