Currently, peeking on a unix datagram socket with an offset larger than len of
the data in the sk receive queue returns immediately with bogus data. That's
because *off is not reset between each skb_queue_walk().

This patch fixes this so that the behavior is the same as peeking with no
offset on an empty queue: the caller blocks.

Signed-off-by: Benjamin Poirier <redacted>
---
 net/core/datagram.c |   25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 02398ae..6c502b5 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c

@@ -78,9 +78,10 @@ static int receiver_wake_function(wait_queue_t *wait, unsigned int mode, int syn
 	return autoremove_wake_function(wait, mode, sync, key);
 }
 /*
- * Wait for a packet..
+ * Wait for the last received packet to be different from skb
  */
-static int wait_for_packet(struct sock *sk, int *err, long *timeo_p)
+static int wait_for_more_packets(struct sock *sk, int *err, long *timeo_p,
+				 struct sk_buff *skb)

 {
 	int error;
 	DEFINE_WAIT_FUNC(wait, receiver_wake_function);

@@ -92,7 +93,7 @@ static int wait_for_packet(struct sock *sk, int *err, long *timeo_p)
 	if (error)
 		goto out_err;
 
-	if (!skb_queue_empty(&sk->sk_receive_queue))
+	if ((struct sk_buff *)sk->sk_receive_queue.prev != skb)

 		goto out;
 
 	/* Socket shut down? */

@@ -131,9 +132,9 @@ out_noerr:
  *	__skb_recv_datagram - Receive a datagram skbuff
  *	@sk: socket
  *	@flags: MSG_ flags
- *	@off: an offset in bytes to peek skb from. Returns an offset
- *	      within an skb where data actually starts
  *	@peeked: returns non-zero if this packet has been seen before
+ *	@_off: an offset in bytes to peek skb from. Returns an offset
+ *	       within an skb where data actually starts
  *	@err: error code returned
  *
  *	Get a datagram skbuff, understands the peeking, nonblocking wakeups

@@ -159,9 +160,9 @@ out_noerr:
  *	the standard around please.
  */
 struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
-				    int *peeked, int *off, int *err)
+				    int *peeked, int *_off, int *err)
 {
-	struct sk_buff *skb;
+	struct sk_buff *skb, *last;
 	long timeo;
 	/*
 	 * Caller is allowed not to check sk->sk_err before skb_recv_datagram()

@@ -182,13 +183,16 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
 		 */
 		unsigned long cpu_flags;
 		struct sk_buff_head *queue = &sk->sk_receive_queue;
+		int off = *_off;
 
 		spin_lock_irqsave(&queue->lock, cpu_flags);
+		last = (struct sk_buff *)queue;

 		skb_queue_walk(queue, skb) {
+			last = skb;
 			*peeked = skb->peeked;
 			if (flags & MSG_PEEK) {
-				if (*off >= skb->len && (skb->len || *off)) {
-					*off -= skb->len;
+				if (off >= skb->len && (skb->len || off)) {
+					off -= skb->len;
 					continue;
 				}
 				skb->peeked = 1;

@@ -197,6 +201,7 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
 				__skb_unlink(skb, queue);
 
 			spin_unlock_irqrestore(&queue->lock, cpu_flags);
+			*_off = off;
 			return skb;
 		}
 		spin_unlock_irqrestore(&queue->lock, cpu_flags);

@@ -206,7 +211,7 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
 		if (!timeo)
 			goto no_packet;
 
-	} while (!wait_for_packet(sk, err, &timeo));
+	} while (!wait_for_more_packets(sk, err, &timeo, last));
 
 	return NULL;