syscall: drop dummy byte for oob in unixgram SendmsgN

This commit relaxes SendmsgN behavior of introducing a dummy 1-byte
payload when sending ancillary-only messages.
The fake payload is not needed for SOCK_DGRAM type sockets, and actually
breaks interoperability with other fd-passing software (journald is one
known example). This introduces an additional check to avoid injecting
dummy payload in such case.

Full reference at https:/golang.org/issue/6476#issue-51285243

Fixes #6476

Change-Id: I19a974b4e7920e002bd0556259ab766572358520
Reviewed-on: https://go-review.googlesource.com/45872Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/creds_test.go

@@ -19,7 +19,21 @@ import (
 // sockets. The SO_PASSCRED socket option is enabled on the sending
 // socket for this to work.
 func TestSCMCredentials(t *testing.T) {
-	fds, err := syscall.Socketpair(syscall.AF_LOCAL, syscall.SOCK_STREAM, 0)
+	socketTypeTests := []struct {
+		socketType int
+		dataLen    int
+	}{
+		{
+			syscall.SOCK_STREAM,
+			1,
+		}, {
+			syscall.SOCK_DGRAM,
+			0,
+		},
+	}
+
+	for _, tt := range socketTypeTests {
+		fds, err := syscall.Socketpair(syscall.AF_LOCAL, tt.socketType, 0)
 		if err != nil {
 			t.Fatalf("Socketpair: %v", err)
 		}
@@ -72,7 +86,7 @@ func TestSCMCredentials(t *testing.T) {
 		ucred.Gid = uint32(os.Getgid())
 		oob := syscall.UnixCredentials(&ucred)
 
-	// this is going to send a dummy byte
+		// On SOCK_STREAM, this is internally going to send a dummy byte
 		n, oobn, err := cli.(*net.UnixConn).WriteMsgUnix(nil, oob, nil)
 		if err != nil {
 			t.Fatalf("WriteMsgUnix: %v", err)
@@ -92,8 +106,8 @@ func TestSCMCredentials(t *testing.T) {
 		if flags != 0 {
 			t.Fatalf("ReadMsgUnix flags = 0x%x, want 0", flags)
 		}
-	if n != 1 {
-		t.Fatalf("ReadMsgUnix n = %d, want 1 (dummy byte)", n)
+		if n != tt.dataLen {
+			t.Fatalf("ReadMsgUnix n = %d, want %d", n, tt.dataLen)
 		}
 		if oobn2 != oobn {
 			// without SO_PASSCRED set on the socket, ReadMsgUnix will
@@ -116,4 +130,5 @@ func TestSCMCredentials(t *testing.T) {
 		if *newUcred != ucred {
 			t.Fatalf("ParseUnixCredentials = %+v, want %+v", newUcred, ucred)
 		}
+	}
 }

src/syscall/syscall_linux.go

@@ -537,8 +537,13 @@ func Recvmsg(fd int, p, oob []byte, flags int) (n, oobn int, recvflags int, from
 	}
 	var dummy byte
 	if len(oob) > 0 {
+		var sockType int
+		sockType, err = GetsockoptInt(fd, SOL_SOCKET, SO_TYPE)
+		if err != nil {
+			return
+		}
 		// receive at least one normal byte
-		if len(p) == 0 {
+		if sockType != SOCK_DGRAM && len(p) == 0 {
 			iov.Base = &dummy
 			iov.SetLen(1)
 		}
@@ -584,8 +589,13 @@ func SendmsgN(fd int, p, oob []byte, to Sockaddr, flags int) (n int, err error)
 	}
 	var dummy byte
 	if len(oob) > 0 {
+		var sockType int
+		sockType, err = GetsockoptInt(fd, SOL_SOCKET, SO_TYPE)
+		if err != nil {
+			return 0, err
+		}
 		// send at least one normal byte
-		if len(p) == 0 {
+		if sockType != SOCK_DGRAM && len(p) == 0 {
 			iov.Base = &dummy
 			iov.SetLen(1)
 		}