[RFC net-next] pppoe: remove session socket receive support

[Qingfang Deng <qingfang.deng@linux.dev>]

No known userspace PPPoE implementation reads from an AF_PPPOX
socket. In particular, pppd uses the socket only to establish a
session and obtain a generic PPP channel with PPPIOCGCHAN. Packet I/O
then uses /dev/ppp.

Before this change, packets received before PPPIOCGCHAN set
PPPOX_BOUND were queued on sk_receive_queue. Since no known
implementation consumed them, they remained there until socket
destruction and unnecessarily consumed memory.

Pass every matched session packet directly to ppp_input(). On a
successful connect, the channel is registered before the socket lock
is released. Generic PPP can therefore queue early packets until
userspace attaches /dev/ppp, preserving packets that arrive between
connect() and PPPIOCGCHAN.

Replace pppoe_recvmsg() with sock_no_recvmsg(). Like PPTP, omit the
poll callback because the socket no longer receives packets. Remove
pppoe_destruct() because the receive queue is no longer used.

Assisted-by: Codex:GPT-5.6
Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
---
 drivers/net/ppp/pppoe.c | 50 ++---------------------------------------
 1 file changed, 2 insertions(+), 48 deletions(-)

diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index bf7414b46a26..e3d425943c76 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -350,23 +350,9 @@ static int pppoe_rcv_core(struct sock *sk, struct sk_buff *skb)
 {
 	struct pppox_sock *po = pppox_sk(sk);
 
-	/* Backlog receive. Semantics of backlog rcv preclude any code from
-	 * executing in lock_sock()/release_sock() bounds; meaning sk->sk_state
-	 * can't change.
-	 */
-
-	if (sk->sk_state & PPPOX_BOUND) {
-		ppp_input(&po->chan, skb);
-	} else {
-		if (sock_queue_rcv_skb(sk, skb))
-			goto abort_kfree;
-	}
+	ppp_input(&po->chan, skb);
 
 	return NET_RX_SUCCESS;
-
-abort_kfree:
-	kfree_skb(skb);
-	return NET_RX_DROP;
 }
 
 /************************************************************************
@@ -498,11 +484,6 @@ static struct proto pppoe_sk_proto __read_mostly = {
 	.obj_size = sizeof(struct pppox_sock),
 };
 
-static void pppoe_destruct(struct sock *sk)
-{
-	skb_queue_purge(&sk->sk_receive_queue);
-}
-
 /***********************************************************************
  *
  * Initialize a new struct sock.
@@ -523,7 +504,6 @@ static int pppoe_create(struct net *net, struct socket *sock, int kern)
 	sock->ops	= &pppoe_ops;
 
 	sk->sk_backlog_rcv	= pppoe_rcv_core;
-	sk->sk_destruct		= pppoe_destruct;
 	sk->sk_state		= PPPOX_NONE;
 	sk->sk_type		= SOCK_STREAM;
 	sk->sk_family		= PF_PPPOX;
@@ -921,31 +901,6 @@ static const struct ppp_channel_ops pppoe_chan_ops = {
 	.fill_forward_path = pppoe_fill_forward_path,
 };
 
-static int pppoe_recvmsg(struct socket *sock, struct msghdr *m,
-			 size_t total_len, int flags)
-{
-	struct sock *sk = sock->sk;
-	struct sk_buff *skb;
-	int error = 0;
-
-	if (sk->sk_state & PPPOX_BOUND)
-		return -EIO;
-
-	skb = skb_recv_datagram(sk, flags, &error);
-	if (!skb)
-		return error;
-
-	total_len = min_t(size_t, total_len, skb->len);
-	error = skb_copy_datagram_msg(skb, 0, m, total_len);
-	if (error == 0) {
-		consume_skb(skb);
-		return total_len;
-	}
-
-	kfree_skb(skb);
-	return error;
-}
-
 #ifdef CONFIG_PROC_FS
 static int pppoe_seq_show(struct seq_file *seq, void *v)
 {
@@ -1046,11 +1001,10 @@ static const struct proto_ops pppoe_ops = {
 	.socketpair	= sock_no_socketpair,
 	.accept		= sock_no_accept,
 	.getname	= pppoe_getname,
-	.poll		= datagram_poll,
 	.listen		= sock_no_listen,
 	.shutdown	= sock_no_shutdown,
 	.sendmsg	= pppoe_sendmsg,
-	.recvmsg	= pppoe_recvmsg,
+	.recvmsg	= sock_no_recvmsg,
 	.mmap		= sock_no_mmap,
 	.ioctl		= pppox_ioctl,
 #ifdef CONFIG_COMPAT
-- 
2.43.0