Implement the MSG_CTRUNC msghdr flag for Unix sockets.

Updates google/gvisor#206

PiperOrigin-RevId: 245880573
Change-Id: Ifa715e98d47f64b8a32b04ae9378d6cd6bd4025e

4 files changed, 21 insertions, 16 deletions

diff --git a/pkg/sentry/fs/host/control.go b/pkg/sentry/fs/host/control.go
index 480f0c8f4..9ebb9bbb3 100644
--- a/pkg/sentry/fs/host/control.go
+++ b/pkg/sentry/fs/host/control.go
@@ -32,17 +32,20 @@ func newSCMRights(fds []int) control.SCMRights {
 }
 
 // Files implements control.SCMRights.Files.
-func (c *scmRights) Files(ctx context.Context, max int) control.RightsFiles {
+func (c *scmRights) Files(ctx context.Context, max int) (control.RightsFiles, bool) {
 	n := max
+	var trunc bool
 	if l := len(c.fds); n > l {
 		n = l
+	} else if n < l {
+		trunc = true
 	}
 
 	rf := control.RightsFiles(fdsToFiles(ctx, c.fds[:n]))
 
 	// Only consume converted FDs (fdsToFiles may convert fewer than n FDs).
 	c.fds = c.fds[len(rf):]
-	return rf
+	return rf, trunc
 }
 
 // Clone implements transport.RightsControlMessage.Clone.
diff --git a/pkg/sentry/fs/host/socket.go b/pkg/sentry/fs/host/socket.go
index 3034e9441..3ed137006 100644
--- a/pkg/sentry/fs/host/socket.go
+++ b/pkg/sentry/fs/host/socket.go
@@ -282,11 +282,11 @@ func (c *ConnectedEndpoint) EventUpdate() {
 }
 
 // Recv implements transport.Receiver.Recv.
-func (c *ConnectedEndpoint) Recv(data [][]byte, creds bool, numRights uintptr, peek bool) (uintptr, uintptr, transport.ControlMessages, tcpip.FullAddress, bool, *syserr.Error) {
+func (c *ConnectedEndpoint) Recv(data [][]byte, creds bool, numRights uintptr, peek bool) (uintptr, uintptr, transport.ControlMessages, bool, tcpip.FullAddress, bool, *syserr.Error) {
 	c.mu.RLock()
 	defer c.mu.RUnlock()
 	if c.readClosed {
-		return 0, 0, transport.ControlMessages{}, tcpip.FullAddress{}, false, syserr.ErrClosedForReceive
+		return 0, 0, transport.ControlMessages{}, false, tcpip.FullAddress{}, false, syserr.ErrClosedForReceive
 	}
 
 	var cm unet.ControlMessage
@@ -296,7 +296,7 @@ func (c *ConnectedEndpoint) Recv(data [][]byte, creds bool, numRights uintptr, p
 
 	// N.B. Unix sockets don't have a receive buffer, the send buffer
 	// serves both purposes.
-	rl, ml, cl, err := fdReadVec(c.file.FD(), data, []byte(cm), peek, c.sndbuf)
+	rl, ml, cl, cTrunc, err := fdReadVec(c.file.FD(), data, []byte(cm), peek, c.sndbuf)
 	if rl > 0 && err != nil {
 		// We got some data, so all we need to do on error is return
 		// the data that we got. Short reads are fine, no need to
@@ -304,7 +304,7 @@ func (c *ConnectedEndpoint) Recv(data [][]byte, creds bool, numRights uintptr, p
 		err = nil
 	}
 	if err != nil {
-		return 0, 0, transport.ControlMessages{}, tcpip.FullAddress{}, false, syserr.FromError(err)
+		return 0, 0, transport.ControlMessages{}, false, tcpip.FullAddress{}, false, syserr.FromError(err)
 	}
 
 	// There is no need for the callee to call RecvNotify because fdReadVec uses
@@ -317,18 +317,18 @@ func (c *ConnectedEndpoint) Recv(data [][]byte, creds bool, numRights uintptr, p
 
 	// Avoid extra allocations in the case where there isn't any control data.
 	if len(cm) == 0 {
-		return rl, ml, transport.ControlMessages{}, tcpip.FullAddress{Addr: tcpip.Address(c.path)}, false, nil
+		return rl, ml, transport.ControlMessages{}, cTrunc, tcpip.FullAddress{Addr: tcpip.Address(c.path)}, false, nil
 	}
 
 	fds, err := cm.ExtractFDs()
 	if err != nil {
-		return 0, 0, transport.ControlMessages{}, tcpip.FullAddress{}, false, syserr.FromError(err)
+		return 0, 0, transport.ControlMessages{}, false, tcpip.FullAddress{}, false, syserr.FromError(err)
 	}
 
 	if len(fds) == 0 {
-		return rl, ml, transport.ControlMessages{}, tcpip.FullAddress{Addr: tcpip.Address(c.path)}, false, nil
+		return rl, ml, transport.ControlMessages{}, cTrunc, tcpip.FullAddress{Addr: tcpip.Address(c.path)}, false, nil
 	}
-	return rl, ml, control.New(nil, nil, newSCMRights(fds)), tcpip.FullAddress{Addr: tcpip.Address(c.path)}, false, nil
+	return rl, ml, control.New(nil, nil, newSCMRights(fds)), cTrunc, tcpip.FullAddress{Addr: tcpip.Address(c.path)}, false, nil
 }
 
 // close releases all resources related to the endpoint.
diff --git a/pkg/sentry/fs/host/socket_test.go b/pkg/sentry/fs/host/socket_test.go
index cc760a7e1..06392a65a 100644
--- a/pkg/sentry/fs/host/socket_test.go
+++ b/pkg/sentry/fs/host/socket_test.go
@@ -207,7 +207,7 @@ func TestSend(t *testing.T) {
 
 func TestRecv(t *testing.T) {
 	e := ConnectedEndpoint{readClosed: true}
-	if _, _, _, _, _, err := e.Recv(nil, false, 0, false); err != syserr.ErrClosedForReceive {
+	if _, _, _, _, _, _, err := e.Recv(nil, false, 0, false); err != syserr.ErrClosedForReceive {
 		t.Errorf("Got %#v.Recv() = %v, want = %v", e, err, syserr.ErrClosedForReceive)
 	}
 }
diff --git a/pkg/sentry/fs/host/socket_unsafe.go b/pkg/sentry/fs/host/socket_unsafe.go
index 8873705c0..e57be0506 100644
--- a/pkg/sentry/fs/host/socket_unsafe.go
+++ b/pkg/sentry/fs/host/socket_unsafe.go
@@ -23,7 +23,7 @@ import (
 //
 // If the total length of bufs is > maxlen, fdReadVec will do a partial read
 // and err will indicate why the message was truncated.
-func fdReadVec(fd int, bufs [][]byte, control []byte, peek bool, maxlen int) (readLen uintptr, msgLen uintptr, controlLen uint64, err error) {
+func fdReadVec(fd int, bufs [][]byte, control []byte, peek bool, maxlen int) (readLen uintptr, msgLen uintptr, controlLen uint64, controlTrunc bool, err error) {
 	flags := uintptr(syscall.MSG_DONTWAIT | syscall.MSG_TRUNC)
 	if peek {
 		flags |= syscall.MSG_PEEK
@@ -34,7 +34,7 @@ func fdReadVec(fd int, bufs [][]byte, control []byte, peek bool, maxlen int) (re
 	length, iovecs, intermediate, err := buildIovec(bufs, maxlen, true)
 	if err != nil && len(iovecs) == 0 {
 		// No partial write to do, return error immediately.
-		return 0, 0, 0, err
+		return 0, 0, 0, false, err
 	}
 
 	var msg syscall.Msghdr
@@ -51,7 +51,7 @@ func fdReadVec(fd int, bufs [][]byte, control []byte, peek bool, maxlen int) (re
 	n, _, e := syscall.RawSyscall(syscall.SYS_RECVMSG, uintptr(fd), uintptr(unsafe.Pointer(&msg)), flags)
 	if e != 0 {
 		// N.B. prioritize the syscall error over the buildIovec error.
-		return 0, 0, 0, e
+		return 0, 0, 0, false, e
 	}
 
 	// Copy data back to bufs.
@@ -59,11 +59,13 @@ func fdReadVec(fd int, bufs [][]byte, control []byte, peek bool, maxlen int) (re
 		copyToMulti(bufs, intermediate)
 	}
 
+	controlTrunc = msg.Flags&syscall.MSG_CTRUNC == syscall.MSG_CTRUNC
+
 	if n > length {
-		return length, n, msg.Controllen, err
+		return length, n, msg.Controllen, controlTrunc, err
 	}
 
-	return n, n, msg.Controllen, err
+	return n, n, msg.Controllen, controlTrunc, err
 }
 
 // fdWriteVec sends from bufs to fd.