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udp: introduce sk_for_each_rcu_safenext()

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Corey Minyard found a race added in commit 271b72c7fa
(udp: RCU handling for Unicast packets.)

 "If the socket is moved from one list to another list in-between the
 time the hash is calculated and the next field is accessed, and the
 socket has moved to the end of the new list, the traversal will not
 complete properly on the list it should have, since the socket will
 be on the end of the new list and there's not a way to tell it's on a
 new list and restart the list traversal.  I think that this can be
 solved by pre-fetching the "next" field (with proper barriers) before
 checking the hash."

This patch corrects this problem, introducing a new
sk_for_each_rcu_safenext() macro.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet
2008-10-29 11:19:58 -07:00
committed by David S. Miller
parent f52b5054ec
commit 96631ed16c
4 changed files with 23 additions and 6 deletions
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4
net/ipv4/udp.c
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@@ -256,7 +256,7 @@ static struct sock *__udp4_lib_lookup(struct net *net, __be32 saddr,
int dif, struct udp_table *udptable)
{
struct sock *sk, *result;
struct hlist_node *node;
struct hlist_node *node, *next;
unsigned short hnum = ntohs(dport);
unsigned int hash = udp_hashfn(net, hnum);
struct udp_hslot *hslot = &udptable->hash[hash];
@@ -266,7 +266,7 @@ static struct sock *__udp4_lib_lookup(struct net *net, __be32 saddr,
begin:
result = NULL;
badness = -1;
sk_for_each_rcu(sk, node, &hslot->head) {
sk_for_each_rcu_safenext(sk, node, &hslot->head, next) {
/*
* lockless reader, and SLAB_DESTROY_BY_RCU items:
* We must check this item was not moved to another chain