tls: Add receive path documentation

Add documentation on rx path setup and cmsg interface.

Signed-off-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/tls.txt

@@ -48,6 +48,9 @@ the transmit and the receive into the kernel.
 
   setsockopt(sock, SOL_TLS, TLS_TX, &crypto_info, sizeof(crypto_info));
 
+Transmit and receive are set separately, but the setup is the same, using either
+TLS_TX or TLS_RX.
+
 Sending TLS application data
 ----------------------------
 
@@ -79,6 +82,28 @@ for memory), or the encryption will always succeed.  If send() returns
 -ENOMEM and some data was left on the socket buffer from a previous
 call using MSG_MORE, the MSG_MORE data is left on the socket buffer.
 
+Receiving TLS application data
+------------------------------
+
+After setting the TLS_RX socket option, all recv family socket calls
+are decrypted using TLS parameters provided.  A full TLS record must
+be received before decryption can happen.
+
+  char buffer[16384];
+  recv(sock, buffer, 16384);
+
+Received data is decrypted directly in to the user buffer if it is
+large enough, and no additional allocations occur.  If the userspace
+buffer is too small, data is decrypted in the kernel and copied to
+userspace.
+
+EINVAL is returned if the TLS version in the received message does not
+match the version passed in setsockopt.
+
+EMSGSIZE is returned if the received message is too big.
+
+EBADMSG is returned if decryption failed for any other reason.
+
 Send TLS control messages
 -------------------------
 
@@ -118,6 +143,43 @@ using a record of type @record_type.
 Control message data should be provided unencrypted, and will be
 encrypted by the kernel.
 
+Receiving TLS control messages
+------------------------------
+
+TLS control messages are passed in the userspace buffer, with message
+type passed via cmsg.  If no cmsg buffer is provided, an error is
+returned if a control message is received.  Data messages may be
+received without a cmsg buffer set.
+
+  char buffer[16384];
+  char cmsg[CMSG_SPACE(sizeof(unsigned char))];
+  struct msghdr msg = {0};
+  msg.msg_control = cmsg;
+  msg.msg_controllen = sizeof(cmsg);
+
+  struct iovec msg_iov;
+  msg_iov.iov_base = buffer;
+  msg_iov.iov_len = 16384;
+
+  msg.msg_iov = &msg_iov;
+  msg.msg_iovlen = 1;
+
+  int ret = recvmsg(sock, &msg, 0 /* flags */);
+
+  struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+  if (cmsg->cmsg_level == SOL_TLS &&
+      cmsg->cmsg_type == TLS_GET_RECORD_TYPE) {
+      int record_type = *((unsigned char *)CMSG_DATA(cmsg));
+      // Do something with record_type, and control message data in
+      // buffer.
+      //
+      // Note that record_type may be == to application data (23).
+  } else {
+      // Buffer contains application data.
+  }
+
+recv will never return data from mixed types of TLS records.
+
 Integrating in to userspace TLS library
 ---------------------------------------
 
@@ -126,10 +188,10 @@ layer of a userspace TLS library.
 
 A patchset to OpenSSL to use ktls as the record layer is here:
 
-https://github.com/Mellanox/tls-openssl
+https://github.com/Mellanox/openssl/commits/tls_rx2
 
 An example of calling send directly after a handshake using
 gnutls.  Since it doesn't implement a full record layer, control
 messages are not supported:
 
-https://github.com/Mellanox/tls-af_ktls_tool
+https://github.com/ktls/af_ktls-tool/commits/RX