Merge "Implement nlmsg<T>"

/*

 * Copyright (C) 2020 The Android Open Source Project

 *

 * Licensed under the Apache License, Version 2.0 (the "License");

 * you may not use this file except in compliance with the License.

 * You may obtain a copy of the License at

 *

 *      http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

#pragma once

#include <libnl++/nlbuf.h>

namespace android::nl {

/**

 * In-place Netlink message parser.

 *

 * This is a C++-style, memory safe(r) implementation of linux/netlink.h macros accessing Netlink

 * message contents. The class doesn't own the underlying data, so the instance is valid as long as

 * the source buffer is allocated and unmodified.

 */

template <typename T>

class nlmsg {

  public:

    /**

     * Validate buffer contents as a message carrying T data and create instance of nlmsg.

     *

     * \param buf Buffer containing the message.

     * \return Parsed message or nullopt, if the buffer data is invalid.

     */

    static std::optional<nlmsg<T>> parse(nlbuf<nlmsghdr> buf) {

        const auto& [nlOk, nlHeader] = buf.getFirst();

        if (!nlOk) return std::nullopt;

        const auto& [dataOk, dataHeader] = buf.data<T>().getFirst();

        if (!dataOk) return std::nullopt;

        const auto attributes = buf.data<nlattr>(sizeof(T));

        return nlmsg<T>(nlHeader, dataHeader, attributes);

    }

    /**

     * Validate buffer contents as a message of a given type and create instance of nlmsg.

     *

     * \param buf Buffer containing the message.

     * \param msgtypes Acceptable message types (within a specific Netlink protocol)

     * \return Parsed message or nullopt, if the buffer data is invalid or message type

     *         doesn't match.

     */

    static std::optional<nlmsg<T>> parse(nlbuf<nlmsghdr> buf, std::set<nlmsgtype_t> msgtypes) {

        const auto& [nlOk, nlHeader] = buf.getFirst();  // we're doing it twice, but it's fine

        if (!nlOk) return std::nullopt;

        if (msgtypes.count(nlHeader.nlmsg_type) == 0) return std::nullopt;

        return parse(buf);

    }

    /**

     * Netlink message header.

     *

     * This is a generic Netlink header containing information such as message flags.

     */

    const nlmsghdr& header;

    /**

     * Netlink message data.

     *

     * This is a payload specific to a given message type.

     */

    const T& data;

    /**

     * Netlink message attributes.

     */

    const nlbuf<nlattr> attributes;

    const T* operator->() const { return &data; }

  private:

    nlmsg(const nlmsghdr& nlHeader, const T& dataHeader, nlbuf<nlattr> attributes)

        : header(nlHeader), data(dataHeader), attributes(attributes) {}

};

}  // namespace android::nl