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, 6 insertions, 0 deletions

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
index 558302a64dd9..b8e66466184f 100644
--- a/drivers/char/misc.c
+++ b/drivers/char/misc.c
@@ -210,6 +210,12 @@ int misc_register(struct miscdevice *misc)
 	int err = 0;
 	bool is_dynamic = (misc->minor == MISC_DYNAMIC_MINOR);
 
+	if (misc->minor > MISC_DYNAMIC_MINOR) {
+		pr_err("Invalid fixed minor %d for miscdevice '%s'\n",
+		       misc->minor, misc->name);
+		return -EINVAL;
+	}
+
 	INIT_LIST_HEAD(&misc->list);
 
 	mutex_lock(&misc_mtx);