Windows® Internals, Sixth Edition, Part 2

The cookie value that is calculated is also saved for use with the EncodeSystemPointer and DecodeSystemPointer APIs, which implement pointer encoding. When an application or a DLL has static pointers that are dynamically called, it runs the risk of having malicious code overwrite the pointer values with code that the malware controls. By encoding all pointers with the cookie value and then decoding them, when malicious code sets a nonencoded pointer, the application will still attempt to decode the pointer, resulting in a corrupted value and causing the program to crash. The EncodePointer and DecodePointer APIs provide similar protection but with a per-process cookie (created on demand) instead of a per-system cookie.

Note

The system cookie is a combination of the system time at generation, the stack value of the saved system time, the number of page faults, and the current interrupt time.

Copy-on-Write

Copy-on-write page protection is an optimization the memory manager uses to conserve physical memory. When a process maps a copy-on-write view of a section object that contains read/write pages, instead of making a process private copy at the time the view is mapped, the memory manager defers making a copy of the pages until the page is written to. For example, as shown in Figure 10-3, two processes are sharing three pages, each marked copy-on-write, but neither of the two processes has attempted to modify any data on the pages.

Figure 10-3. The “before” of copy-on-write

If a thread in either process writes to a page, a memory management fault is generated. The memory manager sees that the write is to a copy-on-write page, so instead of reporting the fault as an access violation, it allocates a new read/write page in physical memory, copies the contents of the original page to the new page, updates the corresponding page-mapping information (explained later in this chapter) in this process to point to the new location, and dismisses the exception, thus causing the instruction that generated the fault to be reexecuted. This time, the write operation succeeds, but as shown in Figure 10-4, the newly copied page is now private to the process that did the writing and isn’t visible to the other process still sharing the copy-on-write page. Each new process that writes to that same shared page will also get its own private copy.

Figure 10-4. The “after” of copy-on-write

One application of copy-on-write is to implement breakpoint support in debuggers. For example, by default, code pages start out as execute-only. If a programmer sets a breakpoint while debugging a program, however, the debugger must add a breakpoint instruction to the code. It does this by first changing the protection on the page to PAGE_EXECUTE_READWRITE and then changing the instruction stream. Because the code page is part of a mapped section, the memory manager creates a private copy for the process with the breakpoint set, while other processes continue using the unmodified code page.

Copy-on-write is one example of an evaluation technique known as lazy evaluation that the memory manager uses as often as possible. Lazy-evaluation algorithms avoid performing an expensive operation until absolutely required—if the operation is never required, no time is wasted on it.

To examine the rate of copy-on-write faults, see the performance counter Memory: Write Copies/sec.

Address Windowing Extensions

Although the 32-bit version of Windows can support up to 64 GB of physical memory (as shown in Table 2-2 in Part 1), each 32-bit user process has by default only a 2-GB virtual address space. (This can be configured up to 3 GB when using the increaseuserva BCD option, described in the upcoming section User Address Space Layout.) An application that needs to make more than 2 GB (or 3 GB) of data easily available in a single process could do so via file mapping, remapping a part of its address space into various portions of a large file. However, significant paging would be involved upon each remap.

For higher performance (and also more fine-grained control), Windows provides a set of functions called Address Windowing Extensions (AWE). These functions allow a process to allocate more physical memory than can be represented in its virtual address space. It then can access the physical memory by mapping a portion of its virtual address space into selected portions of the physical memory at various times.

Allocating and using memory via the AWE functions is done in three steps:

Allocating the physical memory to be used. The application uses the Windows functions AllocateUserPhysicalPages or AllocateUserPhysicalPagesNuma. (These require the Lock Pages In Memory user right.)