[PATCH] char: misc: restore MISC_DYNAMIC_MINOR on device_create() failure

[Vladimir Zapolskiy]

On attempt to register a dynamic minor misc device its minor number is
updated to a virtual minor number prior to device_create() call,
however on error path misc->minor == MISC_DYNAMIC_MINOR is not
restored.

Following the rule of thumb that a function returning an error must
not change the state of the caller, assign MISC_DYNAMIC_MINOR back.

The problem is met in a sutuation, when subsys_initcall(misc_init) is
not yet called and misc_class is not created, but misc_register()
modifies statically defined ".minor = MISC_DYNAMIC_MINOR", therefore
implicitly changing the client's logic on next attempt (e.g. retrying
from deferred list) to register a misc device, whose minor number is
converted from dynamic to some unknown static one.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@xxxxxxxxxx>
---
 drivers/char/misc.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
index 9fd5a91..5113409 100644
--- a/drivers/char/misc.c
+++ b/drivers/char/misc.c
@@ -186,12 +186,13 @@ int misc_register(struct miscdevice * misc)
 {
 	dev_t dev;
 	int err = 0;
+	bool is_dynamic = (misc->minor == MISC_DYNAMIC_MINOR);
 
 	INIT_LIST_HEAD(&misc->list);
 
 	mutex_lock(&misc_mtx);
 
-	if (misc->minor == MISC_DYNAMIC_MINOR) {
+	if (is_dynamic) {
 		int i = find_first_zero_bit(misc_minors, DYNAMIC_MINORS);
 		if (i >= DYNAMIC_MINORS) {
 			err = -EBUSY;
@@ -216,9 +217,13 @@ int misc_register(struct miscdevice * misc)
 		device_create_with_groups(misc_class, misc->parent, dev,
 					  misc, misc->groups, "%s", misc->name);
 	if (IS_ERR(misc->this_device)) {
-		int i = DYNAMIC_MINORS - misc->minor - 1;
-		if (i < DYNAMIC_MINORS && i >= 0)
-			clear_bit(i, misc_minors);
+		if (is_dynamic) {
+			int i = DYNAMIC_MINORS - misc->minor - 1;
+
+			if (i < DYNAMIC_MINORS && i >= 0)
+				clear_bit(i, misc_minors);
+			misc->minor = MISC_DYNAMIC_MINOR;
+		}
 		err = PTR_ERR(misc->this_device);
 		goto out;
 	}
-- 
2.1.4

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