hwrng: remember rng chosen by user

When a user chooses a rng source via sysfs attribute
this rng should be sticky, even when other sources
with better quality to register. This patch introduces
a simple way to remember the user's choice. This is
reflected by a new sysfs attribute file 'rng_selected'
which shows if the current rng has been chosen by
userspace. The new attribute file shows '1' for user
selected rng and '0' otherwise.

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/core.c

@@ -28,6 +28,8 @@
 #define RNG_MODULE_NAME		"hw_random"
 
 static struct hwrng *current_rng;
+/* the current rng has been explicitly chosen by user via sysfs */
+static int cur_rng_set_by_user;
 static struct task_struct *hwrng_fill;
 /* list of registered rngs, sorted decending by quality */
 static LIST_HEAD(rng_list);
@@ -304,6 +306,7 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
 	list_for_each_entry(rng, &rng_list, list) {
 		if (sysfs_streq(rng->name, buf)) {
 			err = 0;
+			cur_rng_set_by_user = 1;
 			if (rng != current_rng)
 				err = set_current_rng(rng);
 			break;
@@ -352,16 +355,27 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
 	return strlen(buf);
 }
 
+static ssize_t hwrng_attr_selected_show(struct device *dev,
+					struct device_attribute *attr,
+					char *buf)
+{
+	return snprintf(buf, PAGE_SIZE, "%d\n", cur_rng_set_by_user);
+}
+
 static DEVICE_ATTR(rng_current, S_IRUGO | S_IWUSR,
 		   hwrng_attr_current_show,
 		   hwrng_attr_current_store);
 static DEVICE_ATTR(rng_available, S_IRUGO,
 		   hwrng_attr_available_show,
 		   NULL);
+static DEVICE_ATTR(rng_selected, S_IRUGO,
+		   hwrng_attr_selected_show,
+		   NULL);
 
 static struct attribute *rng_dev_attrs[] = {
 	&dev_attr_rng_current.attr,
 	&dev_attr_rng_available.attr,
+	&dev_attr_rng_selected.attr,
 	NULL
 };
 
@@ -444,10 +458,12 @@ int hwrng_register(struct hwrng *rng)
 
 	old_rng = current_rng;
 	err = 0;
-	if (!old_rng || (rng->quality > old_rng->quality)) {
+	if (!old_rng ||
+	    (!cur_rng_set_by_user && rng->quality > old_rng->quality)) {
 		/*
 		 * Set new rng as current as the new rng source
-		 * provides better entropy quality.
+		 * provides better entropy quality and was not
+		 * chosen by userspace.
 		 */
 		err = set_current_rng(rng);
 		if (err)
@@ -479,6 +495,7 @@ void hwrng_unregister(struct hwrng *rng)
 	list_del(&rng->list);
 	if (current_rng == rng) {
 		drop_current_rng();
+		cur_rng_set_by_user = 0;
 		/* rng_list is sorted by quality, use the best (=first) one */
 		if (!list_empty(&rng_list)) {
 			struct hwrng *new_rng;