tailscale/net/dns/utf.go

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// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package dns
// This code is only used in Windows builds, but is in an
// OS-independent file so tests can run all the time.
import (
"bytes"
"encoding/binary"
"unicode/utf16"
)
// maybeUnUTF16 tries to detect whether bs contains UTF-16, and if so
// translates it to regular UTF-8.
//
// Some of wsl.exe's output get printed as UTF-16, which breaks a
// bunch of things. Try to detect this by looking for a zero byte in
// the first few bytes of output (which will appear if any of those
// codepoints are basic ASCII - very likely). From that we can infer
// that UTF-16 is being printed, and the byte order in use, and we
// decode that back to UTF-8.
//
// https://github.com/microsoft/WSL/issues/4607
func maybeUnUTF16(bs []byte) []byte {
if len(bs)%2 != 0 {
// Can't be complete UTF-16.
return bs
}
checkLen := 20
if len(bs) < checkLen {
checkLen = len(bs)
}
zeroOff := bytes.IndexByte(bs[:checkLen], 0)
if zeroOff == -1 {
return bs
}
// We assume wsl.exe is trying to print an ASCII codepoint,
// meaning the zero byte is in the upper 8 bits of the
// codepoint. That means we can use the zero's byte offset to
// work out if we're seeing little-endian or big-endian
// UTF-16.
var endian binary.ByteOrder = binary.LittleEndian
if zeroOff%2 == 0 {
endian = binary.BigEndian
}
var u16 []uint16
for i := 0; i < len(bs); i += 2 {
u16 = append(u16, endian.Uint16(bs[i:]))
}
return []byte(string(utf16.Decode(u16)))
}