[PATCH v9 14/30] tracing: Add a trace remote module for testing

Vincent Donnefort
2025-12-02T09:36:46Z

Add a module to help testing the tracefs support for trace remotes. This
module:

* Use simple_ring_buffer to write into a ring-buffer.
* Declare a single "selftest" event that can be triggered from
user-space.
* Register a "test" trace remote.

This is intended to be used by trace remote selftests.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 0c023941a316..aa305be834f9 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -665,6 +665,7 @@ struct dentry *tracefs_create_file(const char *name, umode_t mode,
fsnotify_create(d_inode(dentry->d_parent), dentry);
return tracefs_end_creating(dentry);
}
+EXPORT_SYMBOL_GPL(tracefs_create_file);

static struct dentry *__create_dir(const char *name, struct dentry *parent,
const struct inode_operations *ops)
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 918afcc1fcaf..52131d89993c 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -1244,4 +1244,12 @@ config TRACE_REMOTE
config SIMPLE_RING_BUFFER
bool

+config TRACE_REMOTE_TEST
+ tristate "Test module for remote tracing"
+ select TRACE_REMOTE
+ select SIMPLE_RING_BUFFER
+ help
+ This trace remote includes a ring-buffer writer implementation using
+ "simple_ring_buffer". This is solely intending for testing.
+
endif # FTRACE
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 03d7d80a9436..53534447e70b 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -112,4 +112,6 @@ obj-$(CONFIG_RV) += rv/

obj-$(CONFIG_TRACE_REMOTE) += trace_remote.o
obj-$(CONFIG_SIMPLE_RING_BUFFER) += simple_ring_buffer.o
+obj-$(CONFIG_TRACE_REMOTE_TEST) += remote_test.o
+
libftrace-y := ftrace.o
diff --git a/kernel/trace/remote_test.c b/kernel/trace/remote_test.c
new file mode 100644
index 000000000000..059127489c99
--- /dev/null
+++ b/kernel/trace/remote_test.c
@@ -0,0 +1,259 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2025 - Google LLC
+ * Author: Vincent Donnefort <vdonnefort@google.com>
+ */
+
+#include <linux/module.h>
+#include <linux/simple_ring_buffer.h>
+#include <linux/trace_remote.h>
+#include <linux/tracefs.h>
+#include <linux/types.h>
+
+#define REMOTE_EVENT_INCLUDE_FILE kernel/trace/remote_test_events.h
+#include <trace/define_remote_events.h>
+
+static DEFINE_PER_CPU(struct simple_rb_per_cpu *, simple_rbs);
+static struct trace_buffer_desc *remote_test_buffer_desc;
+
+/*
+ * The trace_remote lock already serializes accesses from the trace_remote_callbacks.
+ * However write_event can still race with load/unload.
+ */
+static DEFINE_MUTEX(simple_rbs_lock);
+
+static int remote_test_load_simple_rb(int cpu, struct ring_buffer_desc *rb_desc)
+{
+ struct simple_rb_per_cpu *cpu_buffer;
+ struct simple_buffer_page *bpages;
+ int ret = -ENOMEM;
+
+ cpu_buffer = kmalloc(sizeof(*cpu_buffer), GFP_KERNEL);
+ if (!cpu_buffer)
+ return ret;
+
+ bpages = kmalloc_array(rb_desc->nr_page_va, sizeof(*bpages), GFP_KERNEL);
+ if (!bpages)
+ goto err_free_cpu_buffer;
+
+ ret = simple_ring_buffer_init(cpu_buffer, bpages, rb_desc);
+ if (ret)
+ goto err_free_bpages;
+
+ scoped_guard(mutex, &simple_rbs_lock)
+ *per_cpu_ptr(&simple_rbs, cpu) = cpu_buffer;
+
+ return 0;
+
+err_free_bpages:
+ kfree(bpages);
+
+err_free_cpu_buffer:
+ kfree(cpu_buffer);
+
+ return ret;
+}
+
+static void remote_test_unload_simple_rb(int cpu)
+{
+ struct simple_rb_per_cpu *cpu_buffer = *per_cpu_ptr(&simple_rbs, cpu);
+ struct simple_buffer_page *bpages;
+
+ if (!cpu_buffer)
+ return;
+
+ guard(mutex)(&simple_rbs_lock);
+
+ bpages = cpu_buffer->bpages;
+ simple_ring_buffer_unload(cpu_buffer);
+ kfree(bpages);
+ kfree(cpu_buffer);
+ *per_cpu_ptr(&simple_rbs, cpu) = NULL;
+}
+
+static struct trace_buffer_desc *remote_test_load(unsigned long size, void *unused)
+{
+ struct ring_buffer_desc *rb_desc;
+ struct trace_buffer_desc *desc;
+ size_t desc_size;
+ int cpu, ret;
+
+ if (WARN_ON(remote_test_buffer_desc))
+ return ERR_PTR(-EINVAL);
+
+ desc_size = trace_buffer_desc_size(size, num_possible_cpus());
+ if (desc_size == SIZE_MAX) {
+ ret = -E2BIG;
+ goto err_unlock_cpus;
+ }
+
+ desc = kmalloc(desc_size, GFP_KERNEL);
+ if (!desc) {
+ ret = -ENOMEM;
+ goto err_unlock_cpus;
+ }
+
+ ret = trace_remote_alloc_buffer(desc, desc_size, size, cpu_possible_mask);
+ if (ret)
+ goto err_free_desc;
+
+ for_each_ring_buffer_desc(rb_desc, cpu, desc) {
+ ret = remote_test_load_simple_rb(rb_desc->cpu, rb_desc);
+ if (ret)
+ goto err;
+ }
+
+ remote_test_buffer_desc = desc;
+
+ return remote_test_buffer_desc;
+
+err:
+ for_each_ring_buffer_desc(rb_desc, cpu, remote_test_buffer_desc)
+ remote_test_unload_simple_rb(rb_desc->cpu);
+ trace_remote_free_buffer(remote_test_buffer_desc);
+
+err_free_desc:
+ kfree(desc);
+
+err_unlock_cpus:
+ cpus_read_unlock();
+
+ return ERR_PTR(ret);
+}
+
+static void remote_test_unload(struct trace_buffer_desc *desc, void *unused)
+{
+ struct ring_buffer_desc *rb_desc;
+ int cpu;
+
+ if (WARN_ON(desc != remote_test_buffer_desc))
+ return;
+
+ for_each_ring_buffer_desc(rb_desc, cpu, desc)
+ remote_test_unload_simple_rb(rb_desc->cpu);
+
+ remote_test_buffer_desc = NULL;
+ trace_remote_free_buffer(desc);
+ kfree(desc);
+}
+
+static int remote_test_enable_tracing(bool enable, void *unused)
+{
+ struct ring_buffer_desc *rb_desc;
+ int cpu;
+
+ if (!remote_test_buffer_desc)
+ return -ENODEV;
+
+ for_each_ring_buffer_desc(rb_desc, cpu, remote_test_buffer_desc)
+ WARN_ON(simple_ring_buffer_enable_tracing(*per_cpu_ptr(&simple_rbs, rb_desc->cpu),
+ enable));
+ return 0;
+}
+
+static int remote_test_swap_reader_page(unsigned int cpu, void *unused)
+{
+ struct simple_rb_per_cpu *cpu_buffer;
+
+ if (cpu >= NR_CPUS)
+ return -EINVAL;
+
+ cpu_buffer = *per_cpu_ptr(&simple_rbs, cpu);
+ if (!cpu_buffer)
+ return -EINVAL;
+
+ return simple_ring_buffer_swap_reader_page(cpu_buffer);
+}
+
+static int remote_test_reset(unsigned int cpu, void *unused)
+{
+ struct simple_rb_per_cpu *cpu_buffer;
+
+ if (cpu >= NR_CPUS)
+ return -EINVAL;
+
+ cpu_buffer = *per_cpu_ptr(&simple_rbs, cpu);
+ if (!cpu_buffer)
+ return -EINVAL;
+
+ return simple_ring_buffer_reset(cpu_buffer);
+}
+
+static int remote_test_enable_event(unsigned short id, bool enable, void *unused)
+{
+ if (id != REMOTE_TEST_EVENT_ID)
+ return -EINVAL;
+
+ /*
+ * Let's just use the struct remote_event enabled field that is turned on and off by
+ * trace_remote. This is a bit racy but good enough for a simple test module.
+ */
+ return 0;
+}
+
+static ssize_t
+write_event_write(struct file *filp, const char __user *ubuf, size_t cnt, loff_t *pos)
+{
+ struct remote_event_format_selftest *evt_test;
+ struct simple_rb_per_cpu *cpu_buffer;
+ unsigned long val;
+ int ret;
+
+ ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
+ if (ret)
+ return ret;
+
+ guard(mutex)(&simple_rbs_lock);
+
+ if (!remote_event_selftest.enabled)
+ return -ENODEV;
+
+ cpu_buffer = *this_cpu_ptr(&simple_rbs);
+ if (!cpu_buffer)
+ return -ENODEV;
+
+ evt_test = simple_ring_buffer_reserve(cpu_buffer,
+ sizeof(struct remote_event_format_selftest),
+ trace_clock_global());
+ if (!evt_test)
+ return -ENODEV;
+
+ evt_test->hdr.id = REMOTE_TEST_EVENT_ID;
+ evt_test->id = val;
+
+ simple_ring_buffer_commit(cpu_buffer);
+
+ return cnt;
+}
+
+static const struct file_operations write_event_fops = {
+ .write = write_event_write,
+};
+
+static int remote_test_init_tracefs(struct dentry *d, void *unused)
+{
+ return tracefs_create_file("write_event", 0200, d, NULL, &write_event_fops) ?
+ 0 : -ENOMEM;
+}
+
+static struct trace_remote_callbacks trace_remote_callbacks = {
+ .init = remote_test_init_tracefs,
+ .load_trace_buffer = remote_test_load,
+ .unload_trace_buffer = remote_test_unload,
+ .enable_tracing = remote_test_enable_tracing,
+ .swap_reader_page = remote_test_swap_reader_page,
+ .reset = remote_test_reset,
+ .enable_event = remote_test_enable_event,
+};
+
+static int __init remote_test_init(void)
+{
+ return trace_remote_register("test", &trace_remote_callbacks, NULL,
+ &remote_event_selftest, 1);
+}
+
+module_init(remote_test_init);
+
+MODULE_DESCRIPTION("Test module for the trace remote interface");
+MODULE_AUTHOR("Vincent Donnefort");
+MODULE_LICENSE("GPL");
diff --git a/kernel/trace/remote_test_events.h b/kernel/trace/remote_test_events.h
new file mode 100644
index 000000000000..26b93b3406fc
--- /dev/null
+++ b/kernel/trace/remote_test_events.h
@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#define REMOTE_TEST_EVENT_ID 1
+
+REMOTE_EVENT(selftest, REMOTE_TEST_EVENT_ID,
+ RE_STRUCT(
+ re_field(u64, id)
+ ),
+ RE_PRINTK("id=%llu", __entry->id)
+);