syscall: allow abstract unix socket to use the full Path len

The previous implementation forced all Unix socket to have a name
strictly shorter than len(sa.raw.Path) to allow a terminating NULL
byte to be added. This requirement does not apply to abstract socket
names under Linux, so for this case we allow the full length.

Fixes #21965

Change-Id: I1d1f58b6b6172d589428c7230cfeae984de78b4b
Reviewed-on: https://go-review.googlesource.com/66190
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/unixsock_test.go

@@ -516,3 +516,57 @@ func TestUnixUnlink(t *testing.T) {
 		l.Close()
 	})
 }
+
+func TestUnixLinuxAbstractLongName(t *testing.T) {
+	if runtime.GOOS != "linux" || !testableNetwork("unixgram") {
+		t.Skip("abstract unix socket long name test")
+	}
+
+	// Create an abstract socket name whose length is exactly
+	// the maximum RawSockkaddrUnix Path len
+	rsu := syscall.RawSockaddrUnix{}
+	addrBytes := make([]byte, len(rsu.Path))
+	copy(addrBytes, "@abstract_test")
+	addr := string(addrBytes)
+
+	la, err := ResolveUnixAddr("unixgram", addr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	c, err := ListenUnixgram("unixgram", la)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.Close()
+
+	off := make(chan bool)
+	data := [5]byte{1, 2, 3, 4, 5}
+	go func() {
+		defer func() { off <- true }()
+		s, err := syscall.Socket(syscall.AF_UNIX, syscall.SOCK_DGRAM, 0)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		defer syscall.Close(s)
+		rsa := &syscall.SockaddrUnix{Name: addr}
+		if err := syscall.Sendto(s, data[:], 0, rsa); err != nil {
+			t.Error(err)
+			return
+		}
+	}()
+
+	<-off
+	b := make([]byte, 64)
+	c.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+	n, from, err := c.ReadFrom(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if from != nil {
+		t.Fatalf("unexpected peer address: %v", from)
+	}
+	if !bytes.Equal(b[:n], data[:]) {
+		t.Fatalf("got %v; want %v", b[:n], data[:])
+	}
+}

src/syscall/syscall_linux.go

@@ -295,7 +295,10 @@ func (sa *SockaddrInet6) sockaddr() (unsafe.Pointer, _Socklen, error) {
 func (sa *SockaddrUnix) sockaddr() (unsafe.Pointer, _Socklen, error) {
 	name := sa.Name
 	n := len(name)
-	if n >= len(sa.raw.Path) {
+	if n > len(sa.raw.Path) {
+		return nil, 0, EINVAL
+	}
+	if n == len(sa.raw.Path) && name[0] != '@' {
 		return nil, 0, EINVAL
 	}
 	sa.raw.Family = AF_UNIX