Granting access to S3 resources based on role name

IAM policy variables are quite cool and let you create generic policys to, for example, give users access to paths in an S3 bucket based on their username, like this:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Action": ["s3:GetObject","s3:PutObject","s3:DeleteObject"],
            "Effect": "Allow",
            "Resource": "arn:aws:s3:::fooCorp-user-files/${aws:username}/*"
        },
        {
            "Action": "s3:ListBucket",
            "Effect": "Allow",
            "Resource": "arn:aws:s3:::fooCorp-user-files"
        }
    ]
}

My question is, how can this be done using roles (attached to EC2 instances) instead of user accounts?

I have a number of app servers with unique IAM user accounts that are linked to a generic policy similar to the one above. This isolates the files accessible by each user/app without creating multiple policies.

I want switch these servers to use roles instead but there doesn't seem to be an equivalent IAM variable like aws:rolename.

The docs indicate that when using a role assigned to an EC2 instance the aws:username variable isn't set and aws:userid is [role-id]:[ec2-instance-id] (which isn't helpful either).

This really seems like something you should be able to do.. or am I coming at this the wrong way?

(Cross-posted to AWS S3 IAM policy for role for restricting few instances to connect to S3 bucket based in instance tag or instance id)

Instead of using aws:SourceArn, use aws:userid!

The Request Information That You Can Use for Policy Variables documentation that you mentioned has a table showing various values of aws:userid including:

For Role assigned to an Amazon EC2 instance, it is set to role-id:ec2-instance-id

Therefore, you could use the Role ID of the role that is used to launch the Amazon EC2 instance to permit access OR the Instance ID.

For example, this one is based on a Role ID:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "SID123",
            "Effect": "Allow",
            "Action": [
                "s3:*"
            ],
            "Resource": [
                "*"
            ],
            "Condition": {
                "StringLike": {
                    "aws:userid": [
                        "AROAIIPEUJOUGITIU5BB6*"
                    ]
                }
            }
        }
    ]
}

Of course, if you are going to assign permission based on a Role ID, then you can just as easily grant permissions within the Role itself.

This one is based on an Instance ID:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "SID123",
            "Effect": "Allow",
            "Action": [
                "s3:*"
            ],
            "Resource": [
                "*"
            ],
            "Condition": {
                "StringLike": {
                    "aws:userid": [
                        "*:i-03c9a5f3fae4b630a"
                    ]
                }
            }
        }
    ]
}

The Instance ID will remain with the instance, but a new one will be assigned if a new instance is launched, even from the same Amazon Machine Image (AMI).