linux-kernel - Re: [PATCH] ftrace: Fix missing synchronize_rcu() removing trampoline from kallsyms

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Date:   Fri, 11 Sep 2020 15:26:24 +0200
From:   peterz@...radead.org
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     rostedt@...dmis.org, mingo@...hat.com, mhiramat@...nel.org,
        mbenes@...e.cz, linux-kernel@...r.kernel.org,
        Paul McKenney <paulmck@...nel.org>
Subject: Re: [PATCH] ftrace: Fix missing synchronize_rcu() removing
 trampoline from kallsyms

On Fri, Sep 11, 2020 at 03:55:22PM +0300, Adrian Hunter wrote:
> On 11/09/20 2:41 pm, peterz@...radead.org wrote:
> > On Tue, Sep 01, 2020 at 12:16:17PM +0300, Adrian Hunter wrote:
> >> Add synchronize_rcu() after list_del_rcu() in
> >> ftrace_remove_trampoline_from_kallsyms() to protect readers of
> >> ftrace_ops_trampoline_list (in ftrace_get_trampoline_kallsym)
> >> which is used when kallsyms is read.
> >>
> >> Fixes: fc0ea795f53c8d ("ftrace: Add symbols for ftrace trampolines")
> >> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> >> ---
> >>  kernel/trace/ftrace.c | 1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> >> index 275441254bb5..4e64367c9774 100644
> >> --- a/kernel/trace/ftrace.c
> >> +++ b/kernel/trace/ftrace.c
> >> @@ -2782,6 +2782,7 @@ static void ftrace_remove_trampoline_from_kallsyms(struct ftrace_ops *ops)
> >>  {
> >>  	lockdep_assert_held(&ftrace_lock);
> >>  	list_del_rcu(&ops->list);
> >> +	synchronize_rcu();
> >>  }
> > 
> > 
> > Hurmph, we've just done a ton of that:
> > 
> > 
> > 	ftrace_shutdown()
> > 	  synchronize_rcu_tasks_rude()
> > 	  ftrace_trampoline_free()
> > 	    ftrace_remove_trampoline_from_kallsyms()
> > 
> > 
> > So would it not be better to move that call before the existing
> > synchronize_rcu_tasks stuff rather than adding another synchronize_rcu()
> > call?
> 
> Doesn't that mean removing the symbol while the trampoline is potentially
> still in use?

Hurm.. potentially yes. OK, lets do this first.