[PATCH 05/13] tracing/remotes: Add printk tracefs file

[Vincent Donnefort]

When enabled, the printk tracefs file enables the redirection of all
events to dmesg. This is similar to tp_printk.

Signed-off-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>

diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
index 21583fae1bd9..1bf0ba159c92 100644
--- a/kernel/trace/trace_remote.c
+++ b/kernel/trace/trace_remote.c
@@ -21,6 +21,7 @@
 enum tri_type {
 	TRI_CONSUMING,
 	TRI_NONCONSUMING,
+	TRI_PRINTK,
 };
 
 struct trace_remote_iterator {
@@ -42,6 +43,7 @@ struct trace_remote {
 	void				*priv;
 	struct trace_buffer		*trace_buffer;
 	struct trace_buffer_desc	*trace_buffer_desc;
+	struct trace_remote_iterator	*printk;
 	struct dentry			*dentry;
 	struct eventfs_inode		*eventfs_root;
 	struct eventfs_inode		*eventfs_subdir;
@@ -335,6 +337,8 @@ static int __alloc_ring_buffer_iter(struct trace_remote_iterator *iter, int cpu)
 	return 0;
 }
 
+static void trace_remote_do_printk(struct trace_remote *remote);
+
 static void __poll_remote(struct work_struct *work)
 {
 	struct delayed_work *dwork = to_delayed_work(work);
@@ -342,6 +346,7 @@ static void __poll_remote(struct work_struct *work)
 
 	remote = container_of(dwork, struct trace_remote, poll_work);
 	ring_buffer_poll_remote(remote->trace_buffer, RING_BUFFER_ALL_CPUS);
+	trace_remote_do_printk(remote);
 
 	schedule_delayed_work(dwork, msecs_to_jiffies(remote->poll_ms));
 }
@@ -351,6 +356,8 @@ static void trace_remote_inc_poll(struct trace_remote *remote)
 	/* poll_cnt <= nr_readers, inherits its overflow protection */
 	if (!remote->poll_cnt++) {
 		ring_buffer_poll_remote(remote->trace_buffer, RING_BUFFER_ALL_CPUS);
+		trace_remote_do_printk(remote);
+
 		schedule_delayed_work(&remote->poll_work, msecs_to_jiffies(remote->poll_ms));
 	}
 }
@@ -393,6 +400,14 @@ static struct trace_remote_iterator
 		trace_seq_init(&iter->seq);
 
 		switch (type) {
+		case TRI_PRINTK:
+			/* only one printk iter allowed */
+			if (WARN_ON_ONCE(remote->printk)) {
+				ret = -EBUSY;
+				break;
+			}
+			smp_store_release(&remote->printk, iter);
+			fallthrough;
 		case TRI_CONSUMING:
 			trace_remote_inc_poll(remote);
 			break;
@@ -427,6 +442,11 @@ static void trace_remote_iter_free(struct trace_remote_iterator *iter)
 	lockdep_assert_held(&remote->lock);
 
 	switch (iter->type) {
+	case TRI_PRINTK:
+		WARN_ON_ONCE(remote->printk != iter);
+		smp_store_release(&remote->printk, NULL);
+		flush_delayed_work(&remote->poll_work);
+		fallthrough;
 	case TRI_CONSUMING:
 		trace_remote_dec_poll(remote);
 		break;
@@ -504,6 +524,7 @@ __peek_event(struct trace_remote_iterator *iter, int cpu, u64 *ts, unsigned long
 	struct ring_buffer_iter *rb_iter;
 
 	switch (iter->type) {
+	case TRI_PRINTK:
 	case TRI_CONSUMING:
 		return ring_buffer_peek(iter->remote->trace_buffer, cpu, ts, lost_events);
 	case TRI_NONCONSUMING:
@@ -571,6 +592,7 @@ static void trace_remote_iter_move(struct trace_remote_iterator *iter)
 	struct trace_buffer *trace_buffer = iter->remote->trace_buffer;
 
 	switch (iter->type) {
+	case TRI_PRINTK:
 	case TRI_CONSUMING:
 		ring_buffer_consume(trace_buffer, iter->evt_cpu, NULL, NULL);
 		break;
@@ -814,6 +836,80 @@ static const struct file_operations trace_fops = {
 	.release	= trace_release,
 };
 
+static void trace_remote_do_printk(struct trace_remote *remote)
+{
+	struct trace_remote_iterator *iter = smp_load_acquire(&remote->printk);
+
+	if (!iter)
+		return;
+
+	trace_remote_iter_read_start(iter);
+
+	while (trace_remote_iter_read_event(iter)) {
+		trace_seq_init(&iter->seq);
+
+		trace_remote_iter_print_event(iter);
+		if (!pr_emerg("%s", iter->seq.buffer))
+			break;
+
+		trace_remote_iter_move(iter);
+	}
+
+	trace_remote_iter_read_finished(iter);
+}
+
+static int trace_remote_enable_printk(struct trace_remote *remote, bool enable)
+{
+	struct trace_remote_iterator *iter = remote->printk;
+
+	lockdep_assert_held(&remote->lock);
+
+	if (enable == !!iter)
+		return 0;
+
+	if (enable) {
+		iter = trace_remote_iter(remote, RING_BUFFER_ALL_CPUS, TRI_PRINTK);
+		if (IS_ERR(iter))
+			return PTR_ERR(iter);
+	} else {
+		trace_remote_iter_free(remote->printk);
+		/* trace_remote_iter_free has reset remote->printk */
+	}
+
+	return 0;
+}
+
+static ssize_t
+printk_write(struct file *filp, const char __user *ubuf, size_t cnt, loff_t *ppos)
+{
+	struct seq_file *seq = filp->private_data;
+	struct trace_remote *remote = seq->private;
+	bool val;
+	int ret;
+
+	ret = kstrtobool_from_user(ubuf, cnt, &val);
+	if (ret)
+		return ret;
+
+	guard(mutex)(&remote->lock);
+
+	ret = trace_remote_enable_printk(remote, val);
+	if (ret)
+		return ret;
+
+	return cnt;
+}
+
+static int printk_show(struct seq_file *s, void *unused)
+{
+	struct trace_remote *remote = s->private;
+
+	seq_printf(s, "%d\n", !!remote->printk);
+
+	return 0;
+}
+DEFINE_SHOW_STORE_ATTRIBUTE(printk);
+
 static struct dentry *tracefs_root;
 static DEFINE_MUTEX(tracefs_lock);
 static u64 tracefs_root_count;
@@ -858,6 +954,10 @@ static int trace_remote_init_tracefs(const char *name, struct trace_remote *remo
 	if (!d)
 		goto err;
 
+	d = trace_create_file("printk", TRACEFS_MODE_WRITE, remote_d, remote, &printk_fops);
+	if (!d)
+		goto err;
+
 	percpu_d = tracefs_create_dir("per_cpu", remote_d);
 	if (!percpu_d) {
 		pr_err("Failed to create tracefs dir "TRACEFS_DIR"%s/per_cpu/\n", name);
-- 
2.54.0.1032.g2f8565e1d1-goog