How does the Linux kernel respond to system calls?
When a user-space application wants to perform a privileged operation or access a resource that requires kernel-level permissions, it makes a system call to the Linux kernel. System calls provide a controlled interface for user-space programs to request services from the kernel. Here's how the Linux kernel responds to system calls:
User-Space Request: A user-space application initiates a system call by invoking a specific function call in its code. These function calls are usually defined in system call wrapper libraries, such as the GNU C Library (glibc).
User-to-Kernel Transition: When the user-space application makes a system call, the CPU transitions from user mode to kernel mode. This transition involves changing the CPU's execution mode, switching to a different stack, and setting up the kernel's execution context.
System Call Number: Each system call is identified by a unique number. The user-space application specifies the system call it wants to invoke by passing the corresponding system call number as an argument.
Parameter Passing: System calls often require additional parameters to perform their intended tasks. These parameters are passed by the user-space application, usually through CPU registers or the stack.
Kernel Dispatch: The Linux kernel has a table called the "system call table" that maps system call numbers to the corresponding kernel functions. When a system call is invoked, the kernel uses the provided system call number to look up the appropriate kernel function to handle the request.
Execution of Kernel Code: The kernel executes the corresponding kernel function to carry out the requested action. This function runs in kernel mode and has direct access to system resources and hardware.
Execution and Validation: Within the kernel function, the provided parameters are validated and processed. The kernel ensures that the requested action is allowed and safe to perform.
Resource Management: The kernel may allocate or deallocate resources, such as memory or I/O buffers, as required by the system call. It also enforces access controls and security checks to prevent unauthorized operations.
Execution Completion: Once the kernel has completed the requested operation, it prepares the result (if any) and returns control to the user-space application.
Return to User Space: The CPU transitions back to user mode, switching to the user-space stack, and restoring the user-space execution context.
System Call Return: The result of the system call is made available to the user-space application. If the system call was successful, the return value typically contains a positive value indicating success. If an error occurred, a negative value is returned, and the specific error code is set.
In summary, the Linux kernel responds to system calls by executing the appropriate kernel function associated with the requested system call number. It performs necessary checks, manages resources, and returns results or error codes back to the user-space application, ensuring controlled and secure interaction between user programs and the kernel.