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Date: Wed, 16 Aug 2017 11:28:42 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Matthew Dawson <matthew@...systems.ca>,
Network Development <netdev@...r.kernel.org>,
"Macieira, Thiago" <thiago.macieira@...el.com>
Subject: Re: [PATCH net] datagram: When peeking datagrams with offset < 0
don't skip empty skbs
On Tue, 2017-08-15 at 13:00 -0400, Willem de Bruijn wrote:
> > There is another difference between reading sk_peek_offset in the
> > caller or in __skb_try_recv_from_queue. The latter is called repeatedly
> > when it returns NULL. Each call can modify *off. I believe that it needs
> > to restart with _off at sk->sk_peek_off each time, as it restarts from the
> > head of the queue each time.
>
> I made a mistake here. *off is not updated when returning NULL.
>
> In that case, it is better to read sk_peek_offset once, than to read
> it each time __skb_try_recv_from_queue is entered.
If I read the above correctly, you are arguining in favor of the
addittional flag version, right?
Regarding the MSG flag exaustion, there are a bunch of flags defined
but apparently unused (MSG_FIN, MSG_SYN, MSG_RST) since long time (if
I'm not too low on coffee).
We can shadow one of them (or ev. drop the above defines, if really
unused).
I think that the MSG_PEEK_OFF should be explicitly cleared in
sk_peek_offset() when the sk_peek_off is negative, to avoid beeing
fooled by stray bits passed by the us, like in the following:
---
diff --git a/include/linux/socket.h b/include/linux/socket.h
index 8b13db5163cc..3b7f53b9cc08 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 MSG_FIN /* Peeking with offset for datagram sockets */
#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..452f9aac2e6a 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -504,12 +504,17 @@ 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;
+ }
+
+ /* clear evental stray bits in the user-provided bitmask */
+ *flags &= ~MSG_PEEK_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/core/sock.c b/net/core/sock.c
index ac2a404c73eb..0c123540b02f 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2408,9 +2408,7 @@ EXPORT_SYMBOL(__sk_mem_reclaim);
int sk_set_peek_off(struct sock *sk, int val)
{
- if (val < 0)
- return -EINVAL;
-
+ /* a negative value will disable peeking with offset */
sk->sk_peek_off = val;
return 0;
}
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;
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 578142b7ca3e..86fb4ff8934c 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -362,7 +362,7 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
return ipv6_recv_rxpmtu(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;
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 7b52a380d710..06c740939d9d 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2124,7 +2124,7 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
do {
mutex_lock(&u->iolock);
- skip = sk_peek_offset(sk, flags);
+ skip = sk_peek_offset(sk, &flags);
skb = __skb_try_recv_datagram(sk, flags, NULL, &peeked, &skip,
&err, &last);
if (skb)
@@ -2304,10 +2304,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
*/
mutex_lock(&u->iolock);
- if (flags & MSG_PEEK)
- skip = sk_peek_offset(sk, flags);
- else
- skip = 0;
+ skip = sk_peek_offset(sk, &flags);
do {
int chunk;
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