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The Linux Kernel Module is a computer software programming language used to create device drivers for the Linux kernel. It was first developed in 1991 by Linus Torvalds, the creator of Linux. The language is based on C, and it is used to write code that interfaces with the Linux kernel. Kernel modules are loaded and unloaded dynamically at runtime, and they can be used to add new features to the kernel or to modify existing features. References: * [Linux Kernel Module](https://en.wikipedia.org/wiki/Linux_kernel_module) * [Linus Torvalds](https://en.wikipedia.org/wiki/Linus_Torvalds)

Ask the right questions secure the right Linux Kernel Module talent among an increasingly shrinking pool of talent.

First 20 minutes

General Linux Kernel Module app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in Linux Kernel Module application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

What are the basic components of a Linux Kernel Module?

The basic components of a Linux Kernel Module are the init and exit functions, module metadata, and the module parameters.

How would you compile a Linux Kernel Module?

To compile a Linux Kernel Module, you would typically use the make command. You would need a Makefile that specifies the kernel module to be built and the necessary dependencies.

Describe the difference between a monolithic kernel and a microkernel.

A monolithic kernel is a single large process running entirely in a single address space. It includes device drivers, file system management, system server calls, etc. A microkernel, on the other hand, only includes minimal functionality like IPC, basic scheduling, and memory handling. Other functionalities are implemented as separate processes running in user space.

What is the role of the init function in a Linux Kernel Module?

The init function is used to initialize the module into the kernel. It is called when the module is inserted into the kernel.

How would you debug a Linux Kernel Module?

Debugging a Linux Kernel Module can be done using printk function for logging debug information, using the /proc file system for dynamic inspection, or using tools like kgdb or gdb for interactive debugging.

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What you’re looking for early on

Does the candidate have a strong understanding of Linux Kernel architecture?

Has the candidate demonstrated experience with C programming language?

Can the candidate debug and solve complex problems?

Does the candidate have experience with version control systems, especially Git?

Next 20 minutes

Specific Linux Kernel Module development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

What are the steps to load and unload a Linux Kernel Module?

To load a Linux Kernel Module, the insmod command is used. To unload a module, the rmmod command is used.

Describe the difference between insmod and modprobe commands.

insmod command inserts a module into the kernel without considering dependencies. modprobe, on the other hand, is more intelligent and handles dependencies.

How would you handle dependencies in a Linux Kernel Module?

Dependencies in a Linux Kernel Module can be handled using the modprobe command which automatically loads all the dependent modules, or by manually loading the dependent modules using insmod before loading the module.

What is the role of the exit function in a Linux Kernel Module?

The exit function is called when the module is removed from the kernel. It is used to clean up any resources that the module has acquired.

How would you handle errors in a Linux Kernel Module?

Errors in a Linux Kernel Module can be handled using error codes and error handling functions like ERR_PTR and PTR_ERR. The module should also clean up any resources it has acquired before returning an error.

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At this point, a skilled Linux Kernel Module engineer should demonstrate strong problem-solving abilities, proficiency in Linux Kernel Module programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with Linux Kernel Module.

What does this basic Linux kernel module code do?
#include 
#include 

int init_module(void)
{
printk(KERN_INFO "Hello world.\n");
return 0;
}

void cleanup_module(void)
{
printk(KERN_INFO "Goodbye world.\n");
}
This code is a simple hello world Linux kernel module. When loaded, it prints 'Hello world' to the kernel log, and when unloaded, it prints 'Goodbye world' to the kernel log.
What does this piece of code do in a Linux kernel module?
#include 
#include 

static int my_init(void) {
return 0;
}

static void my_exit(void) {
}

module_init(my_init);
module_exit(my_exit);
This code defines a Linux kernel module with initialization function 'my_init' and exit function 'my_exit'. The 'module_init' macro is used to load 'my_init' as the initialization function for the module, and 'module_exit' is used to load 'my_exit' as the exit function for the module.
What will be the output of this code snippet on the Linux kernel?
#include 

int *my_array;

my_array = kmalloc(10 * sizeof(*my_array), GFP_KERNEL);
if (!my_array)
printk("kmalloc failed\n");

kfree(my_array);
This code allocates memory for an array of 10 integers in the kernel space using 'kmalloc'. If the allocation fails, it prints 'kmalloc failed'. At the end, it frees the allocated memory using 'kfree'. No output will be produced unless the memory allocation fails.
What does this Linux kernel code do related to threading?
#include 
#include 

struct task_struct *ts;

int my_thread_fn(void *data)
{
while (!kthread_should_stop()) {
ssleep(1);
}
return 0;
}

ts = kthread_run(my_thread_fn, NULL, "my thread");
This code creates a kernel thread named 'my thread' that runs the function 'my_thread_fn'. The thread function sleeps for 1 second in a loop until 'kthread_should_stop' returns true, which is when the thread should stop.

Wrap-up questions

Final candidate for Linux Kernel Module Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle Linux Kernel Module application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

What are the risks of developing Linux Kernel Modules?

Developing Linux Kernel Modules can be risky because a bug in a module can crash the entire system. Also, modules run in kernel space, so they can directly access hardware and memory, which can lead to security issues.

Describe the difference between a process and a thread in Linux.

A process is an instance of a running program, with its own memory space. A thread is a subset of a process, sharing the same memory space with other threads in the same process, but having its own stack and instruction counter.

How would you ensure synchronization in a Linux Kernel Module?

Synchronization in a Linux Kernel Module can be ensured using various mechanisms provided by the kernel, such as semaphores, spinlocks, mutexes, and read-write locks.

Linux Kernel Module application related

Product Perfect's Linux Kernel Module development capabilities

Beyond hiring for your Linux Kernel Module engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in Linux Kernel Module projects, and can engage in multiple capacities.

Linux Kernel Module Consultive assessments

Linux Kernel Module Application Conversion Assessment Linux Kernel Module Application Integration Assessment Linux Kernel Module Data & Database Integration Assessment

Custom Linux Kernel Module development projects

Custom Linux Kernel Module App Conversion Linux Kernel Module Custom App Development Process App UI Design for Linux Kernel Module Facelifts

Hiring guide for Linux Kernel Module Engineers

Linux Kernel Module Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

What you’re looking to see on the Linux Kernel Module engineer at this point.

Linux Kernel Module Consultive assessments

Custom Linux Kernel Module development projects