[PATCH RFC batadv v4 1/8] batman-adv: tt: remove only the entry which was looked up from the hash

batadv_hash_remove() searches a bucket with a compare callback and unlinks
the first matching entry. batadv_compare_tt() matches any entry for the
same MAC address and VLAN, not the object which was passed in, and the
callers in batadv_tt_local_remove() and batadv_tt_global_free() are not
serialized against the rest of the translation table in any way.

So when the looked up entry was already unlinked by another context and a
new entry for the same client was added in the meantime, these two
functions unlink that new entry instead.

Add batadv_compare_tt_entry(), which matches the very object which is
searched for, and use it for both removals. Nothing is unlinked when the
entry is gone already, batadv_hash_remove() then simply returns NULL and
only the reference of the calling context is dropped.

As a side effect the returned hlist_node can no longer belong to a
different object, so both functions can operate on the entry they were
given.

Fixes: af912d77181f ("batman-adv: protect tt_local_entry from concurrent delete events")
Signed-off-by: Sven Eckelmann <[email protected]>
---
 net/batman-adv/translation-table.c | 38 ++++++++++++++++++++++++++------------
 1 file changed, 26 insertions(+), 12 deletions(-)

diff --git a/net/batman-adv/translation-table.c b/net/batman-adv/translation-table.c
index 4f47c97b..af40ff81 100644
--- a/net/batman-adv/translation-table.c
+++ b/net/batman-adv/translation-table.c
@@ -136,6 +136,26 @@ static bool batadv_compare_tt(const struct hlist_node *node, const void *data2)
 	return (tt1->vid == tt2->vid) && batadv_compare_eth(data1, data2);
 }
 
+/**
+ * batadv_compare_tt_entry() - check if a hash node is a specific TT entry
+ * @node: the list element pointer of the TT entry stored in the bucket
+ * @data2: pointer to the tt_common_entry which is looked for
+ *
+ * Unlike batadv_compare_tt(), this only matches the very object which is
+ * passed as @data2 and not just any entry for the same TT client. It is meant
+ * for batadv_hash_remove() callers which must not unlink an entry they did not
+ * look up themselves.
+ *
+ * Return: true if @node belongs to @data2, false otherwise
+ */
+static bool batadv_compare_tt_entry(const struct hlist_node *node,
+				    const void *data2)
+{
+	const struct batadv_tt_common_entry *tt = data2;
+
+	return node == &tt->hash_entry;
+}
+
 /**
  * batadv_choose_tt() - return the index of the tt entry in the hash table
  * @data: pointer to the tt_common_entry object to map
@@ -628,7 +648,6 @@ static void batadv_tt_global_free(struct batadv_priv *bat_priv,
 				  struct batadv_tt_global_entry *tt_global,
 				  const char *message)
 {
-	struct batadv_tt_global_entry *tt_removed_entry;
 	struct hlist_node *tt_removed_node;
 
 	batadv_dbg(BATADV_DBG_TT, bat_priv,
@@ -636,18 +655,16 @@ static void batadv_tt_global_free(struct batadv_priv *bat_priv,
 		   tt_global->common.addr,
 		   batadv_print_vid(tt_global->common.vid), message);
 
+	/* remove exactly this object when still present in hash */
 	tt_removed_node = batadv_hash_remove(bat_priv->tt.global_hash,
-					     batadv_compare_tt,
+					     batadv_compare_tt_entry,
 					     batadv_choose_tt,
 					     &tt_global->common);
 	if (!tt_removed_node)
 		return;
 
 	/* drop reference of remove hash entry */
-	tt_removed_entry = hlist_entry(tt_removed_node,
-				       struct batadv_tt_global_entry,
-				       common.hash_entry);
-	batadv_tt_global_entry_put(tt_removed_entry);
+	batadv_tt_global_entry_put(tt_global);
 }
 
 /**
@@ -1345,7 +1362,6 @@ u16 batadv_tt_local_remove(struct batadv_priv *bat_priv, const u8 *addr,
 			   unsigned short vid, const char *message,
 			   bool roaming)
 {
-	struct batadv_tt_local_entry *tt_removed_entry;
 	struct batadv_tt_local_entry *tt_local_entry;
 	struct hlist_node *tt_removed_node;
 	u16 curr_flags = BATADV_NO_FLAGS;
@@ -1378,18 +1394,16 @@ u16 batadv_tt_local_remove(struct batadv_priv *bat_priv, const u8 *addr,
 	 */
 	batadv_tt_local_event(bat_priv, tt_local_entry, BATADV_TT_CLIENT_DEL);
 
+	/* remove exactly this object when still present in hash */
 	tt_removed_node = batadv_hash_remove(bat_priv->tt.local_hash,
-					     batadv_compare_tt,
+					     batadv_compare_tt_entry,
 					     batadv_choose_tt,
 					     &tt_local_entry->common);
 	if (!tt_removed_node)
 		goto out;
 
 	/* drop reference of remove hash entry */
-	tt_removed_entry = hlist_entry(tt_removed_node,
-				       struct batadv_tt_local_entry,
-				       common.hash_entry);
-	batadv_tt_local_entry_put(tt_removed_entry);
+	batadv_tt_local_entry_put(tt_local_entry);
 
 out:
 	batadv_tt_local_entry_put(tt_local_entry);

-- 
2.47.3