What does the KMDF verifier do?


When you turn on the KMDF verifier(KMDFDV), a whole slew of behaviors are
verified. Unlike the WDM driver verifier (DV) which allows you to select which
behaviors to verify, KMDFDV is nearly an all or nothing affair. Future versions of
KMDFDV will probably expose settings to enable selective verification. After
KMDFDV has been enabled on your driver, certain debugger extensions in the wdfkd
debugger extension can now give additional data. This
document
on WHDC describes each of the extension’s commands. Enabling KMDFDV is not cheap,
it can slow your driver down by as much as 50% (but DV also has the same affect).

Let’s look at the output from !wdfkd.wdfdriverinfo, which I first presented in
yesterday’s
post
to get a better insight into what KMDFDV checks for.

1: kd> !wdfdriverinfo wdfrawbusenumtest fff
----------------------------------
Default driver image name: wdfrawbusenumtest
[...]
----------------------------------
WDF Verifier settings for wdfrawbusenumtest.sys is ON
Pool tracking is ON
Handle verification is ON
IO verification is ON
Lock verification is ON

The list is pretty self explanatory from a top level POV. Let's go into
some of the details of each setting:

  • Pool tracking: KMDF will track most (allocations that
    are near PAGE_SIZE or greater are not tracked) allocations made
    on behalf of the driver writer. You can see the tracking information by
    running !wdfpoolused on your driver.

  • Handle verification: KMDF will perform sanity checks on
    each KMDF handle passed to a KMDF DDI.

  • IO verification: KMDF will perform state and contract checks on
    WDFREQUESTs and WDFQUEUEs. For instance, you can only call
    WdfDeviceEnqueueRequest()
    within the context of your
    EvtIoInCallerContext()
    callback. Verifying that you are following the
    this rule in a production environment wastes cycles, so we only make the check
    when KMDF DV has been enabled.

  • Lock verification: This setting is the least relevant to a KMDF driver
    writer. When lock verification is enabled, each lock in KMDF is assigned a
    position in a locking hierarchy and upon each lock acquisition, KMDF verifies
    that the hierarchy has not been violated. Since you as the driver writer cannot
    create an entry into this hierarchy, there is no way for you to plug into
    this functionality.

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