[PATCH v2] tracefs: Add show_events_dentries

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

Add a file in tracefs that shows the "events" allocated entries and the
dentries that are attached to them. This is used to see what dentries have
been dynamically allocated as well as their current ref counts.

 ~# cat /sys/kernel/tracing/events/sched/sched_switch/enable
 0
 ~# grep -A4 sched_switch /sys/kernel/tracing/show_events_dentries
 sched_switch/ dentry: (1)
 enable dentry: (0)
 id
 filter
 trigger

The first value is the name of the file or directory. If a dentry is
allocated, then a "dentry: (<ref-count>)" is displayed showing the address
of the dentry as well as its ref count.

Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
Changes since v1:  https://lore.kernel.org/linux-trace-kernel/20230913030021.550427998@xxxxxxxxxxx

 - Fixed the missed deletion of the structures from the event_inode.c file
   that was incorrectly in another patch

 - Removed addresses (no need to expose KASLR)

 - Added comments to explain how it works!

 - Now a stand alone patch to go in for this release

 fs/tracefs/Makefile         |   1 +
 fs/tracefs/event_inode.c    |  42 +----------
 fs/tracefs/event_show.c     | 147 ++++++++++++++++++++++++++++++++++++
 fs/tracefs/internal.h       |  44 +++++++++++
 include/linux/tracefs.h     |   2 +
 kernel/trace/trace_events.c |   3 +
 6 files changed, 198 insertions(+), 41 deletions(-)
 create mode 100644 fs/tracefs/event_show.c

diff --git a/fs/tracefs/Makefile b/fs/tracefs/Makefile
index 73c56da8e284..8f48f4fc6698 100644
--- a/fs/tracefs/Makefile
+++ b/fs/tracefs/Makefile
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 tracefs-objs	:= inode.o
 tracefs-objs	+= event_inode.o
+tracefs-objs	+= event_show.o
 
 obj-$(CONFIG_TRACING)	+= tracefs.o
 
diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 9f64e7332796..b23bb0957bb4 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -23,47 +23,7 @@
 #include <linux/delay.h>
 #include "internal.h"
 
-struct eventfs_inode {
-	struct list_head	e_top_files;
-};
-
-/*
- * struct eventfs_file - hold the properties of the eventfs files and
- *                       directories.
- * @name:	the name of the file or directory to create
- * @d_parent:   holds parent's dentry
- * @dentry:     once accessed holds dentry
- * @list:	file or directory to be added to parent directory
- * @ei:		list of files and directories within directory
- * @fop:	file_operations for file or directory
- * @iop:	inode_operations for file or directory
- * @data:	something that the caller will want to get to later on
- * @mode:	the permission that the file or directory should have
- */
-struct eventfs_file {
-	const char			*name;
-	struct dentry			*d_parent;
-	struct dentry			*dentry;
-	struct list_head		list;
-	struct eventfs_inode		*ei;
-	const struct file_operations	*fop;
-	const struct inode_operations	*iop;
-	/*
-	 * Union - used for deletion
-	 * @del_list:	list of eventfs_file to delete
-	 * @rcu:	eventfs_file to delete in RCU
-	 * @is_freed:	node is freed if one of the above is set
-	 */
-	union {
-		struct list_head	del_list;
-		struct rcu_head		rcu;
-		unsigned long		is_freed;
-	};
-	void				*data;
-	umode_t				mode;
-};
-
-static DEFINE_MUTEX(eventfs_mutex);
+DEFINE_MUTEX(eventfs_mutex);
 DEFINE_STATIC_SRCU(eventfs_srcu);
 
 static struct dentry *eventfs_root_lookup(struct inode *dir,
diff --git a/fs/tracefs/event_show.c b/fs/tracefs/event_show.c
new file mode 100644
index 000000000000..66dece7cc810
--- /dev/null
+++ b/fs/tracefs/event_show.c
@@ -0,0 +1,147 @@
+#include <linux/seq_file.h>
+#include <linux/tracefs.h>
+#include "internal.h"
+
+/*
+ * This will iterate three lists that correspond to the directory level
+ * of the eventfs directory.
+ *
+ * level 0 : /sys/kernel/tracing/events
+ * level 1 : /sys/kernel/tracing/events/<system>
+ * level 2 : /sys/kernel/tracing/events/<system>/event
+ *
+ * The iterator needs to see all levels as they all contain dynamically
+ * allocated dentries and inodes.
+ */
+struct event_list {
+	int			level;
+	struct list_head	*head[3];
+	struct list_head	*next[3];
+};
+
+static void *e_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	struct event_list *elist = m->private;
+	int level = elist->level;
+	struct list_head *head = elist->head[level];
+	struct list_head *next = elist->next[level];
+	struct eventfs_file *ef;
+
+	(*pos)++;
+
+	/* If next is equal to head, then the list is complete */
+	while (next == head) {
+		if (!level)
+			return NULL;
+
+		/* sublevel below top level, go up one */
+		elist->level = --level;
+		head = elist->head[level];
+		/* Going down does not update next, so do it here */
+		next = elist->next[level]->next;
+		elist->next[level] = next;
+	}
+
+	ef = list_entry(next, struct eventfs_file, list);
+
+	/* For each entry (not at the bottom) do a breadth first search */
+	if (ef->ei && !list_empty(&ef->ei->e_top_files) && level < 2) {
+		elist->level = ++level;
+		head = &ef->ei->e_top_files;
+		elist->head[level] = head;
+		next = head;
+		/*
+		 * Note, next is now pointing to the next sub level.
+		 * Need to update the next for the previous level on the way up.
+		 */
+	}
+
+	elist->next[level] = next->next;
+	return ef;
+}
+
+static void *e_start(struct seq_file *m, loff_t *pos)
+{
+	struct event_list *elist = m->private;
+	struct eventfs_file *ef = NULL;
+	loff_t l;
+
+	mutex_lock(&eventfs_mutex);
+
+	elist->level = 0;
+	elist->next[0] = elist->head[0]->next;
+
+	for (l = 0; l <= *pos; ) {
+		ef = e_next(m, ef, &l);
+		if (!ef)
+			break;
+	}
+	return ef;
+}
+
+static int e_show(struct seq_file *m, void *v)
+{
+	struct eventfs_file *ef = v;
+
+	seq_printf(m, "%s", ef->name);
+	if (ef->ei)
+		seq_putc(m, '/');
+
+	if (ef->dentry)
+		seq_printf(m, " dentry: (%d)", d_count(ef->dentry));
+	seq_putc(m, '\n');
+
+	return 0;
+}
+
+static void e_stop(struct seq_file *m, void *p)
+{
+	mutex_unlock(&eventfs_mutex);
+}
+
+static const struct seq_operations eventfs_show_dentry_seq_ops = {
+	.start = e_start,
+	.next = e_next,
+	.show = e_show,
+	.stop = e_stop,
+};
+
+static int
+eventfs_show_dentry_open(struct inode *inode, struct file *file)
+{
+	const struct seq_operations *seq_ops = &eventfs_show_dentry_seq_ops;
+	struct event_list *elist;
+	struct tracefs_inode *ti;
+	struct eventfs_inode *ei;
+	struct dentry *dentry;
+
+	/* The inode private should have the dentry of the "events" directory */
+	dentry = inode->i_private;
+	if (!dentry)
+		return -EINVAL;
+
+	elist = __seq_open_private(file, seq_ops, sizeof(*elist));
+	if (!elist)
+		return -ENOMEM;
+
+	ti = get_tracefs(dentry->d_inode);
+	ei = ti->private;
+
+	/*
+	 * Start off at level 0 (/sys/kernel/tracing/events)
+	 * Initialize head to the events files and next to the
+	 * first file.
+	 */
+	elist->level = 0;
+	elist->head[0] = &ei->e_top_files;
+	elist->next[0] = ei->e_top_files.next;
+
+	return 0;
+}
+
+const struct file_operations eventfs_show_dentry_fops = {
+	.open = eventfs_show_dentry_open,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = seq_release,
+};
diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index 4f2e49e2197b..461920f0133f 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -2,11 +2,55 @@
 #ifndef _TRACEFS_INTERNAL_H
 #define _TRACEFS_INTERNAL_H
 
+#include <linux/mutex.h>
+
 enum {
 	TRACEFS_EVENT_INODE		= BIT(1),
 	TRACEFS_EVENT_TOP_INODE		= BIT(2),
 };
 
+struct eventfs_inode {
+	struct list_head	e_top_files;
+};
+
+/*
+ * struct eventfs_file - hold the properties of the eventfs files and
+ *                       directories.
+ * @name:	the name of the file or directory to create
+ * @d_parent:   holds parent's dentry
+ * @dentry:     once accessed holds dentry
+ * @list:	file or directory to be added to parent directory
+ * @ei:		list of files and directories within directory
+ * @fop:	file_operations for file or directory
+ * @iop:	inode_operations for file or directory
+ * @data:	something that the caller will want to get to later on
+ * @mode:	the permission that the file or directory should have
+ */
+struct eventfs_file {
+	const char			*name;
+	struct dentry			*d_parent;
+	struct dentry			*dentry;
+	struct list_head		list;
+	struct eventfs_inode		*ei;
+	const struct file_operations	*fop;
+	const struct inode_operations	*iop;
+	/*
+	 * Union - used for deletion
+	 * @del_list:	list of eventfs_file to delete
+	 * @rcu:	eventfs_file to delete in RCU
+	 * @is_freed:	node is freed if one of the above is set
+	 */
+	union {
+		struct list_head	del_list;
+		struct rcu_head		rcu;
+		unsigned long		is_freed;
+	};
+	void				*data;
+	umode_t				mode;
+};
+
+extern struct mutex eventfs_mutex;
+
 struct tracefs_inode {
 	unsigned long           flags;
 	void                    *private;
diff --git a/include/linux/tracefs.h b/include/linux/tracefs.h
index 009072792fa3..f76c7d74b23d 100644
--- a/include/linux/tracefs.h
+++ b/include/linux/tracefs.h
@@ -58,6 +58,8 @@ struct dentry *tracefs_create_instance_dir(const char *name, struct dentry *pare
 
 bool tracefs_initialized(void);
 
+extern const struct file_operations eventfs_show_dentry_fops;
+
 #endif /* CONFIG_TRACING */
 
 #endif
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 91951d038ba4..5b0cc40910b2 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -3639,6 +3639,9 @@ create_event_toplevel_files(struct dentry *parent, struct trace_array *tr)
 		return -ENOMEM;
 	}
 
+	trace_create_file("show_events_dentries", TRACE_MODE_READ, parent, d_events,
+			  &eventfs_show_dentry_fops);
+
 	error = eventfs_add_events_file("enable", TRACE_MODE_WRITE, d_events,
 				  tr, &ftrace_tr_enable_fops);
 	if (error)
-- 
2.40.1