Re: [PATCH net 2/3] unix/dgram: fix peeking with an offset larger than data in queue

Eric Dumazet
2013-04-25T18:58:23Z

On Thu, 2013-04-25 at 09:47 -0400, Benjamin Poirier wrote:
> 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>
> ---
> 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)

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 ((struct sk_buff *)sk->sk_receive_queue.prev != skb)

Why is the cast needed ?

> 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;

This could be done before spin_lock

> 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;
>

Other than that, patch seems fine.