random: Return nbytes filled from hw RNG

Currently the function get_random_bytes_arch() has return value 'void'.
If the hw RNG fails we currently fall back to using get_random_bytes().
This defeats the purpose of requesting random material from the hw RNG
in the first place.

There are currently no intree users of get_random_bytes_arch().

Only get random bytes from the hw RNG, make function return the number
of bytes retrieved from the hw RNG.

Acked-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

drivers/char/random.c

@@ -1725,26 +1725,28 @@ EXPORT_SYMBOL(del_random_ready_callback);
  * key known by the NSA).  So it's useful if we need the speed, but
  * only if we're willing to trust the hardware manufacturer not to
  * have put in a back door.
+ *
+ * Return number of bytes filled in.
  */
-void get_random_bytes_arch(void *buf, int nbytes)
+int __must_check get_random_bytes_arch(void *buf, int nbytes)
 {
+	int left = nbytes;
 	char *p = buf;
 
-	trace_get_random_bytes_arch(nbytes, _RET_IP_);
-	while (nbytes) {
+	trace_get_random_bytes_arch(left, _RET_IP_);
+	while (left) {
 		unsigned long v;
-		int chunk = min(nbytes, (int)sizeof(unsigned long));
+		int chunk = min_t(int, left, sizeof(unsigned long));
 
 		if (!arch_get_random_long(&v))
 			break;
 
 		memcpy(p, &v, chunk);
 		p += chunk;
-		nbytes -= chunk;
+		left -= chunk;
 	}
 
-	if (nbytes)
-		get_random_bytes(p, nbytes);
+	return nbytes - left;
 }
 EXPORT_SYMBOL(get_random_bytes_arch);
 

include/linux/random.h

@@ -38,7 +38,7 @@ extern void get_random_bytes(void *buf, int nbytes);
 extern int wait_for_random_bytes(void);
 extern int add_random_ready_callback(struct random_ready_callback *rdy);
 extern void del_random_ready_callback(struct random_ready_callback *rdy);
-extern void get_random_bytes_arch(void *buf, int nbytes);
+extern int __must_check get_random_bytes_arch(void *buf, int nbytes);
 
 #ifndef MODULE
 extern const struct file_operations random_fops, urandom_fops;