linux-kernel - Re: [PATCH v2 1/2] tracing: Make the backup instance readonly

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Message-Id: <20260113093100.e91c5be8b8fa5f1e64899ba4@kernel.org>
Date: Tue, 13 Jan 2026 09:31:00 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] tracing: Make the backup instance readonly

On Thu,  8 Jan 2026 23:23:03 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:

> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 9b07ad9eb284..741b16b54d90 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -1379,6 +1379,9 @@ static int __ftrace_set_clr_event(struct trace_array *tr, const char *match,
>  {
>  	int ret;
>  
> +	if (trace_array_is_readonly(tr))
> +		return -EPERM;
> +
>  	mutex_lock(&event_mutex);
>  	ret = __ftrace_set_clr_event_nolock(tr, match, sub, event, set, mod);
>  	mutex_unlock(&event_mutex);
> @@ -2817,8 +2820,8 @@ event_subsystem_dir(struct trace_array *tr, const char *name,
>  	} else
>  		__get_system(system);
>  
> -	/* ftrace only has directories no files */
> -	if (strcmp(name, "ftrace") == 0)
> +	/* ftrace only has directories no files, readonly instance too. */
> +	if (strcmp(name, "ftrace") == 0 || trace_array_is_readonly(tr))
>  		nr_entries = 0;
>  	else
>  		nr_entries = ARRAY_SIZE(system_entries);
> @@ -2979,7 +2982,6 @@ event_create_dir(struct eventfs_inode *parent, struct trace_event_file *file)
>  	struct eventfs_inode *e_events;
>  	struct eventfs_inode *ei;
>  	const char *name;
> -	int nr_entries;
>  	int ret;
>  	static struct eventfs_entry event_entries[] = {
>  		{
> @@ -3024,6 +3026,18 @@ event_create_dir(struct eventfs_inode *parent, struct trace_event_file *file)
>  		},
>  #endif
>  	};
> +	static struct eventfs_entry event_ro_entries[] = {
> +		{
> +			.name		= "format",
> +			.callback	= event_callback,
> +		},
> +#ifdef CONFIG_PERF_EVENTS
> +		{
> +			.name		= "id",
> +			.callback	= event_callback,
> +		},
> +#endif
> +	};

Thinking about this hack again, it would be easier to maintain if I add
a readonly flag to each eventfs_entry and specify readonly when creating
the eventfs top directory.

Let me update it.

Thank you,


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