linux-kernel - Re: [PATCH] tracing: fprobe: use ftrace if CONFIG_DYNAMIC_FTRACE_WITH

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Message-Id: <20251030094047.9f8aa77cb90897a14c8adada@kernel.org>
Date: Thu, 30 Oct 2025 09:40:47 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: rostedt@...dmis.org, jolsa@...nel.org, mathieu.desnoyers@...icios.com,
 jiang.biao@...ux.dev, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: fprobe: use ftrace if
 CONFIG_DYNAMIC_FTRACE_WITH_ARGS

On Wed, 29 Oct 2025 10:15:14 +0800
Menglong Dong <menglong8.dong@...il.com> wrote:

> For now, we will use ftrace for the fprobe if fp->exit_handler not exists
> and CONFIG_DYNAMIC_FTRACE_WITH_REGS is enabled.
> 
> However, CONFIG_DYNAMIC_FTRACE_WITH_REGS is not supported by some arch,
> such as arm. What we need in the fprobe is the function arguments, so we
> can use ftrace for fprobe if CONFIG_DYNAMIC_FTRACE_WITH_ARGS is enabled.
> 
> Therefore, use ftrace if CONFIG_DYNAMIC_FTRACE_WITH_REGS or
> CONFIG_DYNAMIC_FTRACE_WITH_ARGS enabled.
> 
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> ---
>  kernel/trace/fprobe.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index ecd623eef68b..9fad0569f521 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -254,7 +254,11 @@ static inline int __fprobe_kprobe_handler(unsigned long ip, unsigned long parent
>  	return ret;
>  }
>  
> -#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +#if defined(CONFIG_DYNAMIC_FTRACE_WITH_ARGS) || defined(CONFIG_DYNAMIC_FTRACE_WITH_REGS)
> +#define FPROBE_USE_FTRACE
> +#endif
> +
> +#ifdef FPROBE_USE_FTRACE
>  /* ftrace_ops callback, this processes fprobes which have only entry_handler. */
>  static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip,
>  	struct ftrace_ops *ops, struct ftrace_regs *fregs)
> @@ -295,7 +299,7 @@ NOKPROBE_SYMBOL(fprobe_ftrace_entry);
>  
>  static struct ftrace_ops fprobe_ftrace_ops = {
>  	.func	= fprobe_ftrace_entry,
> -	.flags	= FTRACE_OPS_FL_SAVE_REGS,
> +	.flags	= FTRACE_OPS_FL_SAVE_ARGS,
>  };
>  static int fprobe_ftrace_active;
>  
> @@ -349,7 +353,7 @@ static bool fprobe_is_ftrace(struct fprobe *fp)
>  {
>  	return false;
>  }
> -#endif
> +#endif /* !FPROBE_USE_FTRACE */
>  
>  /* fgraph_ops callback, this processes fprobes which have exit_handler. */
>  static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops *gops,
> @@ -599,7 +603,7 @@ static int fprobe_module_callback(struct notifier_block *nb,
>  	if (alist.index > 0) {
>  		ftrace_set_filter_ips(&fprobe_graph_ops.ops,
>  				      alist.addrs, alist.index, 1, 0);
> -#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +#ifdef FPROBE_USE_FTRACE
>  		ftrace_set_filter_ips(&fprobe_ftrace_ops,
>  				      alist.addrs, alist.index, 1, 0);
>  #endif

Instead of introducing FPROBE_USE_FTRACE, I think it is better to add another
wrapper to be called here.

#if defined(CONFIG_DYNAMIC_FTRACE_WITH_ARGS) || defined(CONFIG_DYNAMIC_FTRACE_WITH_REGS)

static void fprobe_set_ips(unsigned long *ips, unsigned int cnt, int remove, int reset)
{
		ftrace_set_filter_ips(&fprobe_graph_ops.ops,
				      alist.addrs, alist.index, 1, 0);
		ftrace_set_filter_ips(&fprobe_ftrace_ops,
				      alist.addrs, alist.index, 1, 0);
}
...
#else
static void fprobe_set_ips(unsigned long *ips, unsigned int cnt, int remove, int reset)
{
		ftrace_set_filter_ips(&fprobe_graph_ops.ops,
				      alist.addrs, alist.index, 1, 0);
}
#endif

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>