General protection fault

This article is about the Intel x86 computing exception. For the webcomic, see General Protection Fault (webcomic).

A general protection fault (GPF) in the Intel x86 and AMD x86-64 architectures, and other unrelated architectures, is a fault (a type of interrupt) that can encompass several cases in which protection mechanisms within the processor architecture are violated by any of the programs that are running, either the kernel or a user program. The mechanism is first described in section 9.8.13 in the Intel 80386 programmer's reference manual from 1986. A general protection fault is implemented as an interrupt (vector number 13 in decimal) in both x86 and AMD64 architectures.

If the processor detects a protection violation, it stops executing the code and sends a GPF interrupt. In most cases the operating system will simply remove the failing process from the execution queue, signal the user, and continue executing another program. If however the operating system fails to catch the general protection fault, i.e. another protection violation occurs before the operating system returns from the previous GPF interrupt, the processor will signal a double fault (interrupt vector 8, a typical BSOD scenario). If yet another failure occurs, the processor will shut down (see triple fault). It will then only respond to a reset (that is, pressing the reset-button) or init (rebooting the entire system) and non-maskable interrupts (NMIs) (unless it has previously failed when handling NMIs, in which case it will ignore these too).

Behaviour in specific operating systems

In Microsoft Windows, the general protection fault presents with varied language, depending on product version:

In systems such as Unix and Linux, the errors are reported separately (e.g. segmentation fault for memory errors).

Memory errors

In memory errors, the faulting program accesses memory that it should not access. Examples include:

However, many modern operating systems implement their memory access-control schemes via paging instead of segmentation, so it is often the case that invalid memory references in operating systems such as Windows are reported via page faults instead of general protection faults. Operating systems typically provide an abstraction layer (such as exception handling or signals) that hides whatever internal processor mechanism was used to raise a memory access error from a program, for the purposes of providing a standard interface for handling many different types of processor-generated error conditions.

In terms of the x86 architecture, general protection faults are specific to segmentation-based protection when it comes to memory accesses. However, general protection faults are still used to report other protection violations (aside from memory access violations) when paging is used, such as the use of instructions not accessible from the current privilege level (CPL).

While it is theoretically possible for an operating system to utilize both paging and segmentation, for the most part, common operating systems typically rely on paging for the bulk of their memory access control needs.

Privilege errors

There are some things on a computer which are reserved for the exclusive use of the operating system. If a program which is not part of the operating system attempts to use one of these features, it may cause a general protection fault.

Additionally, there are storage locations which are reserved both for the operating system and the processor itself. As a consequence of their reservation, they are read-only and an attempt to write data to them by an unprivileged program is an error.

Technical causes for faults

General protection faults are raised by the processor when a protected instruction is encountered which exceeds the permission level of the currently executing task - either because a user-mode program is attempting a protected instruction, or because the operating system has issued a request which would put the processor into an undefined state.

General protection faults are caught and handled by modern operating systems. Generally, if the fault originated in a user-mode program, the user-mode program is terminated. If, however, the fault originated in a core system driver or the operating system itself, the operating system usually saves diagnostic information either to a file or to the screen, and then either causes a blue screen of death or restarts the computer.

Segment limits exceeded

Segment limits can be exceeded:

Segment permissions violated

Segment permissions can be violated by:

Segments illegally loaded

This can occur when:

Switching

Faults can occur in the task state segment (TSS) structure when:

Miscellaneous

Other causes of general protection faults are:

References

  1. ↑ https://support.microsoft.com/kb/75490
General