Don’t attempt to copy built-in modules

Several distributions (including Ubuntu) ship some of the modules we
rely on compiled into the kernel. Since these modules are listed in
modules.builtin, it’s very easy to detect and skip them.

This change fixes go-debos#13 and supersedes go-debos#36.

Signed-off-by: Andrej Shadura <[email protected]>

machine.go

@@ -4,6 +4,7 @@
 package fakemachine
 
 import (
+	"bufio"
 	"bytes"
 	"fmt"
 	"io/ioutil"
@@ -357,7 +358,30 @@ func (m *Machine) writerKernelModules(w *writerhelper.WriterHelper) error {
 		moddir = "/usr/lib/modules"
 	}
 
+	// build a list of built-in modules so that we don’t attempt to copy them
+	var builtinModules = make(map[string]bool)
+
+	f, err := os.Open(path.Join(moddir, kernelRelease, "modules.builtin"))
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		module := scanner.Text()
+		builtinModules[module] = true
+	}
+
+	if err := scanner.Err(); err != nil {
+		return err
+	}
+
 	for _, v := range modules {
+		if builtinModules[v] {
+			continue
+		}
+
 		modpath := path.Join(moddir, kernelRelease, v)
 
 		if strings.HasSuffix(modpath, ".ko") {