unix/dgram: fix peeking with an offset larger than data in queue

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 <bpoirier@suse.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 8 deletions

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 99c4f525b1d9..b5d48ac2a9c1 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,
+                                 const 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 (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
+ *      @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
- *      @peeked: returns non-zero if this packet has been seen before
  *      @err: error code returned
  *
  *      Get a datagram skbuff, understands the peeking, nonblocking wakeups
@@ -161,7 +162,7 @@ out_noerr:
 struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
                                     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()
@@ -182,14 +183,17 @@ 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;
 
+                last = (struct sk_buff *)queue;
                 spin_lock_irqsave(&queue->lock, cpu_flags);
                 skb_queue_walk(queue, skb) {
+                        last = skb;
                         *peeked = skb->peeked;
                         if (flags & MSG_PEEK) {
-                                if (*off >= skb->len && (skb->len || *off ||
+                                if (_off >= skb->len && (skb->len || _off ||
                                                          skb->peeked)) {
-                                        *off -= skb->len;
+                                        _off -= skb->len;
                                         continue;
                                 }
                                 skb->peeked = 1;
@@ -198,6 +202,7 @@ 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);
@@ -207,7 +212,7 @@ 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;