How to use the special pool feature to isolate pool damage

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Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.
The way the Special Pool utility works is by allocating two pages of virtual memory for every pool allocation requested through ExAllocatePoolWithTag that matches the following criteria:

• The allocation request must be for a size smaller than the maximum allocation that will fit in a pool page.
• The request must match the PoolTag specification in the registry.

For pool overrun detection, the first page is used to contain the allocation at the end of the page. The second page is a guard page. For pool underrun detection, the first page is the guard page. It is followed by a page that contains the allocation at the beginning of the page.

Overrun detection is probably the most used. For overrun detection, the requested allocation is placed at the end of the first page by backing up the request size from the end of the page. The allocation size is rounded up to an 8-byte boundary. A pattern key, the size, and the pool tag information are written to the header at the first eight bytes of the first page. The pattern is also propagated throughout the page. Since the allocation is placed on the nearest 8-byte boundary, there may be as many as seven slop bytes following the allocation. The pattern is also written to the slop bytes following the allocation.

The second page is the guard page. The guard page consists of a special page table entry (PTE) that is marked with no-access protection. By marking this second page with no-access protection, any code attempting to read or write beyond the end of the first page immediately causes an access violation resulting in a Stop 0x0000000A or Stop 0x0000001E error. This enables someone debugging the system to identify the exact instruction causing pool damage.

As a backup check to catch violators that write beyond the end of the allocation but not beyond the end of the page, the slop bytes at the end of the allocation are validated during the free pool request (ExFreePoolWithTag). The slop bytes are compared to the pattern in the allocation header to verify if anything is overwritten in the slop byte area. If the verification check does not work, a Stop 0x00000001A error occurs.

This check does not necessarily identify the exact piece of code causing the pool damage, but it may assist in identifying the component causing the damage.

To activate the Special Pool utility, add the following keys and values to the registry: The pool tag mask is the pool tag ID containing masking characters of the pool in which you want to put in the special pool. This mask must be specified in hexadecimal in reverse order. This mask may also contain "?" to mask a single character or "*" to mask from here to the end of the tag. For example, to monitor all pools with a pool tag beginning with "Nt", specify "2A744E" (without the quotation marks), which represents "*tN".

The following table lists more examples:

   Pool to monitor   Character representation   Pool tag mask
   ----------------------------------------------------------               
   All pools         "*"                        0x2A
   N??s              "s??N"                     0x733F3F4E
				

Allocation size mask specification places all pool allocations of a specified size into the special pool. This is also specified in hexadecimal. For example, if all allocations of 32 bytes are placed in the special pool, specify 0x20.

When zero (0x0) is specified, the Special Pool utility is not initialized. In addition, the Special Pool utility is not initialized if the PoolTag registry value is not defined in the registry. 1 indicates that pool allocation overruns are detected for the tag specified. The allocation is located at the end of the page and the guard page follows.

0 indicates that pool allocation underruns are detected for the tag. The allocation is located at the beginning of the page and the guard page precedes the page that contains the allocation.

Common use examples to create these two registry keys would be:

Note: For Windows NT Terminal Server 4.0, you must disable KStackPool when you use special pool. To do this, add the following registry value: If you do not make this change on Terminal Server computers with 256 MB or more memory, you may receive "STOP 0x00000078 (PHASE0_EXCEPTION)" error messages.

Note: After you make any of the registry changes described in this article, restart the computer to cause the changes to take effect.

If after you enable the Special Pool feature, the computer stops responding (hangs) with an error message on a blue screen during startup, reboot the computer and start it by using the Last Known Good Configuration option. Enabling the Special Pool feature is not written to the Last Known Good Configuration entry in the registry until after a successful logon.