linux-kernel - [PATCH v3 1/3] unix/dgram: peek beyond 0-sized skbs

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Date:	Mon, 29 Apr 2013 17:42:12 -0400
From:	Benjamin Poirier <bpoirier@...e.de>
To:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cong Wang <xiyou.wangcong@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v3 1/3] unix/dgram: peek beyond 0-sized skbs

"77c1090 net: fix infinite loop in __skb_recv_datagram()" (v3.8) introduced a
regression:
After that commit, recv can no longer peek beyond a 0-sized skb in the queue.
__skb_recv_datagram() instead stops at the first skb with len == 0 and results
in the system call failing with -EFAULT via skb_copy_datagram_iovec().

When peeking at an offset with 0-sized skb(s), each one of those is received
only once, in sequence. The offset starts moving forward again after receiving
datagrams with len > 0.

Signed-off-by: Benjamin Poirier <bpoirier@...e.de>

---

* v1 fix the case when SO_PEEK_OFF is used to set sk_peek_off beyond a
  0-sized skb

* v2 also fix the situation when sk_peek_off must advance to and beyond a
  0-sized skb

 net/core/datagram.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 368f9c3..99c4f52 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -187,7 +187,8 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
 		skb_queue_walk(queue, skb) {
 			*peeked = skb->peeked;
 			if (flags & MSG_PEEK) {
-				if (*off >= skb->len && skb->len) {
+				if (*off >= skb->len && (skb->len || *off ||
+							 skb->peeked)) {
 					*off -= skb->len;
 					continue;
 				}
-- 
1.7.10.4

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