在AWS CDK中，如果我们想访问已存储在ARN格式下的Lambda函数，则可以使用function_from_arn方法。然而，在某些情况下，function_from_arn方法可能无法正确执行。

如果遇到此问题，可以尝试使用以下代码示例：

import * as lambda from '@aws-cdk/aws-lambda';
import * as cdk from '@aws-cdk/core';
import * as iam from '@aws-cdk/aws-iam';

const arn = 'arn:aws:lambda:us-east-1:xxxxxxxxxx:function:my-function-name';

export class MyStack extends cdk.Stack {
  constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    const fn = new lambda.Function(this, 'MyFunction', {
      functionName: 'MyFunction',
      runtime: lambda.Runtime.NODEJS_12_X,
      handler: 'index.handler',
      code: lambda.Code.fromAsset('path/to/code')
    });

    const myRole = new iam.Role(this, 'MyFunctionRole', {
      assumedBy: new iam.ServicePrincipal('apigateway.amazonaws.com')
    });

    myRole.addToPolicy(new iam.PolicyStatement({
      effect: iam.Effect.ALLOW,
      actions: ['lambda:InvokeFunction', 'logs:CreateLogGroup', 'logs:CreateLogStream', 'logs:PutLogEvents'],
      resources: [fn.functionArn]
    }));

    const fnFromArn = lambda.Function.fromFunctionArn(this, 'FunctionFromArn', arn);
    const fnFromArnPolicy = new iam.Policy(this, 'PolicyFromArn', {
      statements: [
        new iam.PolicyStatement({
          effect: iam.Effect.ALLOW,
          actions: ['lambda:InvokeFunction'],
          resources: [arn]
        })
      ]
    });

    fnFromArnPolicy.attachToRole(myRole);
  }
}

该示例创建名为“FunctionFromArn”的新Lambda函数并将其策略附加到指定的角色中。Lambda函数ARN的权限与现有函数相同，通过将新函数的角色与现有函数ARN