ftrace: Avoid needless updates of the ftrace function call

  From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

Song Shuai reported:

    The list func (ftrace_ops_list_func) will be patched first
    before the transition between old and new calls are set,
    which fixed the race described in this commit `59338f75`.

    While ftrace_trace_function changes from the list func to a
    ftrace_ops func, like unregistering the klp_ops to leave the only
    global_ops in ftrace_ops_list, the ftrace_[regs]_call will be
    replaced with the list func although it already exists. So there
    should be a condition to avoid this.

And suggested using another variable to keep track of what the ftrace
function is set to. But this could be simplified by using a helper
function that does the same with a static variable.

Link: https://lore.kernel.org/lkml/20221026132039.2236233-1-suagrfillet@gmail.com/

Reported-by: Song Shuai <suagrfillet@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/ftrace.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)
  

On Wed, 23 Nov 2022 10:29:28 +0800
Song Shuai <suagrfillet@gmail.com> wrote:

> > @@ -2783,7 +2796,7 @@ void ftrace_modify_all_code(int command)
> >          * traced.
> >          */
> >         if (update) {
> > -               err = ftrace_update_ftrace_func(ftrace_ops_list_func);
> > +               err = update_ftrace_func(ftrace_ops_list_func);
> >                 if (FTRACE_WARN_ON(err))
> >                         return;
> >         }
> > @@ -2799,7 +2812,7 @@ void ftrace_modify_all_code(int command)
> >                 /* If irqs are disabled, we are in stop machine */
> >                 if (!irqs_disabled())
> >                         smp_call_function(ftrace_sync_ipi, NULL, 1);
> > -               err = ftrace_update_ftrace_func(ftrace_trace_function);
> > +               err = update_ftrace_func(ftrace_trace_function);  
> The helper function should only serve the ftrace_ops_list_func.
> 
> And ftrace_trace_function is always different in this function with
> FTRACE_UPDATE_TRACE_FUNC command.
> 
> So this place should be left as it is.

But it is needed to update the static variable. Without this change,
then save_func will get set to ftrace_ops_list_func and never change
after that and then the update to ftrace_ops_list_func will stop happening.

-- Steve
  

Steven Rostedt <rostedt@goodmis.org> 于2022年11月23日周三 03:17写道：
>
> On Wed, 23 Nov 2022 10:29:28 +0800
> Song Shuai <suagrfillet@gmail.com> wrote:
>
> > > @@ -2783,7 +2796,7 @@ void ftrace_modify_all_code(int command)
> > >          * traced.
> > >          */
> > >         if (update) {
> > > -               err = ftrace_update_ftrace_func(ftrace_ops_list_func);
> > > +               err = update_ftrace_func(ftrace_ops_list_func);
> > >                 if (FTRACE_WARN_ON(err))
> > >                         return;
> > >         }
> > > @@ -2799,7 +2812,7 @@ void ftrace_modify_all_code(int command)
> > >                 /* If irqs are disabled, we are in stop machine */
> > >                 if (!irqs_disabled())
> > >                         smp_call_function(ftrace_sync_ipi, NULL, 1);
> > > -               err = ftrace_update_ftrace_func(ftrace_trace_function);
> > > +               err = update_ftrace_func(ftrace_trace_function);
> > The helper function should only serve the ftrace_ops_list_func.
> >
> > And ftrace_trace_function is always different in this function with
> > FTRACE_UPDATE_TRACE_FUNC command.
> >
> > So this place should be left as it is.
>
> But it is needed to update the static variable. Without this change,
> then save_func will get set to ftrace_ops_list_func and never change
> after that and then the update to ftrace_ops_list_func will stop happening.
>
> -- Steve
Yes, you're right. Thx for your explanation.
-- Song
  

On Tue, 22 Nov 2022 18:09:05 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> Song Shuai reported:
> 
>     The list func (ftrace_ops_list_func) will be patched first
>     before the transition between old and new calls are set,
>     which fixed the race described in this commit `59338f75`.
> 
>     While ftrace_trace_function changes from the list func to a
>     ftrace_ops func, like unregistering the klp_ops to leave the only
>     global_ops in ftrace_ops_list, the ftrace_[regs]_call will be
>     replaced with the list func although it already exists. So there
>     should be a condition to avoid this.
> 
> And suggested using another variable to keep track of what the ftrace
> function is set to. But this could be simplified by using a helper
> function that does the same with a static variable.
> 
> Link: https://lore.kernel.org/lkml/20221026132039.2236233-1-suagrfillet@gmail.com/
> 

This looks good to me.

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

Thank you,

> Reported-by: Song Shuai <suagrfillet@gmail.com>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  kernel/trace/ftrace.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 65a5d36463e0..d04552c0c275 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2763,6 +2763,19 @@ void __weak ftrace_arch_code_modify_post_process(void)
>  {
>  }
>  
> +static int update_ftrace_func(ftrace_func_t func)
> +{
> +	static ftrace_func_t save_func;
> +
> +	/* Avoid updating if it hasn't changed */
> +	if (func == save_func)
> +		return 0;
> +
> +	save_func = func;
> +
> +	return ftrace_update_ftrace_func(func);
> +}
> +
>  void ftrace_modify_all_code(int command)
>  {
>  	int update = command & FTRACE_UPDATE_TRACE_FUNC;
> @@ -2783,7 +2796,7 @@ void ftrace_modify_all_code(int command)
>  	 * traced.
>  	 */
>  	if (update) {
> -		err = ftrace_update_ftrace_func(ftrace_ops_list_func);
> +		err = update_ftrace_func(ftrace_ops_list_func);
>  		if (FTRACE_WARN_ON(err))
>  			return;
>  	}
> @@ -2799,7 +2812,7 @@ void ftrace_modify_all_code(int command)
>  		/* If irqs are disabled, we are in stop machine */
>  		if (!irqs_disabled())
>  			smp_call_function(ftrace_sync_ipi, NULL, 1);
> -		err = ftrace_update_ftrace_func(ftrace_trace_function);
> +		err = update_ftrace_func(ftrace_trace_function);
>  		if (FTRACE_WARN_ON(err))
>  			return;
>  	}
> -- 
> 2.35.1
>