Validation using JSON queries

An earlier section shows how to write a validation policy by using Go types describing Kubernetes objects.

There is another way to write validation logic, by extracting the relevant data from the JSON document using ad-hoc queries.

This "jq-like" approach can be useful when the policy has to look deep inside a Kubernetes object. It's especially helpful when dealing with optional inner objects.

This document re-implements the earlier code using JSON queries instead of unmarshaling the JSON payload into Go types.

The validate function

You can use the policy you just created and change its validate function to not use the Go types that define Kubernetes objects.

You can instead use the gjson library to extract data from the raw JSON object.

Firstly, you need to change the requirement section. This is how the code should look:

import (
    "encoding/json"
    "fmt"

    mapset "github.com/deckarep/golang-set/v2"
    kubewarden "github.com/kubewarden/policy-sdk-go"
    kubewarden_protocol "github.com/kubewarden/policy-sdk-go/protocol"
    "github.com/tidwall/gjson"
)

Change the validate function to look like:

validate function

func validate(payload []byte) ([]byte, error) {
    // Create a ValidationRequest instance from the incoming payload
    validationRequest := kubewarden_protocol.ValidationRequest{}
    err := json.Unmarshal(payload, &validationRequest)
    if err != nil {
        return kubewarden.RejectRequest(
            kubewarden.Message(err.Error()),
            kubewarden.Code(400))
    }

    // Create a Settings instance from the ValidationRequest object
    settings, err := NewSettingsFromValidationReq(&validationRequest)
    if err != nil {
        return kubewarden.RejectRequest(
            kubewarden.Message(err.Error()),
            kubewarden.Code(400))
    }

    // Access the **raw** JSON that describes the object
    podJSON := validationRequest.Request.Object

    // NOTE 1
    data := gjson.GetBytes(
        podJSON,
        "metadata.labels")

    var validationErr error
    labels := mapset.NewThreadUnsafeSet[string]()
    data.ForEach(func(key, value gjson.Result) bool {
        // NOTE 2
        label := key.String()
        labels.Add(label)

        // NOTE 3
        validationErr = validateLabel(label, value.String(), &settings)

        // keep iterating if there are no errors
        return validationErr == nil
    })

    // NOTE 4
    if validationErr != nil {
        return kubewarden.RejectRequest(
            kubewarden.Message(validationErr.Error()),
            kubewarden.NoCode)
    }

    // NOTE 5
    for requiredLabel := range settings.ConstrainedLabels {
        if !labels.Contains(requiredLabel) {
            return kubewarden.RejectRequest(
                kubewarden.Message(fmt.Sprintf("Constrained label %s not found inside of Pod", requiredLabel)),
                kubewarden.NoCode)
        }
    }

    return kubewarden.AcceptRequest()
}

The first part of the validate function is similar as before. 'NOTE' highlights the changes.

1. You use a gjson selector to get the label map provided by the object embedded into the request
2. You use a gjson helper to iterate over the results of the query. If the query has no results, the loop never takes place.
3. You use the validateLabel function to validate the label and its value, as before. You're also adding the labels found in the Pod to a previously defined mapset.Set.
4. If the validation produced an error, you immediately return with a validation rejection reply.
5. As before, you iterate over the constrainedLabels to check all are specified in the Pod. The code has been slightly changed to make use of the previously populated mapset.Set.

Testing the validation code

The unit tests and the end-to-end tests don't need any change, you can run them as before:

make test

go test -v
=== RUN   TestParseValidSettings
--- PASS: TestParseValidSettings (0.00s)
=== RUN   TestParseSettingsWithInvalidRegexp
--- PASS: TestParseSettingsWithInvalidRegexp (0.00s)
=== RUN   TestDetectValidSettings
--- PASS: TestDetectValidSettings (0.00s)
=== RUN   TestDetectNotValidSettingsDueToBrokenRegexp
--- PASS: TestDetectNotValidSettingsDueToBrokenRegexp (0.00s)
=== RUN   TestDetectNotValidSettingsDueToConflictingLabels
--- PASS: TestDetectNotValidSettingsDueToConflictingLabels (0.00s)
=== RUN   TestValidateLabel
--- PASS: TestValidateLabel (0.00s)
PASS
ok      github.com/kubewarden/go-policy-template    0.002s

make e2e-tests

bats e2e.bats
e2e.bats
 ✓ accept when no settings are provided
 ✓ accept because label is satisfying a constraint
 ✓ accept labels are not on deny list
 ✓ reject because label is on deny list
 ✓ reject because label is not satisfying a constraint
 ✓ reject because constrained label is missing
 ✓ fail settings validation because of conflicting labels
 ✓ fail settings validation because of invalid constraint

8 tests, 0 failures

All the tests are working as expected.