[PATCH v4 3/8] char: misc: Disallow registering miscdevice whose minor > MISC_DYNAMIC_MINOR

[Zijun Hu <zijun_hu@icloud.com>]

From: Zijun Hu <zijun.hu@oss.qualcomm.com>

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>
---
 drivers/char/misc.c        | 6 ++++++
 include/linux/miscdevice.h | 8 ++++++++
 2 files changed, 14 insertions(+)

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
index 558302a64dd908aee30341547a5413df1af71459..b8e66466184fa21fb66d968db7950e0b5669ac43 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);
diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
index 3e6deb00fc8535a7571f85489c74979e18385bad..565b88efeb23d02b7d91df1cd7df4bdcf2898224 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 {

-- 
2.34.1