sysctl: make sure to terminate strings with a NUL

This is a slightly more complete fix for the previous minimal sysctl
string fix.  It always terminates the returned string with a NUL, even
if the full result wouldn't fit in the user-supplied buffer.

The returned length is the full untruncated length, so that you can
tell when truncation has occurred.
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

kernel/sysctl.c

@@ -2192,27 +2192,32 @@ int sysctl_string(ctl_table *table, int __user *name, int nlen,
 		  void __user *oldval, size_t __user *oldlenp,
 		  void __user *newval, size_t newlen, void **context)
 {
-	size_t l, len;
-	
 	if (!table->data || !table->maxlen) 
 		return -ENOTDIR;
 	
 	if (oldval && oldlenp) {
-		if (get_user(len, oldlenp))
+		size_t bufsize;
+		if (get_user(bufsize, oldlenp))
 			return -EFAULT;
-		if (len) {
-			l = strlen(table->data)+1;
-			if (len > l) len = l;
-			if (len >= table->maxlen)
+		if (bufsize) {
+			size_t len = strlen(table->data), copied;
+
+			/* This shouldn't trigger for a well-formed sysctl */
+			if (len > table->maxlen)
 				len = table->maxlen;
-			if(copy_to_user(oldval, table->data, len))
+
+			/* Copy up to a max of bufsize-1 bytes of the string */
+			copied = (len >= bufsize) ? bufsize - 1 : len;
+
+			if (copy_to_user(oldval, table->data, copied) ||
+			    put_user(0, (char __user *)(oldval + copied)))
 				return -EFAULT;
-			if(put_user(len, oldlenp))
+			if (put_user(len, oldlenp))
 				return -EFAULT;
 		}
 	}
 	if (newval && newlen) {
-		len = newlen;
+		size_t len = newlen;
 		if (len > table->maxlen)
 			len = table->maxlen;
 		if(copy_from_user(table->data, newval, len))