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// Copyright 2019 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package flipcall
import (
"reflect"
"unsafe"
)
// Packets consist of an 8-byte header followed by an arbitrarily-sized
// datagram. The header consists of:
//
// - A 4-byte native-endian connection state.
//
// - A 4-byte native-endian datagram length in bytes.
const (
sizeofUint32 = unsafe.Sizeof(uint32(0))
// PacketHeaderBytes is the size of a flipcall packet header in bytes. The
// maximum datagram size supported by a flipcall connection is equal to the
// length of the packet window minus PacketHeaderBytes.
//
// PacketHeaderBytes is exported to support its use in constant
// expressions. Non-constant expressions may prefer to use
// PacketWindowLengthForDataCap().
PacketHeaderBytes = 2 * sizeofUint32
)
func (ep *Endpoint) connState() *uint32 {
return (*uint32)((unsafe.Pointer)(ep.packet))
}
func (ep *Endpoint) dataLen() *uint32 {
return (*uint32)((unsafe.Pointer)(ep.packet + sizeofUint32))
}
// Data returns the datagram part of ep's packet window as a byte slice.
//
// Note that the packet window is shared with the potentially-untrusted peer
// Endpoint, which may concurrently mutate the contents of the packet window.
// Thus:
//
// - Readers must not assume that two reads of the same byte in Data() will
// return the same result. In other words, readers should read any given byte
// in Data() at most once.
//
// - Writers must not assume that they will read back the same data that they
// have written. In other words, writers should avoid reading from Data() at
// all.
func (ep *Endpoint) Data() []byte {
var bs []byte
bsReflect := (*reflect.SliceHeader)((unsafe.Pointer)(&bs))
bsReflect.Data = ep.packet + PacketHeaderBytes
bsReflect.Len = int(ep.dataCap)
bsReflect.Cap = int(ep.dataCap)
return bs
}
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