dynamic_debug: allow to work if debugfs is disabled

With the realization that having debugfs enabled on "production" systems
is generally not a good idea, debugfs is being disabled from more and
more platforms over time.  However, the functionality of dynamic
debugging still is needed at times, and since it relies on debugfs for
its user api, having debugfs disabled also forces dynamic debug to be
disabled.

To get around this, also create the "control" file for dynamic_debug in
procfs.  This allows people turn on debugging as needed at runtime for
individual driverfs and subsystems.

Reported-by: many different companies
Cc: Jason Baron <jbaron@akamai.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200210211142.GB1373304@kroah.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/dynamic-debug-howto.rst

@@ -54,6 +54,9 @@ If you make a mistake with the syntax, the write will fail thus::
 				<debugfs>/dynamic_debug/control
   -bash: echo: write error: Invalid argument
 
+Note, for systems without 'debugfs' enabled, the control file can be
+found in ``/proc/dynamic_debug/control``.
+
 Viewing Dynamic Debug Behaviour
 ===============================
 

lib/Kconfig.debug

@@ -98,7 +98,7 @@ config DYNAMIC_DEBUG
 	bool "Enable dynamic printk() support"
 	default n
 	depends on PRINTK
-	depends on DEBUG_FS
+	depends on (DEBUG_FS || PROC_FS)
 	help
 
 	  Compiles debug level messages into the kernel, which would not
@@ -116,8 +116,9 @@ config DYNAMIC_DEBUG
 	  Usage:
 
 	  Dynamic debugging is controlled via the 'dynamic_debug/control' file,
-	  which is contained in the 'debugfs' filesystem. Thus, the debugfs
-	  filesystem must first be mounted before making use of this feature.
+	  which is contained in the 'debugfs' filesystem or procfs.
+	  Thus, the debugfs or procfs filesystem must first be mounted before
+	  making use of this feature.
 	  We refer the control file as: <debugfs>/dynamic_debug/control. This
 	  file contains a list of the debug statements that can be enabled. The
 	  format for each line of the file is:

lib/dynamic_debug.c

@@ -876,6 +876,14 @@ static const struct file_operations ddebug_proc_fops = {
 	.write = ddebug_proc_write
 };
 
+static const struct proc_ops proc_fops = {
+	.proc_open = ddebug_proc_open,
+	.proc_read = seq_read,
+	.proc_lseek = seq_lseek,
+	.proc_release = seq_release_private,
+	.proc_write = ddebug_proc_write
+};
+
 /*
  * Allocate a new ddebug_table for the given module
  * and add it to the global list.
@@ -991,15 +999,25 @@ static void ddebug_remove_all_tables(void)
 
 static __initdata int ddebug_init_success;
 
-static int __init dynamic_debug_init_debugfs(void)
+static int __init dynamic_debug_init_control(void)
 {
-	struct dentry *dir;
+	struct proc_dir_entry *procfs_dir;
+	struct dentry *debugfs_dir;
 
 	if (!ddebug_init_success)
 		return -ENODEV;
 
-	dir = debugfs_create_dir("dynamic_debug", NULL);
-	debugfs_create_file("control", 0644, dir, NULL, &ddebug_proc_fops);
+	/* Create the control file in debugfs if it is enabled */
+	if (debugfs_initialized()) {
+		debugfs_dir = debugfs_create_dir("dynamic_debug", NULL);
+		debugfs_create_file("control", 0644, debugfs_dir, NULL,
+				    &ddebug_proc_fops);
+	}
+
+	/* Also create the control file in procfs */
+	procfs_dir = proc_mkdir("dynamic_debug", NULL);
+	if (procfs_dir)
+		proc_create("control", 0644, procfs_dir, &proc_fops);
 
 	return 0;
 }
@@ -1077,4 +1095,4 @@ static int __init dynamic_debug_init(void)
 early_initcall(dynamic_debug_init);
 
 /* Debugfs setup must be done later */
-fs_initcall(dynamic_debug_init_debugfs);
+fs_initcall(dynamic_debug_init_control);