What version of Go are you using (go version)?

$ go version
go version go1.14.1 linux/amd64

Does this issue reproduce with the latest release?

It reproduces on the latest version of x/sys.

What operating system and processor architecture are you using (go env)?

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/jpas/.cache/go-build"
GOENV="/home/jpas/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/jpas/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/nix/store/gvw1mfpdrk7i82884yhxf9lf5j3c12zm-go-1.14.1/share/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/nix/store/gvw1mfpdrk7i82884yhxf9lf5j3c12zm-go-1.14.1/share/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="/nix/store/k930l9lckxjf4zmafv4aapvdbd2sibmj-gcc-wrapper-9.3.0/bin/cc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build011582959=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
	"fmt"
	"github.com/pkg/errors"
	. "golang.org/x/sys/unix"
	"os"
	"os/exec"
	"strconv"
)

func bug(addr [6]uint8) error {
	fd, err := Socket(AF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP)
	if err != nil {
		return err
	}
	defer Close(fd)

	saIn := SockaddrL2{Addr: addr, PSM: 17}
	if err := bind(fd, &saIn); err != nil {
		return err
	}

	pp, err := Getsockname(fd)
	if err != nil {
		return err
	}

	saOut := pp.(*SockaddrL2)
	fmt.Printf("expected: %#v\n", saIn.Addr)
	fmt.Printf("actual  : %#v\n", saOut.Addr)

	return nil
}

func bind(fd int, sa *SockaddrL2) error {
	// We try to bind twice since bluez's input plugin may cause this address to
	// already be in use.
	if Bind(fd, sa) == nil {
		return nil
	}

	// By restarting the bluetooth service, we introduce a race between our program
	// and bluez to bind.
	exec.Command("systemctl", "restart", "bluetooth").Run()

	return Bind(fd, sa)
}

func parseBdAddr(addr string) ([6]uint8, error) {
	var bd [6]uint8

	for i := range bd {
		j := 3 * i
		b, err := strconv.ParseUint(addr[j:j+2], 16, 8)
		if err != nil {
			return bd, errors.Wrap(err, "addr malformed")
		}
		bd[i] = uint8(b)
	}

	return bd, nil
}

func main() {
	if os.Getuid() != 0 {
		fmt.Println("please run as root")
		os.Exit(1)
	}

	if len(os.Args) != 2 {
		fmt.Printf("usage: %s 11:22:33:44:55:66\n", os.Args[0])
		os.Exit(1)
	}

	addr, err := parseBdAddr(os.Args[1])
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}

	if err := bug(addr); err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
}

There are some underlying issues with the "input" plugin for Bluez and binding to Bluetooth sockets. They can be mitigated by restarting the bluetooth daemon and racing to bind the socket.

What did you expect to see?

expected: [6]uint8{0x80, 0x32, 0x53, 0x37, 0x22, 0x19}
actual  : [6]uint8{0x80, 0x32, 0x53, 0x37, 0x22, 0x19}

What did you see instead?

expected: [6]uint8{0x80, 0x32, 0x53, 0x37, 0x22, 0x19}
actual  : [6]uint8{0x19, 0x22, 0x37, 0x53, 0x32, 0x80}

Change proposals

Remove the three lines reversing the order of the bytes, which would mean that SockaddrL2.Addr will be expected to be in network order, not the order that it would usually appear as a string.

Alternatively, we could add the same three lines to anyToSockaddr here, so that the order is the same as what is expected as input to Bind and Connect.

The first option seems like the better option to me since it is in line with RawSockaddrL2 and the underlying syscall API. But the second one may be "more friendly" as the byte order matches the string's ordering.