cifs: sanitize username handling

Currently, it's not very clear whether you're allowed to have a NULL
vol->username or ses->user_name. Some places check for it and some don't.

Make it clear that a NULL pointer is OK in these fields, and ensure that
all the callers check for that.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
diff --git a/fs/cifs/cifs_spnego.c b/fs/cifs/cifs_spnego.c
index 2272fd5..e622863 100644
--- a/fs/cifs/cifs_spnego.c
+++ b/fs/cifs/cifs_spnego.c
@@ -113,9 +113,11 @@
 		   MAX_MECH_STR_LEN +
 		   UID_KEY_LEN + (sizeof(uid_t) * 2) +
 		   CREDUID_KEY_LEN + (sizeof(uid_t) * 2) +
-		   USER_KEY_LEN + strlen(sesInfo->user_name) +
 		   PID_KEY_LEN + (sizeof(pid_t) * 2) + 1;
 
+	if (sesInfo->user_name)
+		desc_len += USER_KEY_LEN + strlen(sesInfo->user_name);
+
 	spnego_key = ERR_PTR(-ENOMEM);
 	description = kzalloc(desc_len, GFP_KERNEL);
 	if (description == NULL)
@@ -152,8 +154,10 @@
 	dp = description + strlen(description);
 	sprintf(dp, ";creduid=0x%x", sesInfo->cred_uid);
 
-	dp = description + strlen(description);
-	sprintf(dp, ";user=%s", sesInfo->user_name);
+	if (sesInfo->user_name) {
+		dp = description + strlen(description);
+		sprintf(dp, ";user=%s", sesInfo->user_name);
+	}
 
 	dp = description + strlen(description);
 	sprintf(dp, ";pid=0x%x", current->pid);