validate: Check configuration against JSON Schema

runtime-spec publishes a JSON Schema covering the configuration format
(and other JSON related to runtime-spec) [1].  Reduce duplication of
effort by validating configurations against that schema.  For example
this gives us lots of range checking:

  $ ocitools validate
  ...
  1. linux.resources.oomScoreAdj: Must be greater than or equal to -1000
  ...

without us having to duplicate all the range-input work that the
runtime-spec folks have already done for us.

Only validating the JSON Schema is not sufficient, because
--host-specific (e.g. you're running on a Linux box) and
cross-property constraits (e.g. must create a new UTS namespace if you
set hostname) are difficult/impossible to express in JSON Schema.

[1]: https://github.com/opencontainers/runtime-spec/tree/v1.0.0-rc1/schema

Signed-off-by: W. Trevor King <wking@tremily.us>

cmd/ocitools/validate.go

@@ -18,6 +18,7 @@ import (
 	"github.com/blang/semver"
 	rspec "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/urfave/cli"
+	"github.com/xeipuuv/gojsonschema"
 )
 
 type configCheck func(rspec.Spec, string, bool) []string
@@ -61,6 +62,8 @@ var (
 		"CAP_KILL",
 		"CAP_AUDIT_WRITE",
 	}
+
+	configSchemaTemplate = "https://raw.githubusercontent.com/opencontainers/runtime-spec/v%s/schema/schema.json"
 )
 
 var bundleValidateCommand = cli.Command{
@@ -103,6 +106,7 @@ var bundleValidateCommand = cli.Command{
 		checks := []configCheck{
 			checkMandatoryFields,
 			checkSemVer,
+			checkJSONSchema,
 			checkMounts,
 			checkPlatform,
 			checkProcess,
@@ -144,6 +148,44 @@ func checkSemVer(spec rspec.Spec, rootfs string, hostCheck bool) (msgs []string)
 	return
 }
 
+// JSONSchemaURL returns the URL for the JSON Schema specifying the
+// configuration format.  It consumes configSchemaTemplate, but we
+// provide it as a function to isolate consumers from inconsistent
+// naming as runtime-spec evolves.
+func JSONSchemaURL(version string) (url string, err error) {
+	if version != rspec.Version {
+		return "", fmt.Errorf("unrecognized version %s", version)
+	}
+	return fmt.Sprintf(configSchemaTemplate, version), nil
+}
+
+// checkJSONSchema validates the configuration against the
+// runtime-spec JSON Schema, using the version of the schema that
+// matches the configuration's declared version.
+func checkJSONSchema(spec rspec.Spec, rootfs string, hostCheck bool) (msgs []string) {
+	url, err := JSONSchemaURL(spec.Version)
+	if err != nil {
+		msgs = append(msgs, err.Error())
+		return msgs
+	}
+
+	schemaLoader := gojsonschema.NewReferenceLoader(url)
+	documentLoader := gojsonschema.NewGoLoader(spec)
+	result, err := gojsonschema.Validate(schemaLoader, documentLoader)
+	if err != nil {
+		msgs = append(msgs, fmt.Sprintf("internal error: %s", err.Error()))
+		return msgs
+	}
+
+	if !result.Valid() {
+		for _, resultError := range result.Errors() {
+			msgs = append(msgs, fmt.Sprintf("%s", resultError))
+		}
+	}
+
+	return msgs
+}
+
 func checkPlatform(spec rspec.Spec, rootfs string, hostCheck bool) (msgs []string) {
 	logrus.Debugf("check platform")