Add tests, documentation

ee/tuf/finalize_linux.go

@@ -4,6 +4,7 @@
 package tuf
 
 import (
+	"bytes"
 	"context"
 	"debug/elf"
 	"errors"
@@ -14,6 +15,9 @@ import (
 	"github.com/kolide/launcher/pkg/allowedcmd"
 )
 
+// On NixOS, we have to set the interpreter for any non-NixOS executable we want to
+// run. This means the binaries that our updater downloads.
+// See: https://unix.stackexchange.com/a/522823
 func patchExecutable(executableLocation string) error {
 	if !allowedcmd.IsNixOS() {
 		return nil
@@ -60,8 +64,10 @@ func getInterpreter(executableLocation string) (string, error) {
 		return "", fmt.Errorf("reading .interp section: %w", err)
 	}
 
-	// interpData should look something like "/lib64/ld-linux-x86-64.so.2"
-	return filepath.Base(string(interpData)), nil
+	trimmedInterpData := bytes.TrimRight(interpData, "\x00")
+
+	// interpData should look something like "/lib64/ld-linux-x86-64.so.2" -- grab just the filename
+	return filepath.Base(string(trimmedInterpData)), nil
 }
 
 func findInterpreterInNixStore(interpreter string) (string, error) {

ee/tuf/finalize_linux_test.go

@@ -0,0 +1,22 @@
+//go:build linux
+// +build linux
+
+package tuf
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_getInterpreter(t *testing.T) {
+	// Use the current executable in our test
+	currentRunningExecutable, err := os.Executable()
+	require.NoError(t, err, "getting current executable")
+
+	// Confirm we pick the expected interpreter
+	interpreter, err := getInterpreter(currentRunningExecutable)
+	require.NoError(t, err, "expected no error getting interpreter")
+	require.Equal(t, "ld-linux-x86-64.so.2", interpreter)
+}

ee/tuf/finalize_other_test.go

@@ -0,0 +1,15 @@
+//go:build !linux
+// +build !linux
+
+package tuf
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_patchExecutable(t *testing.T) {
+	// patchExecutable is a no-op on windows and darwin
+	require.NoError(t, patchExecutable(""))
+}