linux-kernel - Re: [PATCH] ftrace: avoid replacing the list func with itself

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Date:   Tue, 22 Nov 2022 02:28:25 +0000
From:   Song Shuai <suagrfillet@...il.com>
To:     rostedt@...dmis.org, mingo@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: avoid replacing the list func with itself

Song Shuai <suagrfillet@...il.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@...il.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
>