Allow overriding mount options for /dev and /dev/pts

This is useful to optionally set /dev ro,noexec.

Treat /dev and /dev/pts the same as /proc and /sys.
Make sure the Type is right though. Many config.json snippets
on the Internet suggest /dev is tmpfs, not devtmpfs.

runsc/boot/fs.go

@@ -103,33 +103,28 @@ func addOverlay(ctx context.Context, conf *Config, lower *fs.Inode, name string,
 // mandatory mounts that are required by the OCI specification.
 func compileMounts(spec *specs.Spec) []specs.Mount {
 	// Keep track of whether proc and sys were mounted.
-	var procMounted, sysMounted bool
+	var procMounted, sysMounted, devMounted, devptsMounted bool
 	var mounts []specs.Mount
 
-	// Always mount /dev.
-	mounts = append(mounts, specs.Mount{
-		Type:        devtmpfs.Name,
-		Destination: "/dev",
-	})
-
-	mounts = append(mounts, specs.Mount{
-		Type:        devpts.Name,
-		Destination: "/dev/pts",
-	})
-
 	// Mount all submounts from the spec.
 	for _, m := range spec.Mounts {
 		if !specutils.IsSupportedDevMount(m) {
 			log.Warningf("ignoring dev mount at %q", m.Destination)
 			continue
 		}
-		mounts = append(mounts, m)
 		switch filepath.Clean(m.Destination) {
 		case "/proc":
 			procMounted = true
 		case "/sys":
 			sysMounted = true
+		case "/dev":
+			m.Type = devtmpfs.Name
+			devMounted = true
+		case "/dev/pts":
+			m.Type = devpts.Name
+			devptsMounted = true
 		}
+		mounts = append(mounts, m)
 	}
 
 	// Mount proc and sys even if the user did not ask for it, as the spec
@@ -147,6 +142,18 @@ func compileMounts(spec *specs.Spec) []specs.Mount {
 			Destination: "/sys",
 		})
 	}
+	if !devMounted {
+		mandatoryMounts = append(mandatoryMounts, specs.Mount{
+			Type:        devtmpfs.Name,
+			Destination: "/dev",
+		})
+	}
+	if !devptsMounted {
+		mandatoryMounts = append(mandatoryMounts, specs.Mount{
+			Type:        devpts.Name,
+			Destination: "/dev/pts",
+		})
+	}
 
 	// The mandatory mounts should be ordered right after the root, in case
 	// there are submounts of these mandatory mounts already in the spec.

runsc/specutils/specutils.go

@@ -334,15 +334,9 @@ func IsSupportedDevMount(m specs.Mount) bool {
 	var existingDevices = []string{
 		"/dev/fd", "/dev/stdin", "/dev/stdout", "/dev/stderr",
 		"/dev/null", "/dev/zero", "/dev/full", "/dev/random",
-		"/dev/urandom", "/dev/shm", "/dev/pts", "/dev/ptmx",
+		"/dev/urandom", "/dev/shm", "/dev/ptmx",
 	}
 	dst := filepath.Clean(m.Destination)
-	if dst == "/dev" {
-		// OCI spec uses many different mounts for the things inside of '/dev'. We
-		// have a single mount at '/dev' that is always mounted, regardless of
-		// whether it was asked for, as the spec says we SHOULD.
-		return false
-	}
 	for _, dev := range existingDevices {
 		if dst == dev || strings.HasPrefix(dst, dev+"/") {
 			return false