Sure, we do that

Raymond Chen


The DirectX video driver interface for Windows 95
had a method that each driver exposed called something
like “DoesDriverSupport(REFGUID guidCapability)” where we
handed it a capability GUID and it said whether or not
that feature was supported.

There were various capability GUIDs defined, things like
GUID_CanStretchAlpha to ask the driver whether it was
capable of stretching a bitmap with an alpha channel.

There was one driver that returned TRUE when you called
DoesDriverSupport(GUID_XYZ), but when DirectDraw
tried to use that capability, it failed, and in a pretty
spectacular manner.

So one of the DirectDraw developers called the vendor and
asked them, “So does your card do XYZ?”

Their response: “What’s XYZ?”

Turns out that their driver’s implementation of DoesDriverSupport
was something like this:

BOOL DoesDriverSupport(REFGUID guidCapability)
  return TRUE;

In other words, whenever DirectX asked, “Can you do this?”
they answered, “Sure, we do that,” without even checking what
the question was.

(The driver must have been written by the sales department.)

So the DirectDraw folks changed the way they queried for
driver capabilities. One of the developers went into his
boss’s office, took a network card, extracted the

MAC address
, and then smashed the card with a hammer.

You see, this last step was important: The GUID generation
algorithm is based on a combination of time and space.
When you ask CoCreateGuid to create a new GUID, it encodes
the time of your request in the first part of the GUID
and information that uniquely identifies your machine
(the network card’s MAC address, which is required
to be unique by the standards that apply to network card).

By smashing the network card with a hammer, he prevented
that network card from ever being used to generate a GUID.

Next, he added code to DirectDraw so that when it starts up,
it manufactures a random GUID based on that network card
(which – by its having been destroyed – can never be validly created)
and passes it to DoesDriverSupport.
If the driver says, “Sure, we do that”, DirectDraw says,
“Aha! Caught you! I will not believe anything you say from now on.”


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