syscall: Fix FD passing on OpenBSD

Fixes #3349.

R=bradfitz, dave, minux.ma
CC=golang-dev
https://golang.org/cl/7383056

src/pkg/syscall/passfd_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux darwin freebsd netbsd
+// +build linux darwin freebsd netbsd openbsd
 
 package syscall_test
 
@@ -149,3 +149,49 @@ func passFDChild() {
 		return
 	}
 }
+
+// TestUnixRightsRoundtrip tests that UnixRights, ParseSocketControlMessage,
+// and ParseUnixRights are able to successfully round-trip lists of file descriptors.
+func TestUnixRightsRoundtrip(t *testing.T) {
+	testCases := [...][][]int{
+		{{42}},
+		{{1, 2}},
+		{{3, 4, 5}},
+		{{}},
+		{{1, 2}, {3, 4, 5}, {}, {7}},
+	}
+	for _, testCase := range testCases {
+		b := []byte{}
+		var n int
+		for _, fds := range testCase {
+			// Last assignment to n wins
+			n = len(b) + syscall.CmsgLen(4*len(fds))
+			b = append(b, syscall.UnixRights(fds...)...)
+		}
+		// Truncate b
+		b = b[:n]
+
+		scms, err := syscall.ParseSocketControlMessage(b)
+		if err != nil {
+			t.Fatalf("ParseSocketControlMessage: %v", err)
+		}
+		if len(scms) != len(testCase) {
+			t.Fatalf("expected %v SocketControlMessage; got scms = %#v", len(testCase), scms)
+		}
+		for i, scm := range scms {
+			gotFds, err := syscall.ParseUnixRights(&scm)
+			if err != nil {
+				t.Fatalf("ParseUnixRights: %v", err)
+			}
+			wantFds := testCase[i]
+			if len(gotFds) != len(wantFds) {
+				t.Fatalf("expected %v fds, got %#v", len(wantFds), gotFds)
+			}
+			for j, fd := range gotFds {
+				if fd != wantFds[j] {
+					t.Fatalf("expected fd %v, got %v", wantFds[j], fd)
+				}
+			}
+		}
+	}
+}

src/pkg/syscall/sockcmsg_unix.go

@@ -10,7 +10,7 @@ package syscall
 
 import "unsafe"
 
-// Round the length of a raw sockaddr up to align it propery.
+// Round the length of a raw sockaddr up to align it properly.
 func cmsgAlignOf(salen int) int {
 	salign := sizeofPtr
 	// NOTE: It seems like 64-bit Darwin kernel still requires 32-bit
@@ -18,9 +18,6 @@ func cmsgAlignOf(salen int) int {
 	if darwinAMD64 {
 		salign = 4
 	}
-	if salen == 0 {
-		return salign
-	}
 	return (salen + salign - 1) & ^(salign - 1)
 }
 
@@ -50,14 +47,15 @@ type SocketControlMessage struct {
 // messages.
 func ParseSocketControlMessage(b []byte) ([]SocketControlMessage, error) {
 	var msgs []SocketControlMessage
-	for len(b) >= CmsgLen(0) {
-		h, dbuf, err := socketControlMessageHeaderAndData(b)
+	i := 0
+	for i+CmsgLen(0) <= len(b) {
+		h, dbuf, err := socketControlMessageHeaderAndData(b[i:])
 		if err != nil {
 			return nil, err
 		}
-		m := SocketControlMessage{Header: *h, Data: dbuf[:int(h.Len)-cmsgAlignOf(SizeofCmsghdr)]}
+		m := SocketControlMessage{Header: *h, Data: dbuf}
 		msgs = append(msgs, m)
-		b = b[cmsgAlignOf(int(h.Len)):]
+		i += cmsgAlignOf(int(h.Len))
 	}
 	return msgs, nil
 }
@@ -67,7 +65,7 @@ func socketControlMessageHeaderAndData(b []byte) (*Cmsghdr, []byte, error) {
 	if h.Len < SizeofCmsghdr || int(h.Len) > len(b) {
 		return nil, nil, EINVAL
 	}
-	return h, b[cmsgAlignOf(SizeofCmsghdr):], nil
+	return h, b[cmsgAlignOf(SizeofCmsghdr):h.Len], nil
 }
 
 // UnixRights encodes a set of open file descriptors into a socket