Use PutBucketPolicy with an AWS SDK or CLI

bool AwsDoc::S3::putBucketPolicy(const Aws::String &bucketName,
                                 const Aws::String &policyBody,
                                 const Aws::S3::S3ClientConfiguration &clientConfig) {
    Aws::S3::S3Client s3Client(clientConfig);

    std::shared_ptr<Aws::StringStream> request_body =
            Aws::MakeShared<Aws::StringStream>("");
    *request_body << policyBody;

    Aws::S3::Model::PutBucketPolicyRequest request;
    request.SetBucket(bucketName);
    request.SetBody(request_body);

    Aws::S3::Model::PutBucketPolicyOutcome outcome =
            s3Client.PutBucketPolicy(request);

    if (!outcome.IsSuccess()) {
        std::cerr << "Error: putBucketPolicy: "
                  << outcome.GetError().GetMessage() << std::endl;
    } else {
        std::cout << "Set the following policy body for the bucket '" <<
                  bucketName << "':" << std::endl << std::endl;
        std::cout << policyBody << std::endl;
    }

    return outcome.IsSuccess();
}


//! Build a policy JSON string.
/*!
  \param userArn: Aws user Amazon Resource Name (ARN).
      For more information, see https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_identifiers.html#identifiers-arns.
  \param bucketName: Name of a bucket.
  \return String: Policy as JSON string.
*/

Aws::String getPolicyString(const Aws::String &userArn,
                            const Aws::String &bucketName) {
    return
            "{\n"
            "   \"Version\":\"2012-10-17\",\n"
            "   \"Statement\":[\n"
            "       {\n"
            "           \"Sid\": \"1\",\n"
            "           \"Effect\": \"Allow\",\n"
            "           \"Principal\": {\n"
            "               \"AWS\": \""
            + userArn +
            "\"\n""           },\n"
            "           \"Action\": [ \"s3:getObject\" ],\n"
            "           \"Resource\": [ \"arn:aws:s3:::"
            + bucketName +
            "/*\" ]\n"
            "       }\n"
            "   ]\n"
            "}";
}

This example allows all users to retrieve any object in amzn-s3-demo-bucket except those in the MySecretFolder. It also grants put and delete permission to the root user of the AWS account 1234-5678-9012:

aws s3api put-bucket-policy --bucket amzn-s3-demo-bucket --policy file://policy.json

policy.json:
{
   "Statement": [
      {
         "Effect": "Allow",
         "Principal": "*",
         "Action": "s3:GetObject",
         "Resource": "arn:aws:s3:::amzn-s3-demo-bucket/*"
      },
      {
         "Effect": "Deny",
         "Principal": "*",
         "Action": "s3:GetObject",
         "Resource": "arn:aws:s3:::amzn-s3-demo-bucket/MySecretFolder/*"
      },
      {
         "Effect": "Allow",
         "Principal": {
            "AWS": "arn:aws:iam::123456789012:root"
         },
         "Action": [
            "s3:DeleteObject",
            "s3:PutObject"
         ],
         "Resource": "arn:aws:s3:::amzn-s3-demo-bucket/*"
      }
   ]
}

import software.amazon.awssdk.services.s3.S3Client;
import software.amazon.awssdk.services.s3.model.PutBucketPolicyRequest;
import software.amazon.awssdk.services.s3.model.S3Exception;
import software.amazon.awssdk.regions.Region;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.ObjectMapper;

/**
 * Before running this Java V2 code example, set up your development
 * environment, including your credentials.
 * <p>
 * For more information, see the following documentation topic:
 * <p>
 * https://docs.aws.amazon.com/sdk-for-java/latest/developer-guide/get-started.html
 */
public class SetBucketPolicy {
    public static void main(String[] args) {
        final String usage = """

            Usage:
                <bucketName> <polFile>

            Where:
                bucketName - The Amazon S3 bucket to set the policy on.
                polFile - A JSON file containing the policy (see the Amazon S3 Readme for an example).\s
            """;

        if (args.length != 2) {
            System.out.println(usage);
            System.exit(1);
        }

        String bucketName = args[0];
        String polFile = args[1];
        String policyText = getBucketPolicyFromFile(polFile);
        Region region = Region.US_EAST_1;
        S3Client s3 = S3Client.builder()
            .region(region)
            .build();

        setPolicy(s3, bucketName, policyText);
        s3.close();
    }

    /**
     * Sets the policy for an Amazon S3 bucket.
     *
     * @param s3         the {@link S3Client} object used to interact with the Amazon S3 service
     * @param bucketName the name of the Amazon S3 bucket
     * @param policyText the text of the policy to be set on the bucket
     * @throws S3Exception if there is an error setting the bucket policy
     */
    public static void setPolicy(S3Client s3, String bucketName, String policyText) {
        System.out.println("Setting policy:");
        System.out.println("----");
        System.out.println(policyText);
        System.out.println("----");
        System.out.format("On Amazon S3 bucket: \"%s\"\n", bucketName);

        try {
            PutBucketPolicyRequest policyReq = PutBucketPolicyRequest.builder()
                .bucket(bucketName)
                .policy(policyText)
                .build();

            s3.putBucketPolicy(policyReq);

        } catch (S3Exception e) {
            System.err.println(e.awsErrorDetails().errorMessage());
            System.exit(1);
        }

        System.out.println("Done!");
    }

    /**
     * Retrieves the bucket policy from a specified file.
     *
     * @param policyFile the path to the file containing the bucket policy
     * @return the content of the bucket policy file as a string
     */
    public static String getBucketPolicyFromFile(String policyFile) {
        StringBuilder fileText = new StringBuilder();
        try {
            List<String> lines = Files.readAllLines(Paths.get(policyFile), StandardCharsets.UTF_8);
            for (String line : lines) {
                fileText.append(line);
            }

        } catch (IOException e) {
            System.out.format("Problem reading file: \"%s\"", policyFile);
            System.out.println(e.getMessage());
        }

        try {
            final JsonParser parser = new ObjectMapper().getFactory().createParser(fileText.toString());
            while (parser.nextToken() != null) {
            }

        } catch (IOException jpe) {
            jpe.printStackTrace();
        }
        return fileText.toString();
    }
}

Add the policy.

import {
  PutBucketPolicyCommand,
  S3Client,
  S3ServiceException,
} from "@aws-sdk/client-s3";

/**
 * Grant an IAM role GetObject access to all of the objects
 * in the provided bucket.
 * @param {{ bucketName: string, iamRoleArn: string }}
 */
export const main = async ({ bucketName, iamRoleArn }) => {
  const client = new S3Client({});
  const command = new PutBucketPolicyCommand({
    // This is a resource-based policy. For more information on resource-based policies,
    // see https://docs.aws.amazon.com/IAM/latest/UserGuide/access_policies.html#policies_resource-based.
    Policy: JSON.stringify({
      Version: "2012-10-17",
      Statement: [
        {
          Effect: "Allow",
          Principal: {
            AWS: iamRoleArn,
          },
          Action: "s3:GetObject",
          Resource: `arn:aws:s3:::${bucketName}/*`,
        },
      ],
    }),
    // Apply the preceding policy to this bucket.
    Bucket: bucketName,
  });

  try {
    await client.send(command);
    console.log(
      `GetObject access to the bucket "${bucketName}" was granted to the provided IAM role.`,
    );
  } catch (caught) {
    if (
      caught instanceof S3ServiceException &&
      caught.name === "MalformedPolicy"
    ) {
      console.error(
        `Error from S3 while setting the bucket policy for the bucket "${bucketName}". The policy was malformed.`,
      );
    } else if (caught instanceof S3ServiceException) {
      console.error(
        `Error from S3 while setting the bucket policy for the bucket "${bucketName}". ${caught.name}: ${caught.message}`,
      );
    } else {
      throw caught;
    }
  }
};

class BucketWrapper:
    """Encapsulates S3 bucket actions."""

    def __init__(self, bucket):
        """
        :param bucket: A Boto3 Bucket resource. This is a high-level resource in Boto3
                       that wraps bucket actions in a class-like structure.
        """
        self.bucket = bucket
        self.name = bucket.name


    def put_policy(self, policy):
        """
        Apply a security policy to the bucket. Policies control users' ability
        to perform specific actions, such as listing the objects in the bucket.

        :param policy: The policy to apply to the bucket.
        """
        try:
            self.bucket.Policy().put(Policy=json.dumps(policy))
            logger.info("Put policy %s for bucket '%s'.", policy, self.bucket.name)
        except ClientError:
            logger.exception("Couldn't apply policy to bucket '%s'.", self.bucket.name)
            raise

# Wraps an Amazon S3 bucket policy.
class BucketPolicyWrapper
  attr_reader :bucket_policy

  # @param bucket_policy [Aws::S3::BucketPolicy] A bucket policy object configured with an existing bucket.
  def initialize(bucket_policy)
    @bucket_policy = bucket_policy
  end

  # Sets a policy on a bucket.
  #
  def policy(policy)
    @bucket_policy.put(policy: policy)
    true
  rescue Aws::Errors::ServiceError => e
    puts "Couldn't set the policy for #{@bucket_policy.bucket.name}. Here's why: #{e.message}"
    false
  end

end