Subject: Re: [PATCH net] datagram: When peeking datagrams with offset < 0 don't skip empty skbs

From: Paolo Abeni <[email-masked]>
Date: Tue, 15 Aug 2017 17:40:41 +0200

On Mon, 2017-08-14 at 21:35 -0400, Willem de Bruijn wrote:
> It is not infeasible to fix that and fix up all callers, as Matthew's
> patch does. But perhaps this patch is simpler to reason about. Thoughts?
> 
> +static inline bool sk_peek_at_offset(struct sock *sk)
> +{
> +       return READ_ONCE(sk->sk_peek_off) >= 0;
> +}
> +
>  static inline int sk_peek_offset(struct sock *sk, int flags)
>  {
>         if (unlikely(flags & MSG_PEEK)) {
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index ee5647bd91b3..30b53932af73 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -175,12 +175,14 @@ struct sk_buff *__skb_try_recv_from_queue(struct sock *sk,
>         *last = queue->prev;
>         skb_queue_walk(queue, skb) {
>                 if (flags & MSG_PEEK) {
> -                       if (_off >= skb->len && (skb->len || _off ||
> -                                                skb->peeked)) {
> +                       if (_off >= skb->len && sk_peek_at_offset(sk) &&
> +                           (skb->len || _off || skb->peeked)) {

I like the idea a lot, but I think/fear that bad things could happen
since sk_peek_off is read twice at different times: one in
sk_peek_offset() and one in the above chunk.

If the above is not an issue (I am not sure) I'm very fine with this
approach.

For the record, I thought something like the following (uncomplete,
does not even compile):
---
diff --git a/include/linux/socket.h b/include/linux/socket.h
index 8b13db5163cc..5085cf003b88 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -286,6 +286,7 @@ struct ucred {
 #define MSG_SENDPAGE_NOTLAST 0x20000 /* sendpage() internal : not the last page */
 #define MSG_BATCH	0x40000 /* sendmmsg(): more messages coming */
 #define MSG_EOF         MSG_FIN
+#define MSG_PEEK_OFF	0x80000
 
 #define MSG_FASTOPEN	0x20000000	/* Send data in TCP SYN */
 #define MSG_CMSG_CLOEXEC 0x40000000	/* Set close_on_exec for file
diff --git a/include/net/sock.h b/include/net/sock.h
index 7c0632c7e870..e75e024b55d2 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -504,12 +504,14 @@ enum sk_pacing {
 
 int sk_set_peek_off(struct sock *sk, int val);
 
-static inline int sk_peek_offset(struct sock *sk, int flags)
+static inline int sk_peek_offset(struct sock *sk, int *flags)
 {
-	if (unlikely(flags & MSG_PEEK)) {
+	if (unlikely(*flags & MSG_PEEK)) {
 		s32 off = READ_ONCE(sk->sk_peek_off);
-		if (off >= 0)
+		if (off >= 0) {
+			*flags |= MSG_PEEK_OFF;
 			return off;
+		}
 	}
 
 	return 0;
diff --git a/net/core/datagram.c b/net/core/datagram.c
index ee5647bd91b3..91e1d014d64c 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -175,8 +175,8 @@ struct sk_buff *__skb_try_recv_from_queue(struct sock *sk,
 	*last = queue->prev;
 	skb_queue_walk(queue, skb) {
 		if (flags & MSG_PEEK) {
-			if (_off >= skb->len && (skb->len || _off ||
-						 skb->peeked)) {
+			if (flags & MSG_PEEK_OFF && _off >= skb->len &&
+			    (skb->len || _off || skb->peeked)) {
 				_off -= skb->len;
 				continue;
 			}
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index a7c804f73990..7e1bcd3500f4 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1574,7 +1574,7 @@ int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
 		return ip_recv_error(sk, msg, len, addr_len);
 
 try_again:
-	peeking = off = sk_peek_offset(sk, flags);
+	peeking = off = sk_peek_offset(sk, &flags);
 	skb = __skb_recv_udp(sk, flags, noblock, &peeked, &off, &err);
 	if (!skb)
 		return err;
---
Paolo