firecracker: simplify chroot provisioning (#4868)

Author: bduffany

buildpatches/com_github_firecracker_microvm_firecracker_go_sdk_jailer.patch

@@ -0,0 +1,25 @@
+diff --git a/jailer.go b/jailer.go
+index e261fae..a5e055d 100644
+--- a/jailer.go
++++ b/jailer.go
+@@ -413,10 +413,19 @@ func LinkFilesHandler(kernelImageFileName string) Handler {
+ 			}
+ 
+ 			// copy all drives to the root fs
++			rootfsPrefix := rootfs + string(os.PathSeparator)
+ 			for i, drive := range m.Cfg.Drives {
+ 				hostPath := StringValue(drive.PathOnHost)
+-				driveFileName := filepath.Base(hostPath)
+ 
++				// If the provided host path is already within the rootfs then just
++				// update the drive path to be rootfs-relative.
++				if strings.HasPrefix(hostPath, rootfsPrefix) {
++					rootfsRelativePath := strings.TrimPrefix(hostPath, rootfsPrefix)
++					m.Cfg.Drives[i].PathOnHost = String(rootfsRelativePath)
++					continue
++				}
++
++				driveFileName := filepath.Base(hostPath)
+ 				if err := os.Link(
+ 					hostPath,
+ 					filepath.Join(rootfs, driveFileName),

deps.bzl

@@ -1591,6 +1591,12 @@ def install_go_mod_dependencies(workspace_name = "buildbuddy"):
     go_repository(
         name = "com_github_firecracker_microvm_firecracker_go_sdk",
         importpath = "github.com/firecracker-microvm/firecracker-go-sdk",
+        patch_args = ["-p1"],
+        # TODO(bduffany): when
+        # https://github.com/firecracker-microvm/firecracker-go-sdk/pull/510 is
+        # merged, cherry-pick the final revision into our fork, and remove this
+        # patch.
+        patches = ["@{}//buildpatches:com_github_firecracker_microvm_firecracker_go_sdk_jailer.patch".format(workspace_name)],
         replace = "github.com/buildbuddy-io/firecracker-go-sdk",
         sum = "h1:Wx6fPNZOs0SJ9NpTsLbJsItORvEM9D94k/vr8ZwIBEg=",
         version = "v0.0.0-20230721-1d5c50b",

enterprise/server/remote_execution/containers/firecracker/firecracker.go

@@ -361,7 +361,6 @@ type FirecrackerContainer struct {
 	containerImage   string // the OCI container image. ex "alpine:latest"
 	actionWorkingDir string // the action directory with inputs / outputs
 	containerFSPath  string // the path to the container ext4 image
-	tempDir          string // path for writing disk images before the chroot is created
 	user             string // user to execute all commands as
 
 	rmOnce *sync.Once
@@ -1634,27 +1633,12 @@ func (c *FirecrackerContainer) create(ctx context.Context) error {
 	c.rmOnce = &sync.Once{}
 	c.rmErr = nil
 
-	var err error
-	c.tempDir, err = os.MkdirTemp(c.jailerRoot, "fc-container-*")
-	if err != nil {
-		return err
-	}
 	if err := os.MkdirAll(c.getChroot(), 0755); err != nil {
 		return status.InternalErrorf("failed to create chroot dir: %s", err)
 	}
 
 	scratchFSPath := c.scratchFSPath()
 	workspaceFSPath := c.workspaceFSPath()
-
-	// When mounting the workspace image directly as a block device (rather than
-	// as an NBD), the firecracker go SDK expects the disk images to be outside
-	// the chroot, and will move them to the chroot for us. So we place them in
-	// a temp dir so that the SDK doesn't complain that the chroot paths already
-	// exist when it tries to create them.
-	if !*enableNBD {
-		scratchFSPath = filepath.Join(c.tempDir, scratchFSName)
-		workspaceFSPath = filepath.Join(c.tempDir, workspaceFSName)
-	}
 	if *EnableRootfs {
 		if err := c.initRootfsStore(ctx); err != nil {
 			return status.WrapError(err, "create root image")
@@ -1670,7 +1654,6 @@ func (c *FirecrackerContainer) create(ctx context.Context) error {
 	if err := c.createWorkspaceImage(ctx, "" /*=workspaceDir*/, workspaceFSPath); err != nil {
 		return err
 	}
-	log.CtxDebugf(ctx, "Scratch and workspace disk images written to %q", c.tempDir)
 	log.CtxDebugf(ctx, "Using container image at %q", c.containerFSPath)
 	log.CtxDebugf(ctx, "getChroot() is %q", c.getChroot())
 	fcCfg, err := c.getConfig(ctx, c.containerFSPath, scratchFSPath, workspaceFSPath)
@@ -1969,12 +1952,6 @@ func (c *FirecrackerContainer) remove(ctx context.Context) error {
 		log.CtxErrorf(ctx, "Error cleaning up networking: %s", err)
 		lastErr = err
 	}
-	if c.tempDir != "" {
-		if err := os.RemoveAll(c.tempDir); err != nil {
-			log.CtxErrorf(ctx, "Error removing workspace fs: %s", err)
-			lastErr = err
-		}
-	}
 	if err := os.RemoveAll(filepath.Dir(c.getChroot())); err != nil {
 		log.CtxErrorf(ctx, "Error removing chroot: %s", err)
 		lastErr = err