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/45872
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/syscall/syscall_linux.go b/src/syscall/syscall_linux.go
index 4d5a5e12d3..3c7d378d71 100644
--- a/src/syscall/syscall_linux.go
+++ b/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)
 		}