On Wed, 7 Jan 2026 11:41:33 -0500
Steven Rostedt [off-list ref] wrote:

On Wed,  7 Jan 2026 23:45:59 +0900
"Masami Hiramatsu (Google)" [off-list ref] wrote:

quoted

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Since there is no reason to reuse the backup instance, make it
readonly. 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.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 kernel/trace/trace.c        |   91 ++++++++++++++++++++++++++++++++-----------
 kernel/trace/trace.h        |    6 +++
 kernel/trace/trace_events.c |   14 +++++--
 3 files changed, 84 insertions(+), 27 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 38f7a7a55c23..725930f5980e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c

@@ -4888,6 +4888,9 @@ static int tracing_open(struct inode *inode, struct file *file)
 		int cpu = tracing_get_cpu(inode);
 		struct array_buffer *trace_buf = &tr->array_buffer;
 
+		if (trace_array_is_readonly(tr))
+			return -EPERM;

So this fails if someone opens a file in RDONLY mode?

Why?

Ah, that's a bug. Let me fix it.

quoted

+
 #ifdef CONFIG_TRACER_MAX_TRACE
 		if (tr->current_trace->print_max)
 			trace_buf = &tr->max_buffer;

@@ -6077,6 +6080,9 @@ static int __tracing_resize_ring_buffer(struct trace_array *tr,
 ssize_t tracing_resize_ring_buffer(struct trace_array *tr,
 				  unsigned long size, int cpu_id)
 {
+	if (trace_array_is_readonly(tr))
+		return -EPERM;

In fact, I don't think we need any of these.

Would you mean we should check readonly 

quoted

+
 	guard(mutex)(&trace_types_lock);
 
 	if (cpu_id != RING_BUFFER_ALL_CPUS) {

quoted

@@ -9353,12 +9381,16 @@ static void
 tracing_init_tracefs_percpu(struct trace_array *tr, long cpu)
 {
 	struct dentry *d_percpu = tracing_dentry_percpu(tr, cpu);
+	umode_t writable_mode = TRACE_MODE_WRITE;
 	struct dentry *d_cpu;
 	char cpu_dir[30]; /* 30 characters should be more than enough */
 
 	if (!d_percpu)
 		return;
 
+	if (trace_array_is_readonly(tr))
+		writable_mode = TRACE_MODE_READ;

This is more like what we should do with all the files in a read-only
instance. Just make all files not allow writes.

Actually, that's my first prototype but it did not work (at least on
tracefs).
Superuser can write anything unless the file does not have .write
operation.

We may need to make sure they can't be changed to write as well. But that
will require a change to tracefs (and eventfs).

Ah, you mean the permission check is not correctly done in tracefs/eventfs yet?

Thank you,

-- Steve

quoted

+
 	snprintf(cpu_dir, 30, "cpu%ld", cpu);
 	d_cpu = tracefs_create_dir(cpu_dir, d_percpu);
 	if (!d_cpu) {

@@ -9371,7 +9403,7 @@ tracing_init_tracefs_percpu(struct trace_array *tr, long cpu)
 				tr, cpu, &tracing_pipe_fops);
 
 	/* per cpu trace */
-	trace_create_cpu_file("trace", TRACE_MODE_WRITE, d_cpu,
+	trace_create_cpu_file("trace", writable_mode, d_cpu,
 				tr, cpu, &tracing_fops);
 
 	trace_create_cpu_file("trace_pipe_raw", TRACE_MODE_READ, d_cpu,

-- 
Masami Hiramatsu (Google) [off-list ref]