param: locking for kernel parameters

There may be cases (most obviously, sysfs-writable charp parameters) where
a module needs to prevent sysfs access to parameters.

Rather than express this in terms of a big lock, the functions are
expressed in terms of what they protect against.  This is clearer, esp.
if the implementation changes to a module-level or even param-level lock.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Phil Carmody <ext-phil.2.carmody@nokia.com>

include/linux/moduleparam.h

@@ -130,6 +130,62 @@ __check_old_set_param(int (*oldset)(const char *, struct kernel_param *))
 #define module_param(name, type, perm)				\
 	module_param_named(name, name, type, perm)
 
+/**
+ * kparam_block_sysfs_write - make sure a parameter isn't written via sysfs.
+ * @name: the name of the parameter
+ *
+ * There's no point blocking write on a paramter that isn't writable via sysfs!
+ */
+#define kparam_block_sysfs_write(name)			\
+	do {						\
+		BUG_ON(!(__param_##name.perm & 0222));	\
+		__kernel_param_lock();			\
+	} while (0)
+
+/**
+ * kparam_unblock_sysfs_write - allows sysfs to write to a parameter again.
+ * @name: the name of the parameter
+ */
+#define kparam_unblock_sysfs_write(name)		\
+	do {						\
+		BUG_ON(!(__param_##name.perm & 0222));	\
+		__kernel_param_unlock();		\
+	} while (0)
+
+/**
+ * kparam_block_sysfs_read - make sure a parameter isn't read via sysfs.
+ * @name: the name of the parameter
+ *
+ * This also blocks sysfs writes.
+ */
+#define kparam_block_sysfs_read(name)			\
+	do {						\
+		BUG_ON(!(__param_##name.perm & 0444));	\
+		__kernel_param_lock();			\
+	} while (0)
+
+/**
+ * kparam_unblock_sysfs_read - allows sysfs to read a parameter again.
+ * @name: the name of the parameter
+ */
+#define kparam_unblock_sysfs_read(name)			\
+	do {						\
+		BUG_ON(!(__param_##name.perm & 0444));	\
+		__kernel_param_unlock();		\
+	} while (0)
+
+#ifdef CONFIG_SYSFS
+extern void __kernel_param_lock(void);
+extern void __kernel_param_unlock(void);
+#else
+static inline void __kernel_param_lock(void)
+{
+}
+static inline void __kernel_param_unlock(void)
+{
+}
+#endif
+
 #ifndef MODULE
 /**
  * core_param - define a historical core kernel parameter.

kernel/params.c

@@ -31,12 +31,14 @@
 #define DEBUGP(fmt, a...)
 #endif
 
+/* Protects all parameters, and incidentally kmalloced_param list. */
+static DEFINE_MUTEX(param_lock);
+
 /* This just allows us to keep track of which parameters are kmalloced. */
 struct kmalloced_param {
 	struct list_head list;
 	char val[];
 };
-static DEFINE_MUTEX(param_lock);
 static LIST_HEAD(kmalloced_params);
 
 static void *kmalloc_parameter(unsigned int size)
@@ -47,10 +49,7 @@ static void *kmalloc_parameter(unsigned int size)
 	if (!p)
 		return NULL;
 
-	mutex_lock(&param_lock);
 	list_add(&p->list, &kmalloced_params);
-	mutex_unlock(&param_lock);
-
 	return p->val;
 }
 
@@ -59,7 +58,6 @@ static void maybe_kfree_parameter(void *param)
 {
 	struct kmalloced_param *p;
 
-	mutex_lock(&param_lock);
 	list_for_each_entry(p, &kmalloced_params, list) {
 		if (p->val == param) {
 			list_del(&p->list);
@@ -67,7 +65,6 @@ static void maybe_kfree_parameter(void *param)
 			break;
 		}
 	}
-	mutex_unlock(&param_lock);
 }
 
 static inline char dash2underscore(char c)
@@ -93,6 +90,7 @@ static int parse_one(char *param,
 		     int (*handle_unknown)(char *param, char *val))
 {
 	unsigned int i;
+	int err;
 
 	/* Find parameter */
 	for (i = 0; i < num_params; i++) {
@@ -102,7 +100,10 @@ static int parse_one(char *param,
 				return -EINVAL;
 			DEBUGP("They are equal!  Calling %p\n",
 			       params[i].ops->set);
-			return params[i].ops->set(val, &params[i]);
+			mutex_lock(&param_lock);
+			err = params[i].ops->set(val, &params[i]);
+			mutex_unlock(&param_lock);
+			return err;
 		}
 	}
 
@@ -400,6 +401,7 @@ static int param_array(const char *name,
 		/* nul-terminate and parse */
 		save = val[len];
 		((char *)val)[len] = '\0';
+		BUG_ON(!mutex_is_locked(&param_lock));
 		ret = set(val, &kp);
 
 		if (ret != 0)
@@ -438,6 +440,7 @@ static int param_array_get(char *buffer, const struct kernel_param *kp)
 		if (i)
 			buffer[off++] = ',';
 		p.arg = arr->elem + arr->elemsize * i;
+		BUG_ON(!mutex_is_locked(&param_lock));
 		ret = arr->ops->get(buffer + off, &p);
 		if (ret < 0)
 			return ret;
@@ -522,7 +525,9 @@ static ssize_t param_attr_show(struct module_attribute *mattr,
 	if (!attribute->param->ops->get)
 		return -EPERM;
 
+	mutex_lock(&param_lock);
 	count = attribute->param->ops->get(buf, attribute->param);
+	mutex_unlock(&param_lock);
 	if (count > 0) {
 		strcat(buf, "\n");
 		++count;
@@ -541,7 +546,9 @@ static ssize_t param_attr_store(struct module_attribute *mattr,
 	if (!attribute->param->ops->set)
 		return -EPERM;
 
+	mutex_lock(&param_lock);
 	err = attribute->param->ops->set(buf, attribute->param);
+	mutex_unlock(&param_lock);
 	if (!err)
 		return len;
 	return err;
@@ -555,6 +562,18 @@ static ssize_t param_attr_store(struct module_attribute *mattr,
 #endif
 
 #ifdef CONFIG_SYSFS
+void __kernel_param_lock(void)
+{
+	mutex_lock(&param_lock);
+}
+EXPORT_SYMBOL(__kernel_param_lock);
+
+void __kernel_param_unlock(void)
+{
+	mutex_unlock(&param_lock);
+}
+EXPORT_SYMBOL(__kernel_param_unlock);
+
 /*
  * add_sysfs_param - add a parameter to sysfs
  * @mk: struct module_kobject