tracing: Deprecate auto-mounting tracefs in debugfs

In January 2015, tracefs was created to allow access to the tracing
infrastructure without needing to compile in debugfs. When tracefs is
configured, the directory /sys/kernel/tracing will exist and tooling is
expected to use that path to access the tracing infrastructure.

To allow backward compatibility, when debugfs is mounted, it would
automount tracefs in its "tracing" directory so that tooling that had hard
coded /sys/kernel/debug/tracing would still work.

It has been over 10 years since the new interface was introduced, and all
tooling should now be using it. Start the process of deprecating the old
path so that it doesn't need to be maintained anymore.

A new config is added to allow distributions to disable automounting of
tracefs on debugfs.

If /sys/kernel/debug/tracing is accessed, a pr_warn() will trigger stating:

"NOTICE: Automounting of tracing to debugfs is deprecated and will be removed in 2030"

Expect to remove this feature in 5 years (2030).

Cc: <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Ian Rogers <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

Documentation/ABI/obsolete/automount-tracefs-debugfs

@@ -0,0 +1,20 @@
+What:		/sys/kernel/debug/tracing
+Date:		May 2008
+KernelVersion:	2.6.27
+Contact:	[email protected]
+Description:
+
+	The ftrace was first added to the kernel, its interface was placed
+	into the debugfs file system under the "tracing" directory. Access
+	to the files were in /sys/kernel/debug/tracing. As systems wanted
+	access to the tracing interface without having to enable debugfs, a
+	new interface was created called "tracefs". This was a stand alone
+	file system and was usually mounted in /sys/kernel/tracing.
+
+	To allow older tooling to continue to operate, when mounting
+	debugfs, the tracefs file system would automatically get mounted in
+	the "tracing" directory of debugfs. The tracefs interface was added
+	in January 2015 in the v4.1 kernel.
+
+	All tooling should now be using tracefs directly and the "tracing"
+	directory in debugfs should be removed by January 2030.

kernel/trace/Kconfig

@@ -199,6 +199,19 @@ menuconfig FTRACE
 
 if FTRACE
 
+config TRACEFS_AUTOMOUNT_DEPRECATED
+	bool "Automount tracefs on debugfs [DEPRECATED]"
+	depends on TRACING
+	default y
+	help
+	  The tracing interface was moved from /sys/kernel/debug/tracing
+	  to /sys/kernel/tracing in 2015, but the tracing file system
+	  was still automounted in /sys/kernel/debug for backward
+	  compatibility with tooling.
+
+	  The new interface has been around for more than 10 years and
+	  the old debug mount will soon be removed.
+
 config BOOTTIME_TRACING
 	bool "Boot-time Tracing support"
 	depends on TRACING

kernel/trace/trace.c

@@ -6296,7 +6296,7 @@ static bool tracer_options_updated;
 static void add_tracer_options(struct trace_array *tr, struct tracer *t)
 {
 	/* Only enable if the directory has been created already. */
-	if (!tr->dir)
+	if (!tr->dir && !(tr->flags & TRACE_ARRAY_FL_GLOBAL))
 		return;
 
 	/* Only create trace option files after update_tracer_options finish */
@@ -8977,13 +8977,13 @@ static inline __init int register_snapshot_cmd(void) { return 0; }
 
 static struct dentry *tracing_get_dentry(struct trace_array *tr)
 {
-	if (WARN_ON(!tr->dir))
-		return ERR_PTR(-ENODEV);
-
 	/* Top directory uses NULL as the parent */
 	if (tr->flags & TRACE_ARRAY_FL_GLOBAL)
 		return NULL;
 
+	if (WARN_ON(!tr->dir))
+		return ERR_PTR(-ENODEV);
+
 	/* All sub buffers have a descriptor */
 	return tr->dir;
 }
@@ -10249,6 +10249,7 @@ init_tracer_tracefs(struct trace_array *tr, struct dentry *d_tracer)
 	ftrace_init_tracefs(tr, d_tracer);
 }
 
+#ifdef CONFIG_TRACEFS_AUTOMOUNT_DEPRECATED
 static struct vfsmount *trace_automount(struct dentry *mntpt, void *ingore)
 {
 	struct vfsmount *mnt;
@@ -10270,6 +10271,8 @@ static struct vfsmount *trace_automount(struct dentry *mntpt, void *ingore)
 	if (IS_ERR(fc))
 		return ERR_CAST(fc);
 
+	pr_warn("NOTICE: Automounting of tracing to debugfs is deprecated and will be removed in 2030\n");
+
 	ret = vfs_parse_fs_string(fc, "source",
 				  "tracefs", strlen("tracefs"));
 	if (!ret)
@@ -10280,6 +10283,7 @@ static struct vfsmount *trace_automount(struct dentry *mntpt, void *ingore)
 	put_fs_context(fc);
 	return mnt;
 }
+#endif
 
 /**
  * tracing_init_dentry - initialize top level trace array
@@ -10304,6 +10308,7 @@ int tracing_init_dentry(void)
 	if (WARN_ON(!tracefs_initialized()))
 		return -ENODEV;
 
+#ifdef CONFIG_TRACEFS_AUTOMOUNT_DEPRECATED
 	/*
 	 * As there may still be users that expect the tracing
 	 * files to exist in debugfs/tracing, we must automount
@@ -10312,6 +10317,7 @@ int tracing_init_dentry(void)
 	 */
 	tr->dir = debugfs_create_automount("tracing", NULL,
 					   trace_automount, NULL);
+#endif
 
 	return 0;
 }