linux-kernel - Re: [PATCH v6 2/4] tracing: Make the backup instance non-reusable

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Message-ID: <20260204211721.74e501f0@robin>
Date: Wed, 4 Feb 2026 21:17:21 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/4] tracing: Make the backup instance non-reusable

On Sun,  1 Feb 2026 12:29:15 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> 
> Since there is no reason to reuse the backup instance, make it
> readonly (but erasable).
> Note that only backup instances are readonly, because
> other trace instances will be empty unless it is writable.
> Only backup instances have copy entries from the original.
> 
> With this change, most of the trace control files are removed
> from the backup instance, including eventfs enable/filter etc.
> 
>  # find /sys/kernel/tracing/instances/backup/events/ | wc -l
>  4093
>  # find /sys/kernel/tracing/instances/boot_map/events/ | wc -l
>  9573
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> ---
>  Changes in v6:
>    - Remove tracing_on file from readonly instances.
>    - Remove unused writable_mode from tracing_init_tracefs_percpu().
>    - Cleanup init_tracer_tracefs() and create_event_toplevel_files().
>    - Remove TRACE_MODE_WRITE_MASK.
>    - Add TRACE_ARRAY_FL_RDONLY.
>  Changes in v5:
>    - Rebased on the latest for-next (and hide show_event_filters/triggers
>      if the instance is readonly.
>  Changes in v4:
>   - Make trace data erasable. (not reusable)
>  Changes in v3:
>   - Resuse the beginning part of event_entries for readonly files.
>   - Remove readonly file_operations and checking readonly flag in
>     each write operation.
>  Changes in v2:
>   - Use readonly file_operations to prohibit writing instead of
>     checking flags in write() callbacks.
>   - Remove writable files from eventfs.
> ---
>  kernel/trace/trace.c        |   94 +++++++++++++++++++++++++++++--------------
>  kernel/trace/trace.h        |    7 +++
>  kernel/trace/trace_boot.c   |    5 +-
>  kernel/trace/trace_events.c |   76 ++++++++++++++++++++---------------
>  4 files changed, 117 insertions(+), 65 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 5c3e4a554143..b0efcf1e0809 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -5052,6 +5052,11 @@ static ssize_t
>  tracing_write_stub(struct file *filp, const char __user *ubuf,
>  		   size_t count, loff_t *ppos)
>  {
> +	struct trace_array *tr = file_inode(filp)->i_private;
> +
> +	if (trace_array_is_readonly(tr))
> +		return -EPERM;
> +
>  	return count;
>  }
>  
> @@ -5152,6 +5157,9 @@ tracing_cpumask_write(struct file *filp, const char __user *ubuf,
>  	cpumask_var_t tracing_cpumask_new;
>  	int err;
>  
> +	if (trace_array_is_readonly(tr))
> +		return -EPERM;
> +

Shouldn't these checks be done in the open function? Doing it now is
too late, as -EPERM on a write is confusing when the open for write
succeeds.

-- Steve

>  	if (count == 0 || count > KMALLOC_MAX_SIZE)
>  		return -EINVAL;
>