Re: [RFC][Patch 2/2][take2]kprobe: kprobe-booster against 2.6.14-mm1 for i386

[Keshavamurthy Anil S <anil dot s dot keshavamurthy at intel dot com>]

On Tue, Dec 20, 2005 at 10:21:14PM +0900, Masami Hiramatsu wrote:
> Hi, Anil
> 
> Masami Hiramatsu wrote:
> >>Issue No 1:
> >>	In the above code patch, once you call reset_current_kprobes() and
> >>preempt_enable_no_resched() ( I see preeempt_enable_no_resched()
> >>in your fix to this code), you are not suppose to relay on
> >>&p->ainsn.insn, the reason is that, on the other cpu technically
> >>this memory can get freed due to unregister_kprobe() call.
> >>In other words, by the time this cpu tries to execte the instruction
> >>at regs->eip, that memory might have gone and system is bound to crash.
> > 
> > I think old djprobe has the same issue. So, it will be solved by using
> > safety check routine after removing breakpoint. Current kprobe uses
> > synchronize_sched() for waiting rcu processing in unregister_kprobe().
> > In my opinion, the solution is that introducing safety check same as
> > djprobe instead of synchronize_sched() in unregister_kprobe().
> 
> I found the rcu_barrier() function that works similar to djprobe's
> safety check routine.
> 
> > The safety check routine of djprobe waits until all cpus execute the works,
> > and the works are executed from the keventd.
> 
> The rcu_barrier() function waits until all cpus have gone through a
> quiescent state.
> 
> - The CPU performs a process switch.
> - The CPU starts executing in User Mode.
> - The CPU executes the idle loop
> In each of these cases, We say that the CPU has gone through
> a quiescent state.
> 
> If we call rcu_barrier() after "int3" is removed, we can ensure
> followings after the rcu_barrier() called.
> - interrupt handlers are finished on all CPUs.
> - p->ainsn.insn is not executed on all CPUs.
> And we can remove a boosted kprobe safely.
> 
> Current kprobes uses synchronize_sched(). I think this function
> checks safety of only current CPU. So, it may not ensure above
> conditions. But rcu_barrier() checks safety of all CPUs. So, it
> can ensure above conditions.
> How would you think about this?

You are correct, we need to ensure safety of all CPUs.
rcu_barrier() will work.

> 
> > And also, on preemptible kernel, the booster is not enabled where the
> > kernel preemption is enabled. So, there are no preempted threads on
> > p->ainsn.insn.
Yes, this is also true, you have to disable booster when kernel
preemption is enabled. So in effect booster will work only when
kernel preemption is disabled.


> 
> 
> -- 
> Masami HIRAMATSU
> 2nd Research Dept.
> Hitachi, Ltd., Systems Development Laboratory
> E-mail: hiramatu@sdl.hitachi.co.jp
> 
- Anil