[IP]: Simplify and consolidate MSG_PEEK error handling

When a packet is obtained from skb_recv_datagram with MSG_PEEK enabled
it is left on the socket receive queue.  This means that when we detect
a checksum error we have to be careful when trying to free the packet
as someone could have dequeued it in the time being.

Currently this delicate logic is duplicated three times between UDPv4,
UDPv6 and RAWv6.  This patch moves them into a one place and simplifies
the code somewhat.

This is based on a suggestion by Eric Dumazet.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

4 files changed, 42 insertions, 41 deletions

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 1bcfef51ac5815..f8d322e1ea9276 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -47,6 +47,7 @@
 #include <linux/rtnetlink.h>
 #include <linux/poll.h>
 #include <linux/highmem.h>
+#include <linux/spinlock.h>
 
 #include <net/protocol.h>
 #include <linux/skbuff.h>
@@ -200,6 +201,41 @@ void skb_free_datagram(struct sock *sk, struct sk_buff *skb)
 }
 
 /**
+ *	skb_kill_datagram - Free a datagram skbuff forcibly
+ *	@sk: socket
+ *	@skb: datagram skbuff
+ *	@flags: MSG_ flags
+ *
+ *	This function frees a datagram skbuff that was received by
+ *	skb_recv_datagram.  The flags argument must match the one
+ *	used for skb_recv_datagram.
+ *
+ *	If the MSG_PEEK flag is set, and the packet is still on the
+ *	receive queue of the socket, it will be taken off the queue
+ *	before it is freed.
+ *
+ *	This function currently only disables BH when acquiring the
+ *	sk_receive_queue lock.  Therefore it must not be used in a
+ *	context where that lock is acquired in an IRQ context.
+ */
+
+void skb_kill_datagram(struct sock *sk, struct sk_buff *skb, unsigned int flags)
+{
+	if (flags & MSG_PEEK) {
+		spin_lock_bh(&sk->sk_receive_queue.lock);
+		if (skb == skb_peek(&sk->sk_receive_queue)) {
+			__skb_unlink(skb, &sk->sk_receive_queue);
+			atomic_dec(&skb->users);
+		}
+		spin_unlock_bh(&sk->sk_receive_queue.lock);
+	}
+
+	kfree_skb(skb);
+}
+
+EXPORT_SYMBOL(skb_kill_datagram);
+
+/**
  *	skb_copy_datagram_iovec - Copy a datagram to an iovec.
  *	@skb: buffer to copy
  *	@offset: offset in the buffer to start copying from
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 2422a5f7195d00..012c4621e40afc 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -846,20 +846,7 @@ out:
 csum_copy_err:
 	UDP_INC_STATS_BH(UDP_MIB_INERRORS);
 
-	/* Clear queue. */
-	if (flags&MSG_PEEK) {
-		int clear = 0;
-		spin_lock_bh(&sk->sk_receive_queue.lock);
-		if (skb == skb_peek(&sk->sk_receive_queue)) {
-			__skb_unlink(skb, &sk->sk_receive_queue);
-			clear = 1;
-		}
-		spin_unlock_bh(&sk->sk_receive_queue.lock);
-		if (clear)
-			kfree_skb(skb);
-	}
-
-	skb_free_datagram(sk, skb);
+	skb_kill_datagram(sk, skb, flags);
 
 	if (noblock)
 		return -EAGAIN;	
diff --git a/net/ipv6/raw.c b/net/ipv6/raw.c
index a66900cda2afc7..66f1d12ea57831 100644
--- a/net/ipv6/raw.c
+++ b/net/ipv6/raw.c
@@ -32,6 +32,7 @@
 #include <linux/icmpv6.h>
 #include <linux/netfilter.h>
 #include <linux/netfilter_ipv6.h>
+#include <linux/skbuff.h>
 #include <asm/uaccess.h>
 #include <asm/ioctls.h>
 #include <asm/bug.h>
@@ -433,25 +434,14 @@ out:
 	return err;
 
 csum_copy_err:
-	/* Clear queue. */
-	if (flags&MSG_PEEK) {
-		int clear = 0;
-		spin_lock_bh(&sk->sk_receive_queue.lock);
-		if (skb == skb_peek(&sk->sk_receive_queue)) {
-			__skb_unlink(skb, &sk->sk_receive_queue);
-			clear = 1;
-		}
-		spin_unlock_bh(&sk->sk_receive_queue.lock);
-		if (clear)
-			kfree_skb(skb);
-	}
+	skb_kill_datagram(sk, skb, flags);
 
 	/* Error for blocking case is chosen to masquerade
 	   as some normal condition.
 	 */
 	err = (flags&MSG_DONTWAIT) ? -EAGAIN : -EHOSTUNREACH;
 	/* FIXME: increment a raw6 drops counter here */
-	goto out_free;
+	goto out;
 }
 
 static int rawv6_push_pending_frames(struct sock *sk, struct flowi *fl,
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 5cc8731eb55b85..d8538dcea8130b 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -36,6 +36,7 @@
 #include <linux/ipv6.h>
 #include <linux/icmpv6.h>
 #include <linux/init.h>
+#include <linux/skbuff.h>
 #include <asm/uaccess.h>
 
 #include <net/sock.h>
@@ -300,20 +301,7 @@ out:
 	return err;
 
 csum_copy_err:
-	/* Clear queue. */
-	if (flags&MSG_PEEK) {
-		int clear = 0;
-		spin_lock_bh(&sk->sk_receive_queue.lock);
-		if (skb == skb_peek(&sk->sk_receive_queue)) {
-			__skb_unlink(skb, &sk->sk_receive_queue);
-			clear = 1;
-		}
-		spin_unlock_bh(&sk->sk_receive_queue.lock);
-		if (clear)
-			kfree_skb(skb);
-	}
-
-	skb_free_datagram(sk, skb);
+	skb_kill_datagram(sk, skb, flags);
 
 	if (flags & MSG_DONTWAIT) {
 		UDP6_INC_STATS_USER(UDP_MIB_INERRORS);