Character Device Drivers in deep

Learn how to implement a Linux character device using struct file_operations with open, close, read, and write handlers, including owner, permissions, and translating user-space parameters to driver parameters.

Bind the file_operations structure to a character device by initializing it and attaching it to the module, illustrating how the kernel registers device operations.

Compare cdev_init and cdev_alloc in character device drivers, showing how memory allocation and dynamic setup affect device activation and protection against module unload.

Explore creating a Linux kernel module character device with cdev_alloc, registering with a major number, and handling open, read, and release.

Learn how a Linux kernel module registers file operations with cdev_init, manages memory and permissions, and handles permission denied errors when accessing a character device.

Explore how open and release calls behave in fork scenarios for character device drivers, highlighting that open and release may occur only once and how close is involved.

Explore how many times open and release are invoked when multiple processes access character device drivers.

Examine an example of the struct file in a character device driver, illustrating user-space interactions and the handling of operations and pointers within the driver context.

Explore the inode structure and its file metadata. Understand size, blocks, block size, timestamps, and device numbers, and how stat reveals them and how major and minor numbers identify devices.

Study an example of struct inode in character device drivers, focusing on permissions, the user of the device globally, and major and minor numbers for device identification.

Showcases how struct inode and struct file interact during file open and creation, and highlights behavior when a file exists, is created anew, or is deleted.

Accessing a user space buffer from kernel space is discussed, highlighting security issues and the risks of copying from user space in character device drivers.

Explore copy_to_user in character device drivers, illustrating how data is copied to user space and how it contrasts with copy operations.

Explore how put_user in character device drivers handles 64-bit values, stores data, and manages timing with sleeps to illustrate concurrent device interactions.

Explore get_user in character device drivers, showing how to retrieve device values and manage a global variable versus a local property as the latest values change.

Pass a structure between user space and kernel space by copying data to and from the kernel, using open, read, and write operations on the device.

This lecture examines passing user heap variables to a kernel driver. It shows copying from user space into a kernel buffer and writing to a device file descriptor.

Examine why passing a structure with one member on the stack and the other in the heap fails in character device driver operations, and analyze practical implications.

Learn how kernel utilities print dev_t using print_dev_t and format_dev_t, extract major and minor numbers with macros, write to a buffer, and return the number of characters written.

Explore whether strlen can be safely used in kernel space for character device drivers, including length calculation, memory buffers, user space interactions, and common pitfalls.

Learn how strnlen_user checks the length of a null-terminated string from userspace by passing a userspace pointer and a maximum length, returning the size including the terminating null.

Learn to pass a structure with a stack and a heap member by copying from user twice and using pointers to access both parts.

Reveal a flaw in a character driver when reading data byte by byte. Improper buffer handling and copying to user space can repeat the first character rather than advancing.

Explore creating multiple character device nodes by allocating several device numbers, initializing each device in a loop, and ensuring each uses a separate buffer to prevent data sharing.

Learn how to add support for private data in a character device driver by initializing device messages, linking devices, and accessing the base address, service name, and file pointers.

Explore input and output control for character device drivers, focusing on device-specific custom commands through system calls, including format, reset, and shutdown operations and per-device parameter handling.

Learn how ioctl BLKGETSIZE retrieves a block device size by returning the number of 512-byte blocks, so you can compute disk size.

Learn an ioctl example with BLKGETSIZE64 to obtain a block device size in bytes and sectors, and see how user space can issue device specific custom commands.

learn how to define ioctl commands using unique magic numbers and four-part macros with type, number, size, and direction to prevent clashes and ensure safe device control.

Learn how macros decode information from ioctl commands, extracting the magic number, sequence number, and size from combined values to interpret device driver operations.

Leverage ioctl macros to update character device driver code, using a magic number and defined command codes to read from the device and pass data to a clear operation.

Explore how the access_ok macro validates user-space pointers in a character device driver, including when to verify addresses without data transfer and version-specific argument usage.

Explore how to update character device driver code with the access_ok macro to validate userspace arguments, perform access checks, and safely manage addresses in kernel space.

Explore how ioctl command sizes affect data transfer in character device drivers, showing why mismatched argument sizes can yield incorrect data and how to ensure you pass the exact size.

On a 64-bit machine, the unsigned long size changes, complicating ioctl handling for 32-bit processes. Map or redefine command numbers to match the actual command size.

Explore sending a signal from a module to a process, using the signal function with two arguments to target a specific task, with user-space context and practical examples.

Use open and release to ensure only one process can open the device at a time, avoiding concurrency issues.

Restrict device access to a single user at a time by designating an owner on first open and enforcing authority.

Explore how capabilities assign only the necessary privileges to processes, instead of full privileges, and how extended attributes store these capabilities to safeguard privileged operations.

Explore how capabilities like cap_dac_override govern file-system access, with a practical example of granting or denying access based on user permissions, groups, and capability checks.

Presents an example of using the cap_sys_module capability to load modules within character device drivers. Shows how this capability enables observing system interactions and managing module execution.

When the process terminates, the release function runs automatically, even if you don't call close, to release resources.

Learn to work with character device drivers by allocating major and minor numbers, registering and unregistering devices, using misc devices, and implementing init and exit sequences.

Explore a basic example of a misc driver, showing a single function, its dynamic creation, and how delays and messages illustrate simple driver behavior.