Understanding the Linux Kernel, Second Edition

Exception Handling

Most exceptions issued by the CPU are interpreted by Linux as error conditions. When one of them occurs, the kernel sends a signal to the process that caused the exception to notify it of an anomalous condition. If, for instance, a process performs a division by zero, the CPU raises a “Divide error” exception and the corresponding exception handler sends a SIGFPE signal to the current process, which then takes the necessary steps to recover or (if no signal handler is set for that signal) abort.

There are a couple of cases, however, where Linux exploits CPU exceptions to manage hardware resources more efficiently. A first case is already described in section Section 3.3.4. The “Device not available” exception is used together with the TS flag of the cr0 register to force the kernel to load the floating point registers of the CPU with new values. A second case refers to the Page Fault exception, which is used to defer allocating new page frames to the process until the last possible moment. The corresponding handler is complex because the exception may, or may not, denote an error condition (see Section 8.4).

Exception handlers have a standard structure consisting of three parts:

1. Save the contents of most registers in the Kernel Mode stack (this part is coded in assembly language).

2. Handle the exception by means of a high-level C function.

3. Exit from the handler by means of the ret_from_exception( ) function.

To take advantage of exceptions, the IDT must be properly ...