Adding a resource based policy vs IAM policy

I want to have a lambda accessing secret manager.

For reasons beyond the scope of this question, I want to go with a resource based policy.

So I am considering allowing principal lambda-my-lambda in the specific SM secret.

Will I also need to add a specific IAM policy to the lambda's executor role?

Or will just the resource based policy allow lambda-my-lambda to access the secrets' manager secret?

Either is sufficient on its own.

Identity-based policy = "Entry Pass"

• Permissions policy is attached to your Lambda Role

Resource-based policy = "Guest List"

• Permissions policy is attached to the Secret
• "Principal": <Lambda Role ARN>