char: misc: Disallow registering miscdevice whose minor > MISC_DYNAMIC_MINOR

Currently, It is allowed to register miscdevice with minor > 255
which is defined by macro MISC_DYNAMIC_MINOR, and cause:

- Chaos regarding division and management of minor codes.
- Registering failure if the minor was allocated to other dynamic request.

Fortunately, in-kernel users have not had such usage yet.
Fix by refusing to register miscdevice whose minor > 255.

Also bring in a very simple minor code space division and management:

<  255 : Fixed minor code
== 255 : Indicator to request dynamic minor code
>  255 : Dynamic minor code requested, 1048320 minor codes totally
And all fixed minors allocated should be registered in 'linux/miscdevice.h'

Signed-off-by: Zijun Hu <zijun.hu@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250714-rfc_miscdev-v6-3-2ed949665bde@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
index 3e6deb00fc85..565b88efeb23 100644
--- a/include/linux/miscdevice.h
+++ b/include/linux/miscdevice.h
@@ -71,6 +71,14 @@
 #define USERIO_MINOR		240
 #define VHOST_VSOCK_MINOR	241
 #define RFKILL_MINOR		242
+
+/*
+ * Misc char device minor code space division related to below macro:
+ *
+ * <  255  : Fixed minor code
+ * == 255  : Indicator to request dynamic minor code
+ * >  255  : Dynamic minor code requested, 1048320 minor codes totally.
+ */
 #define MISC_DYNAMIC_MINOR	255
 
 struct miscdevice {