Load minimal spec to extract container root

Instead of loading the full OCI spec when extracting the container root
from the container state, this change ensures that only the Root is
unmarshalled.

Signed-off-by: Evan Lezar <elezar@nvidia.com>

Author: elezar

internal/oci/state.go

@@ -56,26 +56,10 @@ func ReadContainerState(reader io.Reader) (*State, error) {
 	return &s, nil
 }
 
-// LoadSpec loads the OCI spec associated with the container state
-func (s *State) LoadSpec() (*specs.Spec, error) {
-	specFilePath := GetSpecFilePath(s.Bundle)
-	specFile, err := os.Open(specFilePath)
-	if err != nil {
-		return nil, fmt.Errorf("failed to open OCI spec file: %v", err)
-	}
-	defer specFile.Close()
-
-	spec, err := LoadFrom(specFile)
-	if err != nil {
-		return nil, fmt.Errorf("failed to load OCI spec: %v", err)
-	}
-	return spec, nil
-}
-
 // GetContainerRoot returns the root for the container from the associated spec. If the spec is not yet loaded, it is
 // loaded and cached.
 func (s *State) GetContainerRoot() (string, error) {
-	spec, err := s.LoadSpec()
+	spec, err := s.loadMinimalSpec()
 	if err != nil {
 		return "", err
 	}
@@ -91,3 +75,27 @@ func (s *State) GetContainerRoot() (string, error) {
 
 	return filepath.Join(s.Bundle, containerRoot), nil
 }
+
+// loadMinimalSpec loads a reduced OCI spec associated with the container state.
+func (s *State) loadMinimalSpec() (*minimalSpec, error) {
+	specFilePath := GetSpecFilePath(s.Bundle)
+	specFile, err := os.Open(specFilePath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to open OCI spec file: %v", err)
+	}
+	defer specFile.Close()
+
+	ms := &minimalSpec{}
+	if err := json.NewDecoder(specFile).Decode(ms); err != nil {
+		return nil, fmt.Errorf("failed to load minimal OCI spec: %v", err)
+	}
+	return ms, nil
+}
+
+// A minimalSpec is used to return desired properties from the container config.
+// We define this here instead of using specs.Spec as is to avoid decoding
+// unneeded fields in container lifecycle hooks.
+type minimalSpec struct {
+	// Root configures the container's root filesystem.
+	Root *specs.Root `json:"root,omitempty"`
+}