ftrace: avoid replacing the list func with itself

  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.

This patch backups saved_ftrace_func by saved_ftrace_func_old
which will be compared with the list func before trying to patch it.

Signed-off-by: Song Shuai <suagrfillet@gmail.com>
---
 kernel/trace/ftrace.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
  

Song Shuai <suagrfillet@gmail.com> 于2022年10月26日周三 13:20写道：
>
> 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.
>
> This patch backups saved_ftrace_func by saved_ftrace_func_old
> which will be compared with the list func before trying to patch it.
>
Ping...

Thanks,
Song
> Signed-off-by: Song Shuai <suagrfillet@gmail.com>
> ---
>  kernel/trace/ftrace.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index bc921a3f7ea8..56b1a42e1937 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2755,6 +2755,8 @@ void __weak ftrace_arch_code_modify_post_process(void)
>  {
>  }
>
> +static ftrace_func_t saved_ftrace_func_old;
> +
>  void ftrace_modify_all_code(int command)
>  {
>         int update = command & FTRACE_UPDATE_TRACE_FUNC;
> @@ -2774,7 +2776,7 @@ void ftrace_modify_all_code(int command)
>          * to make sure the ops are having the right functions
>          * traced.
>          */
> -       if (update) {
> +       if (update && saved_ftrace_func_old != ftrace_ops_list_func) {
>                 err = ftrace_update_ftrace_func(ftrace_ops_list_func);
>                 if (FTRACE_WARN_ON(err))
>                         return;
> @@ -2918,6 +2920,7 @@ static void ftrace_trampoline_free(struct ftrace_ops *ops)
>  static void ftrace_startup_enable(int command)
>  {
>         if (saved_ftrace_func != ftrace_trace_function) {
> +               saved_ftrace_func_old = saved_ftrace_func;
>                 saved_ftrace_func = ftrace_trace_function;
>                 command |= FTRACE_UPDATE_TRACE_FUNC;
>         }
> @@ -3007,6 +3010,7 @@ int ftrace_shutdown(struct ftrace_ops *ops, int command)
>         ops->flags &= ~FTRACE_OPS_FL_ENABLED;
>
>         if (saved_ftrace_func != ftrace_trace_function) {
> +               saved_ftrace_func_old = saved_ftrace_func;
>                 saved_ftrace_func = ftrace_trace_function;
>                 command |= FTRACE_UPDATE_TRACE_FUNC;
>         }
> @@ -8321,6 +8325,7 @@ static void ftrace_startup_sysctl(void)
>
>         /* Force update next time */
>         saved_ftrace_func = NULL;
> +       saved_ftrace_func_old = NULL;
>         /* ftrace_start_up is true if we want ftrace running */
>         if (ftrace_start_up) {
>                 command = FTRACE_UPDATE_CALLS;
> --
> 2.20.1
>
  

On Tue, 22 Nov 2022 02:28:25 +0000
Song Shuai <suagrfillet@gmail.com> wrote:

> Song Shuai <suagrfillet@gmail.com> 于2022年10月26日周三 13:20写道：
> >
> > 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.
> >
> > This patch backups saved_ftrace_func by saved_ftrace_func_old
> > which will be compared with the list func before trying to patch it.
> >  
> Ping...

Thanks for the ping. I had thought I had replied to this, but I don't
see it in my sent folder. I may have been distracted, and lost the
message.

I'll take a look at it tomorrow.

-- Steve
  

Steven Rostedt <rostedt@goodmis.org> 于2022年11月22日周二 02:34写道：
>
> On Tue, 22 Nov 2022 02:28:25 +0000
> Song Shuai <suagrfillet@gmail.com> wrote:
>
> > Song Shuai <suagrfillet@gmail.com> 于2022年10月26日周三 13:20写道：
> > >
> > > 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.
> > >
> > > This patch backups saved_ftrace_func by saved_ftrace_func_old
> > > which will be compared with the list func before trying to patch it.
> > >
> > Ping...
>
> Thanks for the ping. I had thought I had replied to this, but I don't
> see it in my sent folder. I may have been distracted, and lost the
> message.
>
> I'll take a look at it tomorrow.
>
> -- Steve
No problem, thanks!

-- Song
  

On Tue, 22 Nov 2022 02:28:25 +0000
Song Shuai <suagrfillet@gmail.com> wrote:

> Song Shuai <suagrfillet@gmail.com> 于2022年10月26日周三 13:20写道：
> >
> > 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.
> >
> > This patch backups saved_ftrace_func by saved_ftrace_func_old
> > which will be compared with the list func before trying to patch it.
> >  
> Ping...

Wouldn't something like this be simpler and easier to manage (without
adding another variable to keep track of)?

-- Steve

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;
 	}
  

Steven Rostedt <rostedt@goodmis.org> 于2022年11月22日周二 21:32写道：
>
> On Tue, 22 Nov 2022 02:28:25 +0000
> Song Shuai <suagrfillet@gmail.com> wrote:
>
> > Song Shuai <suagrfillet@gmail.com> 于2022年10月26日周三 13:20写道：
> > >
> > > 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.
> > >
> > > This patch backups saved_ftrace_func by saved_ftrace_func_old
> > > which will be compared with the list func before trying to patch it.
> > >
> > Ping...
>
> Wouldn't something like this be simpler and easier to manage (without
> adding another variable to keep track of)?
>
> -- Steve
>
Thanks for the corrections, this looks great to me.
> 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;
>         }
Thanks,
Song