2 files changed, 49 insertions, 20 deletions

diff --git a/pkg/sentry/socket/control/control.go b/pkg/sentry/socket/control/control.go
index fa3188d51..af1a4e95f 100644
--- a/pkg/sentry/socket/control/control.go
+++ b/pkg/sentry/socket/control/control.go
@@ -348,43 +348,62 @@ func PackTClass(t *kernel.Task, tClass int32, buf []byte) []byte {
 	)
 }
 
-func addSpaceForCmsg(cmsgDataLen int, buf []byte) []byte {
-	newBuf := make([]byte, 0, len(buf)+linux.SizeOfControlMessageHeader+cmsgDataLen)
-	return append(newBuf, buf...)
-}
-
-// PackControlMessages converts the given ControlMessages struct into a buffer.
+// PackControlMessages packs control messages into the given buffer.
+//
 // We skip control messages specific to Unix domain sockets.
-func PackControlMessages(t *kernel.Task, cmsgs socket.ControlMessages) []byte {
-	var buf []byte
-	// The use of t.Arch().Width() is analogous to Linux's use of sizeof(long) in
-	// CMSG_ALIGN.
-	width := t.Arch().Width()
-
+//
+// Note that some control messages may be truncated if they do not fit under
+// the capacity of buf.
+func PackControlMessages(t *kernel.Task, cmsgs socket.ControlMessages, buf []byte) []byte {
 	if cmsgs.IP.HasTimestamp {
-		buf = addSpaceForCmsg(int(width), buf)
 		buf = PackTimestamp(t, cmsgs.IP.Timestamp, buf)
 	}
 
 	if cmsgs.IP.HasInq {
 		// In Linux, TCP_CM_INQ is added after SO_TIMESTAMP.
-		buf = addSpaceForCmsg(AlignUp(linux.SizeOfControlMessageInq, width), buf)
 		buf = PackInq(t, cmsgs.IP.Inq, buf)
 	}
 
 	if cmsgs.IP.HasTOS {
-		buf = addSpaceForCmsg(AlignUp(linux.SizeOfControlMessageTOS, width), buf)
 		buf = PackTOS(t, cmsgs.IP.TOS, buf)
 	}
 
 	if cmsgs.IP.HasTClass {
-		buf = addSpaceForCmsg(AlignUp(linux.SizeOfControlMessageTClass, width), buf)
 		buf = PackTClass(t, cmsgs.IP.TClass, buf)
 	}
 
 	return buf
 }
 
+// cmsgSpace is equivalent to CMSG_SPACE in Linux.
+func cmsgSpace(t *kernel.Task, dataLen int) int {
+	return linux.SizeOfControlMessageHeader + AlignUp(dataLen, t.Arch().Width())
+}
+
+// CmsgsSpace returns the number of bytes needed to fit the control messages
+// represented in cmsgs.
+func CmsgsSpace(t *kernel.Task, cmsgs socket.ControlMessages) int {
+	space := 0
+
+	if cmsgs.IP.HasTimestamp {
+		space += cmsgSpace(t, linux.SizeOfTimeval)
+	}
+
+	if cmsgs.IP.HasInq {
+		space += cmsgSpace(t, linux.SizeOfControlMessageInq)
+	}
+
+	if cmsgs.IP.HasTOS {
+		space += cmsgSpace(t, linux.SizeOfControlMessageTOS)
+	}
+
+	if cmsgs.IP.HasTClass {
+		space += cmsgSpace(t, linux.SizeOfControlMessageTClass)
+	}
+
+	return space
+}
+
 // Parse parses a raw socket control message into portable objects.
 func Parse(t *kernel.Task, socketOrEndpoint interface{}, buf []byte) (socket.ControlMessages, error) {
 	var (
diff --git a/pkg/sentry/socket/hostinet/socket.go b/pkg/sentry/socket/hostinet/socket.go
index a8c152b54..c957b0f1d 100644
--- a/pkg/sentry/socket/hostinet/socket.go
+++ b/pkg/sentry/socket/hostinet/socket.go
@@ -45,7 +45,7 @@ const (
 	sizeofSockaddr = syscall.SizeofSockaddrInet6 // sizeof(sockaddr_in6) > sizeof(sockaddr_in)
 
 	// maxControlLen is the maximum size of a control message buffer used in a
-	// recvmsg syscall.
+	// recvmsg or sendmsg syscall.
 	maxControlLen = 1024
 )
 
@@ -412,9 +412,12 @@ func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 			msg.Namelen = uint32(len(senderAddrBuf))
 		}
 		if controlLen > 0 {
-			controlBuf = make([]byte, maxControlLen)
+			if controlLen > maxControlLen {
+				controlLen = maxControlLen
+			}
+			controlBuf = make([]byte, controlLen)
 			msg.Control = &controlBuf[0]
-			msg.Controllen = maxControlLen
+			msg.Controllen = controlLen
 		}
 		n, err := recvmsg(s.fd, &msg, sysflags)
 		if err != nil {
@@ -489,7 +492,14 @@ func (s *socketOperations) SendMsg(t *kernel.Task, src usermem.IOSequence, to []
 		return 0, syserr.ErrInvalidArgument
 	}
 
-	controlBuf := control.PackControlMessages(t, controlMessages)
+	space := uint64(control.CmsgsSpace(t, controlMessages))
+	if space > maxControlLen {
+		space = maxControlLen
+	}
+	controlBuf := make([]byte, 0, space)
+	// PackControlMessages will append up to space bytes to controlBuf.
+	controlBuf = control.PackControlMessages(t, controlMessages, controlBuf)
+
 	sendmsgFromBlocks := safemem.WriterFunc(func(srcs safemem.BlockSeq) (uint64, error) {
 		// Refuse to do anything if any part of src.Addrs was unusable.
 		if uint64(src.NumBytes()) != srcs.NumBytes() {