[lttng-dev] [PATCH] Expose kernel tracer to user-space (version 6)

[Francis Giraldeau]

By writing to the file /proc/lttng_uevent, a user-space application creates a
kernel event. The event's payload is by default UTF-8 text, but any data can be
written, up to 1024 bytes. Null-character is optional and is not enforced. The
event uses sequence for space efficiency and to store any data as payload.

The feature is enabled when the module lttng-uevent is loaded. The module can
be removed if no tracing sessions with lttng_uevent enabled is active.

Signed-off-by: Francis Giraldeau <francis.giraldeau at gmail.com>
---
 instrumentation/events/lttng-module/uevent.h |   33 +++++++++
 probes/Makefile                              |    1 +
 probes/lttng-uevent.c                        |   97 ++++++++++++++++++++++++++
 3 files changed, 131 insertions(+)
 create mode 100644 instrumentation/events/lttng-module/uevent.h
 create mode 100644 probes/lttng-uevent.c

diff --git a/instrumentation/events/lttng-module/uevent.h b/instrumentation/events/lttng-module/uevent.h
new file mode 100644
index 0000000..f67d901
--- /dev/null
+++ b/instrumentation/events/lttng-module/uevent.h
@@ -0,0 +1,33 @@
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM uevent
+
+#if !defined(UEVENT_H_) || defined(TRACE_HEADER_MULTI_READ)
+#define UEVENT_H_
+
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(lttng_uevent,
+
+	TP_PROTO(const char *str, size_t len),
+
+	TP_ARGS(str, len),
+
+	/*
+	 * Uses sequence to hold variable size data, by default considered
+	 * as text. Null-terminal character is optional and is not enforced.
+	 */
+	TP_STRUCT__entry(
+		__dynamic_array_text(char, text, len)
+	),
+
+	TP_fast_assign(
+		tp_memcpy_dyn_from_user(text, str)
+	),
+
+	TP_printk("")
+)
+
+#endif /* UEVENT_H_ */
+
+/* This part must be outside protection */
+#include "../../../probes/define_trace.h"
diff --git a/probes/Makefile b/probes/Makefile
index 698a9c9..3003535 100644
--- a/probes/Makefile
+++ b/probes/Makefile
@@ -14,6 +14,7 @@ obj-m += lttng-probe-sched.o
 obj-m += lttng-probe-irq.o
 obj-m += lttng-probe-signal.o
 obj-m += lttng-probe-timer.o
+obj-m += lttng-uevent.o
 
 obj-m += lttng-probe-statedump.o
 
diff --git a/probes/lttng-uevent.c b/probes/lttng-uevent.c
new file mode 100644
index 0000000..96fb233
--- /dev/null
+++ b/probes/lttng-uevent.c
@@ -0,0 +1,97 @@
+/*
+ * probes/lttng-uevent.c
+ *
+ * Expose kernel tracer to user-space through /proc/lttng
+ *
+ * Copyright (C) 2009-2012 Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; only
+ * version 2.1 of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <linux/module.h>
+#include <linux/proc_fs.h>
+
+/*
+ * Create lttng_uevent tracepoint probes.
+ */
+
+#define TP_MODULE_NOAUTOLOAD
+#define LTTNG_PACKAGE_BUILD
+#define CREATE_TRACE_POINTS
+#define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
+#include "../instrumentation/events/lttng-module/uevent.h"
+
+#define LTTNG_UEVENT_FILE "lttng_uevent"
+
+/**
+ * lttng_uevent_write - write user-space data into kernel trace
+ * @file: file pointer
+ * @user_buf: user string
+ * @count: length to copy
+ * @ppos: unused
+ *
+ * Copy count bytes into a trace event "lttng_uevent".
+ *
+ * Returns the number of bytes copied from the source.
+ * Notice that there is no guarantee that the event is
+ * actually written in a trace. This can occur in 3
+ * situations:
+ *
+ *   * Buffer overrun
+ *   * No trace session are active
+ *   * The data size is greater than a sub-buffer
+ */
+
+ssize_t lttng_uevent_write(struct file *file, const char __user *ubuf,
+		size_t count, loff_t *fpos)
+{
+	trace_lttng_uevent(ubuf, count);
+	return count;
+}
+
+static const struct file_operations uev_ops = {
+	.owner = THIS_MODULE,
+	.write = lttng_uevent_write
+};
+
+static int __init lttng_uevent_init(void)
+{
+	struct proc_dir_entry *uev_file;
+
+	uev_file = create_proc_entry(LTTNG_UEVENT_FILE, 0444, NULL);
+	if (!uev_file)
+		return -ENOENT;
+	uev_file->proc_fops = &uev_ops;
+	uev_file->mode = S_IFREG | S_IWUGO;
+	uev_file->uid = 0;
+	uev_file->gid = 0;
+	/* register manually the probe */
+	__lttng_events_init__uevent();
+	return 0;
+}
+
+static void __exit lttng_uevent_exit(void)
+{
+	/* unregister manually the probe */
+	__lttng_events_exit__uevent();
+	remove_proc_entry(LTTNG_UEVENT_FILE, NULL);
+}
+
+module_init(lttng_uevent_init);
+module_exit(lttng_uevent_exit);
+
+MODULE_LICENSE("GPL and additional rights");
+MODULE_AUTHOR("Francis Giraldeau <francis.giraldeau at gmail.com>");
+MODULE_DESCRIPTION("Append custom events to kernel trace from user-space");
-- 
1.7.9.5