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    SubjectRe: [PATCH] rcu/kprobes: Comment why rcu_nmi_enter() is marked NOKPROBE
    On Wed, 19 Feb 2020 12:16:09 -0500
    Steven Rostedt <rostedt@goodmis.org> wrote:

    > From: Steven Rostedt (VMware) <rostedt@goodmis.org>
    >
    > It's confusing that rcu_nmi_enter() is marked NOKPROBE and
    > rcu_nmi_exit() is not. One may think that the exit needs to be marked
    > for the same reason the enter is, as rcu_nmi_exit() reverts the RCU
    > state back to what it was before rcu_nmi_enter(). But the reason has
    > nothing to do with the state of RCU.
    >
    > The breakpoint handler (int3 on x86) must not have any kprobe on it
    > until the kprobe handler is called. Otherwise, it can cause an infinite
    > recursion and crash the machine. It just so happens that
    > rcu_nmi_enter() is called by the int3 handler before the kprobe handler
    > can run, and therefore needs to be marked as NOKPROBE.
    >
    > Comment this to remove the confusion to why rcu_nmi_enter() is marked
    > NOKPROBE but rcu_nmi_exit() is not.

    Looks good to me.

    Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>

    Thanks,

    >
    > Link: https://lore.kernel.org/r/20200213163800.5c51a5f1@gandalf.local.home
    > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    > ---
    > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    > index 1694a6b57ad8..ada7b2b638fb 100644
    > --- a/kernel/rcu/tree.c
    > +++ b/kernel/rcu/tree.c
    > @@ -846,6 +846,14 @@ void rcu_nmi_enter(void)
    > {
    > rcu_nmi_enter_common(false);
    > }
    > +/*
    > + * All functions called in the breakpoint trap handler (e.g. do_int3()
    > + * on x86), must not allow kprobes until the kprobe breakpoint handler
    > + * is called, otherwise it can cause an infinite recursion.
    > + * On some archs, rcu_nmi_enter() is called in the breakpoint handler
    > + * before the kprobe breakpoint handler is called, thus it must be
    > + * marked as NOKPROBE.
    > + */
    > NOKPROBE_SYMBOL(rcu_nmi_enter);
    >
    > /**


    --
    Masami Hiramatsu <mhiramat@kernel.org>

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