Unstructured converter panics if the destination struct contains private fields

[Hugo Shaka]

Using the DefaultUnsatructuredConverter with a destination struct containing a non-exported field throws a panic.

I might be in a specific edge case, but we're using protobuf to generate gRPC structs. Those structs contain private fields holding the grpc state and cause the controller to panic, even if the unstructured object only contain the public user-facing fields.

I would expect the converter to ignore private fields. I can send a PR if you confirm this should be fixed and that the fix is to skip the private fields.

Reproducing

Run the following snippet:

package main

import (
	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
	"k8s.io/apimachinery/pkg/runtime"
	"log"
)

type StructuredObject struct {
	Spec StructuredObjectSpec
}

type StructuredObjectSpec struct {
	bar []byte
	Foo string
}

func main() {
	u := &unstructured.Unstructured{
		Object: map[string]interface{}{
			"spec": map[string]interface{}{
				"foo": "value",
			},
		},
	}

	structured := StructuredObject{}
	err := runtime.DefaultUnstructuredConverter.FromUnstructuredWithValidation(
		u.Object,
		&structured,
		true, /* returnUnknownFields */
	)
	if err != nil {
		log.Fatal(err)
	}
}

Observe the panic

panic: reflect: reflect.Value.Set using value obtained using unexported field

goroutine 1 [running]:
reflect.flag.mustBeAssignableSlow(0x140000337a8?)
        /usr/local/go/src/reflect/value.go:269 +0xb4
reflect.flag.mustBeAssignable(...)
        /usr/local/go/src/reflect/value.go:259
reflect.Value.Set({0x100f4c3e0?, 0x14000169800?, 0x14000011168?}, {0x100f4c3e0?, 0x101232e40?, 0x98?})
        /usr/local/go/src/reflect/value.go:2319 +0x58
k8s.io/apimachinery/pkg/runtime.structFromUnstructured({0x100f63c60?, 0x140001697d0?, 0x1012f0248?}, {0x100f7e000?, 0x14000169800?, 0x100c2d5f8?}, 0x14000033e60)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:556 +0x490
k8s.io/apimachinery/pkg/runtime.fromUnstructured({0x100f63c60?, 0x140001697d0?, 0x1400001115c?}, {0x100f7e000?, 0x14000169800?, 0x98?}, 0x14000033e60)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:359 +0x2c4
k8s.io/apimachinery/pkg/runtime.structFromUnstructured({0x100f63c60?, 0x140001697a0?, 0x14000033c48?}, {0x100f71840?, 0x14000169800?, 0x100cabbf4?}, 0x14000033e60)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:550 +0x64c
k8s.io/apimachinery/pkg/runtime.fromUnstructured({0x100f63c60?, 0x140001697a0?, 0x10?}, {0x100f71840?, 0x14000169800?, 0x100c35cec?}, 0x14000033e60)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:359 +0x2c4
k8s.io/apimachinery/pkg/runtime.(*unstructuredConverter).FromUnstructuredWithValidation(0x1011c8df0, 0x140001697a0, {0x100f44b20, 0x14000169800}, 0x1)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:247 +0x20c
main.main()
        /Users/shaka/playground/repro-runtime-panic/main.go:30 +0x110

[Vince Prignano]

This might be an issue for api machinery, rather than controller runtime

[Vince Prignano]

Something to try, use json:"-" tag in private fields

[Hugo Shaka]

Something to try, use json:"-" tag in private fields

The protobuf folks seem opinionated against doing so: golang/protobuf#1379.

I missed that this was in the apimachinery module, sorry for the spam. I'll go open an issue there.

Edit: opened kubernetes/kubernetes#124154