net/tls: enable sk_msg redirect to tls socket egress

Bring back tls_sw_sendpage_locked. sk_msg redirection into a socket
with TLS_TX takes the following path:

tcp_bpf_sendmsg_redir
tcp_bpf_push_locked
tcp_bpf_push
kernel_sendpage_locked
sock->ops->sendpage_locked

Also update the flags test in tls_sw_sendpage_locked to allow flag
MSG_NO_SHARED_FRAGS. bpf_tcp_sendmsg sets this.

Link: https://lore.kernel.org/netdev/[e-mail]/T/#t
Link: https://github.com/wdebruij/kerneltools/commits/icept.2
Fixes: 0608c69c9a80 ("bpf: sk_msg, sock{map|hash} redirect through ULP")
Fixes: f3de19af0f5b ("Revert \"net/tls: remove unused function tls_sw_sendpage_locked\"")
Signed-off-by: Willem de Bruijn <[e-mail]>
Acked-by: John Fastabend <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/tls.h

···
 void tls_sw_strparser_arm(struct sock *sk, struct tls_context *ctx);
 void tls_sw_strparser_done(struct tls_context *tls_ctx);
 int tls_sw_sendmsg(struct sock *sk, struct msghdr *msg, size_t size);
+int tls_sw_sendpage_locked(struct sock *sk, struct page *page,
+			   int offset, size_t size, int flags);
 int tls_sw_sendpage(struct sock *sk, struct page *page,
 		    int offset, size_t size, int flags);
 void tls_sw_cancel_work_tx(struct tls_context *tls_ctx);

net/tls/tls_main.c

···
 {
 	tls_sw_proto_ops = inet_stream_ops;
 	tls_sw_proto_ops.splice_read = tls_sw_splice_read;
+	tls_sw_proto_ops.sendpage_locked   = tls_sw_sendpage_locked,
 
 	tls_device_init();
 	tcp_register_ulp(&tcp_tls_ulp_ops);

net/tls/tls_sw.c

···
 	return copied ? copied : ret;
 }
 
+int tls_sw_sendpage_locked(struct sock *sk, struct page *page,
+			   int offset, size_t size, int flags)
+{
+	if (flags & ~(MSG_MORE | MSG_DONTWAIT | MSG_NOSIGNAL |
+		      MSG_SENDPAGE_NOTLAST | MSG_SENDPAGE_NOPOLICY |
+		      MSG_NO_SHARED_FRAGS))
+		return -ENOTSUPP;
+
+	return tls_sw_do_sendpage(sk, page, offset, size, flags);
+}
+
 int tls_sw_sendpage(struct sock *sk, struct page *page,
 		    int offset, size_t size, int flags)
 {