diff --git a/fs/namespace.c b/fs/namespace.c
index 9e8350d458f25be6711a70713ec45e9a8c347717..2c5cc697a18db4f71191e00dbfc235691b19a247 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -930,7 +930,35 @@ void mark_mounts_for_expiry(struct list_head *mounts)
 
 EXPORT_SYMBOL_GPL(mark_mounts_for_expiry);
 
-int copy_mount_options (const void __user *data, unsigned long *where)
+/*
+ * Some copy_from_user() implementations do not return the exact number of
+ * bytes remaining to copy on a fault.  But copy_mount_options() requires that.
+ * Note that this function differs from copy_from_user() in that it will oops
+ * on bad values of `to', rather than returning a short copy.
+ */
+static long
+exact_copy_from_user(void *to, const void __user *from, unsigned long n)
+{
+	char *t = to;
+	const char __user *f = from;
+	char c;
+
+	if (!access_ok(VERIFY_READ, from, n))
+		return n;
+
+	while (n) {
+		if (__get_user(c, f)) {
+			memset(t, 0, n);
+			break;
+		}
+		*t++ = c;
+		f++;
+		n--;
+	}
+	return n;
+}
+
+int copy_mount_options(const void __user *data, unsigned long *where)
 {
 	int i;
 	unsigned long page;
@@ -952,7 +980,7 @@ int copy_mount_options (const void __user *data, unsigned long *where)
 	if (size > PAGE_SIZE)
 		size = PAGE_SIZE;
 
-	i = size - copy_from_user((void *)page, data, size);
+	i = size - exact_copy_from_user((void *)page, data, size);
 	if (!i) {
 		free_page(page); 
 		return -EFAULT;