> Hi,
>
> Recently I wanted to write an application which receives DSCP value from kernel through recvmsg's ancilliary control data feature, but I had to realize that despite I've used setsockopt with IP_RECVTOS parameter to achieve this, the buffer contained only SCTP level control data. When I looked into the code, I found that in net/sctp/socket.c::sctp_recvmsg() the necessary lines are commented out:
>
> if (sp->subscribe.sctp_data_io_event)
> sctp_ulpevent_read_sndrcvinfo(event, msg);
> #if 0
> /* FIXME: we should be calling IP/IPv6 layers. */
> if (sk->sk_protinfo.af_inet.cmsg_flags)
> ip_cmsg_recv(msg, skb);
> #endif
those code do not run any more, and maybe only run for IPv4 before
it be removed.
> I've tried to search about the reasons, but I couldn't find any answers about why is this left like this. Is there any special reason? At first look this code snippet looks working, but I suppose it's not an accident why it is not included in the official code.
You can try the following un-test patch, only for IPv4.
diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
index 5c9bada..7dcbca8 100644
--- a/include/net/sctp/structs.h
+++ b/include/net/sctp/structs.h
@@ -631,6 +631,8 @@ struct sctp_pf {
struct sock *(*create_accept_sk) (struct sock *sk,
struct sctp_association *asoc);
void (*addr_v4map) (struct sctp_sock *, union sctp_addr *);
+ void (*cmsg_recv_ctl)(struct sock *sk, struct msghdr *msg,
+ struct sk_buff *skb);
struct sctp_af *af;
};
diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
index 865ce7b..01bde2d 100644
--- a/net/sctp/ipv6.c
+++ b/net/sctp/ipv6.c
@@ -893,6 +893,12 @@ static int sctp_inet6_supported_addrs(const struct sctp_sock *opt,
return 1;
}
+static void sctp_v6_cmsg_recv_ctl(struct sock *sk, struct msghdr *msg,
+ struct sk_buff *skb)
+{
+ // skip
+}
+
static const struct proto_ops inet6_seqpacket_ops = {
.family = PF_INET6,
.owner = THIS_MODULE,
@@ -988,6 +994,7 @@ static struct sctp_pf sctp_pf_inet6 = {
.supported_addrs = sctp_inet6_supported_addrs,
.create_accept_sk = sctp_v6_create_accept_sk,
.addr_v4map = sctp_v6_addr_v4map,
+ .cmsg_recv_ctl = sctp_v6_cmsg_recv_ctl,
.af = &sctp_af_inet6,
};
diff --git a/net/sctp/protocol.c b/net/sctp/protocol.c
index d5bf91d..64ec7ed 100644
--- a/net/sctp/protocol.c
+++ b/net/sctp/protocol.c
@@ -846,6 +846,15 @@ static int sctp_inet_supported_addrs(const struct sctp_sock *opt,
return 1;
}
+static void sctp_v4_cmsg_recv_ctl(struct sock *sk, struct msghdr *msg,
+ struct sk_buff *skb)
+{
+ struct inet_sock *inet = inet_sk(sk);
+
+ if (inet->cmsg_flags)
+ ip_cmsg_recv(msg, skb);
+}
+
/* Wrapper routine that calls the ip transmit routine. */
static inline int sctp_v4_xmit(struct sk_buff *skb,
struct sctp_transport *transport)
@@ -876,6 +885,7 @@ static struct sctp_pf sctp_pf_inet = {
.supported_addrs = sctp_inet_supported_addrs,
.create_accept_sk = sctp_v4_create_accept_sk,
.addr_v4map = sctp_v4_addr_v4map,
+ .cmsg_recv_ctl = sctp_v4_cmsg_recv_ctl,
.af = &sctp_af_inet
};
diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index f694ee1..c8bb106 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -1997,11 +1997,8 @@ SCTP_STATIC int sctp_recvmsg(struct kiocb *iocb, struct sock *sk,
/* Check if we allow SCTP_SNDRCVINFO. */
if (sp->subscribe.sctp_data_io_event)
sctp_ulpevent_read_sndrcvinfo(event, msg);
-#if 0
- /* FIXME: we should be calling IP/IPv6 layers. */
- if (sk->sk_protinfo.af_inet.cmsg_flags)
- ip_cmsg_recv(msg, skb);
-#endif
+
+ sp->pf->cmsg_recv_ctl(sk, msg, skb);
err = copied;
--
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