[FIB]: Fix rcu_dereference() abuses in fib_trie.c

node_parent() and tnode_get_child() currently use rcu_dereference().

These functions are called from both
- readers only paths (where rcu_dereference() is needed), and
- writer path (where rcu_dereference() is not needed)

To make explicit where rcu_dereference() is really needed, I
introduced new node_parent_rcu() and tnode_get_child_rcu() functions
which use rcu_dereference(), while node_parent() and tnode_get_child()
dont use it.

Then I changed calling sites where rcu_dereference() was really needed
to call the _rcu() variants.

This should have no impact but for alpha architecture, and may help
future sparse checks.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 21 insertions, 12 deletions

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index fbc80d15827..a52334d30cf 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -165,9 +165,13 @@ static struct kmem_cache *fn_alias_kmem __read_mostly;
 
 static inline struct tnode *node_parent(struct node *node)
 {
-        struct tnode *ret;
+        return (struct tnode *)(node->parent & ~NODE_TYPE_MASK);
+}
+
+static inline struct tnode *node_parent_rcu(struct node *node)
+{
+        struct tnode *ret = node_parent(node);
 
-        ret = (struct tnode *)(node->parent & ~NODE_TYPE_MASK);
         return rcu_dereference(ret);
 }
 
@@ -177,13 +181,18 @@ static inline void node_set_parent(struct node *node, struct tnode *ptr)
                            (unsigned long)ptr | NODE_TYPE(node));
 }
 
-/* rcu_read_lock needs to be hold by caller from readside */
+static inline struct node *tnode_get_child(struct tnode *tn, unsigned int i)
+{
+        BUG_ON(i >= 1U << tn->bits);
 
-static inline struct node *tnode_get_child(struct tnode *tn, int i)
+        return tn->child[i];
+}
+
+static inline struct node *tnode_get_child_rcu(struct tnode *tn, unsigned int i)
 {
-        BUG_ON(i >= 1 << tn->bits);
+        struct node *ret = tnode_get_child(tn, i);
 
-        return rcu_dereference(tn->child[i]);
+        return rcu_dereference(ret);
 }
 
 static inline int tnode_child_length(const struct tnode *tn)
@@ -938,7 +947,7 @@ fib_find_node(struct trie *t, u32 key)
 
                 if (tkey_sub_equals(tn->key, pos, tn->pos-pos, key)) {
                         pos = tn->pos + tn->bits;
-                        n = tnode_get_child(tn, tkey_extract_bits(key, tn->pos, tn->bits));
+                        n = tnode_get_child_rcu(tn, tkey_extract_bits(key, tn->pos, tn->bits));
                 } else
                         break;
         }
@@ -1688,7 +1697,7 @@ static struct leaf *nextleaf(struct trie *t, struct leaf *thisleaf)
 
                 p = (struct tnode*) trie;  /* Start */
         } else
-                p = node_parent(c);
+                p = node_parent_rcu(c);
 
         while (p) {
                 int pos, last;
@@ -1725,7 +1734,7 @@ static struct leaf *nextleaf(struct trie *t, struct leaf *thisleaf)
 up:
                 /* No more children go up one step  */
                 c = (struct node *) p;
-                p = node_parent(c);
+                p = node_parent_rcu(c);
         }
         return NULL; /* Ready. Root of trie */
 }
@@ -1987,7 +1996,7 @@ static struct node *fib_trie_get_next(struct fib_trie_iter *iter)
                  iter->tnode, iter->index, iter->depth);
 rescan:
         while (cindex < (1<<tn->bits)) {
-                struct node *n = tnode_get_child(tn, cindex);
+                struct node *n = tnode_get_child_rcu(tn, cindex);
 
                 if (n) {
                         if (IS_LEAF(n)) {
@@ -2006,7 +2015,7 @@ rescan:
         }
 
         /* Current node exhausted, pop back up */
-        p = node_parent((struct node *)tn);
+        p = node_parent_rcu((struct node *)tn);
         if (p) {
                 cindex = tkey_extract_bits(tn->key, p->pos, p->bits)+1;
                 tn = p;
@@ -2315,7 +2324,7 @@ static int fib_trie_seq_show(struct seq_file *seq, void *v)
         if (v == SEQ_START_TOKEN)
                 return 0;
 
-        if (!node_parent(n)) {
+        if (!node_parent_rcu(n)) {
                 if (iter->trie == iter->trie_local)
                         seq_puts(seq, "<local>:\n");
                 else