Code build role is not authorized to perform: sts:AssumeRole on resource

Question

I have a code build leading to EKS. When it calls this particular command "CREDENTIALS=$(aws sts assume-role --role-arn arn:aws:iam::3318******:role/EksWorkshopCodeBuildKubectlRole  --role-session-name code-build --duration-seconds 900)" I get an error 
"An error occurred (AccessDenied) when calling the AssumeRole operation: User: arn:aws:sts::331*****:assumed-role/codebuild-kubernetes-eks-service-role/AWSCodeBuild-31746234-c1a9-4fe9-9cbc-b0d54264613e is not authorized to perform: sts:AssumeRole on resource: arn:aws:iam::331879450537:role/EksWorkshopCodeBuildKubectlRole"

My code build trusted relationship looks like 
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "Service": "codebuild.amazonaws.com"
            },
            "Action": "sts:AssumeRole",
            "Condition": {}
        }
    ]
}
I have an STS policy attached to my user, group, codebuils service role and eksworkshopkubectlrole.
What could I be doing wrong

Answer

As per the documentation https://docs.aws.amazon.com/codebuild/latest/userguide/troubleshooting.html#troubleshooting-assume-role, the issue could be either STS is not active for the AWS region or CodeBuild service role associated with the build project does not exist or does not have sufficient permissions to trust CodeBuild.

Answer

Would it work if you modify the trusted relationship as follows?

Before
```
"Principal": { "Service": "codebuild.amazonaws.com" }, 
```
After
```
"Principal": { "AWS": "arn:aws:iam::331879450537:role/codebuild-kubernetes-eks-service-role" }, 
```

I think that the principal that call the AssumeRole operation is not codebuild, but `codebuild-kubernetes-eks-service-role`.
Of course, `codebuild-kubernetes-eks-service-role` must have permission to call assume role too.

Code build role is not authorized to perform: sts:AssumeRole on resource

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