Skip to content

Latest commit

 

History

History
51 lines (36 loc) · 4.19 KB

5_ NetlinkGreetCodeWalkReceivingUserSpaceMsg.md

File metadata and controls

51 lines (36 loc) · 4.19 KB

Detailed Notes for Interview Revision: Receiving and Processing Messages in Linux Kernel Module 📝

Resumption of Discussion on Linux Kernel Module 🔄

  1. Receiving Data from User Space:

    • On receiving a message from the user space, a specific function is invoked in the kernel space.
    • The received data from user space is present in the data member of the socket buffer (sk_buff) passed to this function.

  2. Netlink Message Header:

    • Data exchange between user space and kernel space through netlink sockets takes the form of a netlink message header followed by the actual payload.
    • The format is: ![Netlink Message Format]
      • Netlink Message Header
      • Payload
    • By typecasting the data member of the sk_buff to a netlink message header type, we obtain access to the header fields.

  3. Accessing Netlink Message Fields:

    • Use the API netlink_msg_dump to view all fields of the netlink message header sent by the user space application. Note: This isn't a standard API; it's an API crafted for this discussion.
    • One critical field is port_id, which stores the port ID of the user space application. This can be accessed via the netlink_msg_PID member of the netlink message header.

  4. Accessing the Payload:

    • With the netlink message header accessible, to get the payload (data behind the header):
      • Use the standard Linux API: netlink_msg_data(), passing the pointer to the netlink message header. This returns a pointer to the start of the payload.
    • To know the byte size of data sent from user space, refer to the len member of the socket buffer.

  5. Kernel Module Processing:

    • Print various information such as the port ID of the sending user space process, the payload received, and the lengths from both the socket buffer and the netlink message header (these should match).
    • With access to the user space data and knowledge of its size, the kernel module can process this data based on specific requirements or business logic.

  6. Upcoming Discussion:

    • The next topic will delve into how the kernel space can reply back to the user space after processing the received data.

Interview Questions on Receiving and Processing Messages in Linux Kernel Module 🤔❓

Q1: When a Linux kernel module receives a message from user space, which data structure holds the received data? 🧐
A1: When a Linux kernel module receives a message from user space, the received data is present in the data member of the socket buffer (sk_buff) that is passed to the specific function invoked in the kernel space on receiving the message. 📦➡️

Q2: How is data exchanged between the user space and kernel space via netlink sockets? 🔀
A2: Data is exchanged between user space and kernel space via netlink sockets in the form of a netlink message header followed by the payload. The header provides metadata about the message, while the payload carries the actual content or data sent from the user space. 📨

Q3: How can you obtain access to the payload sent from user space after obtaining the netlink message header? 🔍
A3: Once you have access to the netlink message header, you can obtain the payload by using the standard Linux API called netlink_msg_data(). By passing the pointer to the netlink message header to this API, it will return a pointer pointing to the start of the payload, which is the actual data sent from the user space. 📌

Q4: What's the significance of the port_id field in the netlink message header? ⚓
A4: The port_id field in the netlink message header is significant as it stores the port ID of the user space application that sent the message to the kernel. This information is crucial for communication management and ensuring the kernel can appropriately address its response back to the correct user space application. 🔄

Always remember, effectively handling and processing data communication between user space and kernel space is central to the development and functioning of kernel modules. Being familiar with structures and methodologies involved will provide a strong foundation when working with kernel module development. 🛠🧠