Re: [RFC PATCH tip/master] kprobes: Use synchronize_rcu_tasks() for optprobe wit CONFIG_PREEMPT

[Paul E. McKenney]

On Wed, May 24, 2017 at 09:00:03PM +0900, Masami Hiramatsu wrote:
> To enable jump optimized probe with CONFIG_PREEMPT, use
> synchronize_rcu_tasks() to wait for all tasks preempted
> on trampoline code back on track.
> 
> Since the jump optimized kprobes can replace multiple
> instructions, there can be tasks which are preempted
> on the 2nd (or 3rd) instructions. If the kprobe
> replaces those instructions by a jump instruction,
> when those tasks back to the preempted place, it is
> a middle of the jump instruction and causes a kernel
> panic.
> To avoid such tragedies in advance, kprobe optimizer
> prepare a detour route using normal kprobe (e.g.
> int3 breakpoint on x86), and wait for the tasks which
> is interrrupted on such place by synchronize_sched()
> when CONFIG_PREEMPT=n.
> If CONFIG_PREEMPT=y, things be more complicated, because
> such interrupted thread can be preempted (other thread
> can be scheduled in interrupt handler.) So, kprobes
> optimizer has to wait for those tasks scheduled normally.
> In this case we can use synchronize_rcu_tasks() which
> ensures that all preempted tasks back on track and
> schedule it.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

> ---
>  arch/Kconfig     |    2 +-
>  kernel/kprobes.c |   23 ++++++++++++++++++++++-
>  2 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 6c00e5b..2abb8de 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -90,7 +90,7 @@ config STATIC_KEYS_SELFTEST
>  config OPTPROBES
>  	def_bool y
>  	depends on KPROBES && HAVE_OPTPROBES
> -	depends on !PREEMPT
> +	select TASKS_RCU if PREEMPT
> 
>  config KPROBES_ON_FTRACE
>  	def_bool y
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 9f60567..6d69074 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -377,6 +377,23 @@ static inline void copy_kprobe(struct kprobe *ap, struct kprobe *p)
>  static bool kprobes_allow_optimization;
> 
>  /*
> + * Synchronizing wait on trampline code for interrupted tasks/threads.
> + * Since the threads running on dynamically allocated trampline code
> + * can be interrupted, kprobes has to wait for those tasks back on
> + * track and scheduled. If the kernel is preemptive, the thread can be
> + * preempted by other tasks on the trampoline too. For such case, this
> + * calls synchronize_rcu_tasks() to wait for those tasks back on track.
> + */
> +static void synchronize_on_trampoline(void)
> +{
> +#ifdef CONFIG_PREEMPT
> +	synchronize_rcu_tasks();
> +#else
> +	synchronize_sched();
> +#endif
> +}
> +
> +/*
>   * Call all pre_handler on the list, but ignores its return value.
>   * This must be called from arch-dep optimized caller.
>   */
> @@ -578,8 +595,12 @@ static void kprobe_optimizer(struct work_struct *work)
>  	 * there is a chance that Nth instruction is interrupted. In that
>  	 * case, running interrupt can return to 2nd-Nth byte of jump
>  	 * instruction. This wait is for avoiding it.
> +	 * With CONFIG_PREEMPT, the interrupts can leads preemption. To wait
> +	 * for such thread, we will use synchronize_rcu_tasks() which ensures
> +	 * all preeempted tasks are scheduled normally. So we can ensure there
> +	 * is no threads running there.
>  	 */
> -	synchronize_sched();
> +	synchronize_on_trampoline();
> 
>  	/* Step 3: Optimize kprobes after quiesence period */
>  	do_optimize_kprobes();
>