Move some helpers into modules

For functional clarity.

Signed-off-by: Colin Walters <walters@verbum.org>

bib/internal/setup/setup.go

@@ -0,0 +1,78 @@
+package setup
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+
+	"github.com/osbuild/bootc-image-builder/bib/internal/utils"
+)
+
+// EnsureEnvironment mutates external filesystem state as necessary
+// to run in a container environment.  This function is idempotent.
+func EnsureEnvironment() error {
+	osbuildPath := "/usr/bin/osbuild"
+	if utils.IsMountpoint(osbuildPath) {
+		return nil
+	}
+
+	// A hardcoded security label from Fedora derivatives
+	// TODO: Avoid hardcoding this by using either host policy lookup
+	// Or eventually depend on privileged containers just having this capability.
+	//
+	// We need this in order to get `install_t` that has `CAP_MAC_ADMIN` for creating SELinux
+	// labels unknown to the host.
+	//
+	// Note that the transition to `install_t` must happen at this point. Osbuild stages run in `bwrap` that creates
+	// a nosuid, no_new_privs environment. In such an environment, we cannot transition from `unconfined_t` to `install_t`,
+	// because we would get more privileges.
+	installType := "system_u:object_r:install_exec_t:s0"
+	// Where we dump temporary files; this must be an overlayfs as we cannot
+	// write security contexts on overlayfs.
+	runTmp := "/run/osbuild/"
+
+	if err := os.MkdirAll(runTmp, 0o755); err != nil {
+		return err
+	}
+	if !utils.IsMountpoint(runTmp) {
+		if err := exec.Command("mount", "-t", "tmpfs", "tmpfs", runTmp).Run(); err != nil {
+			return fmt.Errorf("failed to mount tmpfs to %s: %w", runTmp, err)
+		}
+	}
+	src, err := os.Open("/usr/bin/osbuild")
+	if err != nil {
+		return err
+	}
+	defer src.Close()
+	destPath := filepath.Join(runTmp, "osbuild")
+	dst, err := os.Create(destPath)
+	if err != nil {
+		return err
+	}
+	defer dst.Close()
+	if _, err := io.Copy(dst, src); err != nil {
+		return err
+	}
+	if err := dst.Chmod(0o755); err != nil {
+		return err
+	}
+	dst.Close()
+	if err := exec.Command("chcon", installType, destPath).Run(); err != nil {
+		return fmt.Errorf("failed to chcon: %w", err)
+	}
+
+	// Create a bind mount into our target location; we can't copy it because
+	// again we have to perserve the SELinux label.
+	if err := exec.Command("mount", "--bind", destPath, osbuildPath).Run(); err != nil {
+		return fmt.Errorf("failed to bind mount to %s: %w", osbuildPath, err)
+	}
+	return nil
+}
+
+// Validate checks that the environment is supported (e.g. caller set up the
+// container correctly)
+func Validate() error {
+	return nil
+}