debugfs: Remove broken no-mount mode

debugfs access modes were added in Linux 5.10 (Dec 2020) [1], but the
no-mount mode has behaved effectively the same as the off mode since
Linux 5.12 (Apr 2021) [2]. The only difference is the specific error
code returned by the debugfs_create_* functions, which is -ENOENT in
no-mount mode and -EPERM in off mode.

Given that no-mount hasn't worked for several years with no complaints,
just remove it.

[1] a24c6f7 ("debugfs: Add access restriction option")

[2] bc6de80 ("debugfs: be more robust at handling improper input in debugfs_lookup()")
    5634856 ("debugfs: do not attempt to create a new file before the filesystem is initalized")

Signed-off-by: Aaron Thompson <dev@aaront.org>
Link: https://patch.msgid.link/20251120102222.18371-3-dev@null.aaront.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dev-aaront-org

Documentation/admin-guide/kernel-parameters.txt

@@ -1113,12 +1113,8 @@
 
 	debugfs=    	[KNL,EARLY] This parameter enables what is exposed to
 			userspace and debugfs internal clients.
-			Format: { on, no-mount, off }
+			Format: { on, off }
 			on: 	All functions are enabled.
-			no-mount:
-				Filesystem is not registered but kernel clients can
-			        access APIs and a crashkernel can be used to read
-				its content. There is nothing to mount.
 			off: 	Filesystem is not registered and clients
 			        get a -EPERM as result when trying to register files
 				or directories within debugfs.

fs/debugfs/inode.c

@@ -35,7 +35,7 @@
 static struct vfsmount *debugfs_mount;
 static int debugfs_mount_count;
 static bool debugfs_registered;
-static unsigned int debugfs_allow __ro_after_init = DEFAULT_DEBUGFS_ALLOW_BITS;
+static bool debugfs_enabled __ro_after_init = IS_ENABLED(DEBUG_FS_ALLOW_ALL);
 
 /*
  * Don't allow access attributes to be changed whilst the kernel is locked down
@@ -365,7 +365,7 @@ static struct dentry *debugfs_start_creating(const char *name,
 	struct dentry *dentry;
 	int error;
 
-	if (!(debugfs_allow & DEBUGFS_ALLOW_API))
+	if (!debugfs_enabled)
 		return ERR_PTR(-EPERM);
 
 	if (!debugfs_initialized())
@@ -885,21 +885,25 @@ static int __init debugfs_kernel(char *str)
 {
 	if (str) {
 		if (!strcmp(str, "on"))
-			debugfs_allow = DEBUGFS_ALLOW_API | DEBUGFS_ALLOW_MOUNT;
-		else if (!strcmp(str, "no-mount"))
-			debugfs_allow = DEBUGFS_ALLOW_API;
+			debugfs_enabled = true;
 		else if (!strcmp(str, "off"))
-			debugfs_allow = 0;
+			debugfs_enabled = false;
+		else if (!strcmp(str, "no-mount")) {
+			pr_notice("debugfs=no-mount is a deprecated alias "
+				  "for debugfs=off\n");
+			debugfs_enabled = false;
+		}
 	}
 
 	return 0;
 }
 early_param("debugfs", debugfs_kernel);
+
 static int __init debugfs_init(void)
 {
 	int retval;
 
-	if (!(debugfs_allow & DEBUGFS_ALLOW_MOUNT))
+	if (!debugfs_enabled)
 		return -EPERM;
 
 	retval = sysfs_create_mount_point(kernel_kobj, "debug");

fs/debugfs/internal.h

@@ -55,17 +55,4 @@ enum {
 	HAS_IOCTL = 16
 };
 
-#define DEBUGFS_ALLOW_API	BIT(0)
-#define DEBUGFS_ALLOW_MOUNT	BIT(1)
-
-#ifdef CONFIG_DEBUG_FS_ALLOW_ALL
-#define DEFAULT_DEBUGFS_ALLOW_BITS (DEBUGFS_ALLOW_MOUNT | DEBUGFS_ALLOW_API)
-#endif
-#ifdef CONFIG_DEBUG_FS_DISALLOW_MOUNT
-#define DEFAULT_DEBUGFS_ALLOW_BITS (DEBUGFS_ALLOW_API)
-#endif
-#ifdef CONFIG_DEBUG_FS_ALLOW_NONE
-#define DEFAULT_DEBUGFS_ALLOW_BITS (0)
-#endif
-
 #endif /* _DEBUGFS_INTERNAL_H_ */

lib/Kconfig.debug

@@ -679,7 +679,7 @@ choice
 	help
 	  This selects the default access restrictions for debugfs.
 	  It can be overridden with kernel command line option
-	  debugfs=[on,no-mount,off]. The restrictions apply for API access
+	  debugfs=[on,off]. The restrictions apply for API access
 	  and filesystem registration.
 
 config DEBUG_FS_ALLOW_ALL
@@ -688,13 +688,6 @@ config DEBUG_FS_ALLOW_ALL
 	  No restrictions apply. Both API and filesystem registration
 	  is on. This is the normal default operation.
 
-config DEBUG_FS_DISALLOW_MOUNT
-	bool "Do not register debugfs as filesystem"
-	help
-	  The API is open but filesystem is not loaded. Clients can still do
-	  their work and read with debug tools that do not need
-	  debugfs filesystem.
-
 config DEBUG_FS_ALLOW_NONE
 	bool "No access"
 	help