| # | Layer | Components |
|---|---|---|
| 1 | Userspace | user application |
| 2 | Userspace | GNU C library (glibc) |
| 3 | Kernelspace | System Call Interface |
| 4 | Kernelspace | Subsystems: virtual filesystem, memory management, process management |
| 5 | Kernelspace | Architecture Dependent Code, device drivers |
| 6 | Hardware | Physical devices |
How
- The application is setting up the system call number and parameters, and it issues a trap instruction
- The execution mode switches from user to kernel, raised the privilege level to kernel mode; the CPU switches to a kernel stack; the user stack and the return address to user space is saved on the kernel stack
- The kernel entry point saves registers on the kernel stack
- The system call dispatcher identifies the system call function and runs it
- The user space registers are restored and execution is switched back to user (e.g. calling IRET); reduce the privilege level back to user mode
- The user space application resumes
Parameters Handling
Pointers have a few important special cases that must be checked:
-
Never allow pointers to kernel-space
-
Check for invalid pointers
-
Check the pointer against the user address space before using it, or
-
Avoid checking the pointer and rely on the MMU to detect when the pointer is invalid and use the page fault handler to determine that the pointer was invalid