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// Copyright 2023 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 checksum
import (
"encoding/binary"
"math/bits"
"unsafe"
)
// Note: odd indicates whether initial is a partial checksum over an odd number
// of bytes.
func calculateChecksum(buf []byte, odd bool, initial uint16) (uint16, bool) {
// Use a larger-than-uint16 accumulator to benefit from parallel summation
// as described in RFC 1071 1.2.C.
acc := uint64(initial)
// Handle an odd number of previously-summed bytes, and get the return
// value for odd.
if odd {
acc += uint64(buf[0])
buf = buf[1:]
}
odd = len(buf)&1 != 0
// Aligning &buf[0] below is much simpler if len(buf) >= 8; special-case
// smaller bufs.
if len(buf) < 8 {
if len(buf) >= 4 {
acc += (uint64(buf[0]) << 8) + uint64(buf[1])
acc += (uint64(buf[2]) << 8) + uint64(buf[3])
buf = buf[4:]
}
if len(buf) >= 2 {
acc += (uint64(buf[0]) << 8) + uint64(buf[1])
buf = buf[2:]
}
if len(buf) >= 1 {
acc += uint64(buf[0]) << 8
// buf = buf[1:] is skipped because it's unused and nogo will
// complain.
}
return reduce(acc), odd
}
// On little-endian architectures, multi-byte loads from buf will load
// bytes in the wrong order. Rather than byte-swap after each load (slow),
// we byte-swap the accumulator before summing any bytes and byte-swap it
// back before returning, which still produces the correct result as
// described in RFC 1071 1.2.B "Byte Order Independence".
//
// acc is at most a uint16 + a uint8, so its upper 32 bits must be 0s. We
// preserve this property by byte-swapping only the lower 32 bits of acc,
// so that additions to acc performed during alignment can't overflow.
acc = uint64(bswapIfLittleEndian32(uint32(acc)))
// Align &buf[0] to an 8-byte boundary.
bswapped := false
if sliceAddr(buf)&1 != 0 {
// Compute the rest of the partial checksum with bytes swapped, and
// swap back before returning; see the last paragraph of
// RFC 1071 1.2.B.
acc = uint64(bits.ReverseBytes32(uint32(acc)))
bswapped = true
// No `<< 8` here due to the byte swap we just did.
acc += uint64(bswapIfLittleEndian16(uint16(buf[0])))
buf = buf[1:]
}
if sliceAddr(buf)&2 != 0 {
acc += uint64(*(*uint16)(unsafe.Pointer(&buf[0])))
buf = buf[2:]
}
if sliceAddr(buf)&4 != 0 {
acc += uint64(*(*uint32)(unsafe.Pointer(&buf[0])))
buf = buf[4:]
}
// Sum 64 bytes at a time. Beyond this point, additions to acc may
// overflow, so we have to handle carrying.
for len(buf) >= 64 {
var carry uint64
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[0])), 0)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[8])), carry)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[16])), carry)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[24])), carry)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[32])), carry)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[40])), carry)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[48])), carry)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[56])), carry)
acc, _ = bits.Add64(acc, 0, carry)
buf = buf[64:]
}
// Sum the remaining 0-63 bytes.
if len(buf) >= 32 {
var carry uint64
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[0])), 0)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[8])), carry)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[16])), carry)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[24])), carry)
acc, _ = bits.Add64(acc, 0, carry)
buf = buf[32:]
}
if len(buf) >= 16 {
var carry uint64
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[0])), 0)
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[8])), carry)
acc, _ = bits.Add64(acc, 0, carry)
buf = buf[16:]
}
if len(buf) >= 8 {
var carry uint64
acc, carry = bits.Add64(acc, *(*uint64)(unsafe.Pointer(&buf[0])), 0)
acc, _ = bits.Add64(acc, 0, carry)
buf = buf[8:]
}
if len(buf) >= 4 {
var carry uint64
acc, carry = bits.Add64(acc, uint64(*(*uint32)(unsafe.Pointer(&buf[0]))), 0)
acc, _ = bits.Add64(acc, 0, carry)
buf = buf[4:]
}
if len(buf) >= 2 {
var carry uint64
acc, carry = bits.Add64(acc, uint64(*(*uint16)(unsafe.Pointer(&buf[0]))), 0)
acc, _ = bits.Add64(acc, 0, carry)
buf = buf[2:]
}
if len(buf) >= 1 {
// bswapIfBigEndian16(buf[0]) == bswapIfLittleEndian16(buf[0]<<8).
var carry uint64
acc, carry = bits.Add64(acc, uint64(bswapIfBigEndian16(uint16(buf[0]))), 0)
acc, _ = bits.Add64(acc, 0, carry)
// buf = buf[1:] is skipped because it's unused and nogo will complain.
}
// Reduce the checksum to 16 bits and undo byte swaps before returning.
acc16 := bswapIfLittleEndian16(reduce(acc))
if bswapped {
acc16 = bits.ReverseBytes16(acc16)
}
return acc16, odd
}
func reduce(acc uint64) uint16 {
// Ideally we would do:
// return uint16(acc>>48) +' uint16(acc>>32) +' uint16(acc>>16) +' uint16(acc)
// for more instruction-level parallelism; however, there is no
// bits.Add16().
acc = (acc >> 32) + (acc & 0xffff_ffff) // at most 0x1_ffff_fffe
acc32 := uint32(acc>>32 + acc) // at most 0xffff_ffff
acc32 = (acc32 >> 16) + (acc32 & 0xffff) // at most 0x1_fffe
return uint16(acc32>>16 + acc32) // at most 0xffff
}
func bswapIfLittleEndian32(val uint32) uint32 {
return binary.BigEndian.Uint32((*[4]byte)(unsafe.Pointer(&val))[:])
}
func bswapIfLittleEndian16(val uint16) uint16 {
return binary.BigEndian.Uint16((*[2]byte)(unsafe.Pointer(&val))[:])
}
func bswapIfBigEndian16(val uint16) uint16 {
return binary.LittleEndian.Uint16((*[2]byte)(unsafe.Pointer(&val))[:])
}
func sliceAddr(buf []byte) uintptr {
return uintptr(unsafe.Pointer(unsafe.SliceData(buf)))
}
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