The NDIS Driver Library (NDL)

The NDIS Driver Library (NDL) is a header-only library that makes it easier to write NDIS drivers.

Features:

• Easy to integrate: just drop a header into your C or C++ code and you're ready to go
• Wide support: you can use this library in a driver that targets any version of Windows since Windows Vista
• Fast: all these routines are designed for speed, and compete well with their hand-written equivalents
• Battle-hardened: parts of the OS — including NDIS.SYS itself — use these headers
• Written by the experts: these headers were written and tested by the OS Networking team at Microsoft

Example usage

If your LWF injects packets on the send path, you must pluck them back out of the send-complete path. But cutting apart NBL chains is a tiresome chore — why not let the library do it for you?

#include <ndis.h>
#include <ndis/ndl/nblqueue.h>
#include <ndis/ndl/nblclassify.h>

NDIS_NBL_CLASSIFICATION_INDEX_CALLBACK IsMyNbl;

VOID FilterSendNetBufferListsComplete(NDIS_HANDLE FilterContext, NET_BUFFER_LIST *NblChain, ...)
{
    MY_FILTER *Filter = (MY_FILTER*)FilterContext;

    NBL_QUEUE Mine;
    NBL_QUEUE Theirs;

    NdisInitializeNblQueue(&Mine);
    NdisInitializeNblQueue(&Theirs);

    //
    // Demux the NBL chain into 2 queues, by SourceHandle
    //
    NdisClassifyNblChain2(NblChain, IsMyNbl, Filter, &Theirs, &Mine);

    if (!NdisIsNblQueueEmpty(&Theirs)) {
        NdisFSendNetBufferListsComplete(Filter->NdisHandle, NdisGetNblChainFromNblQueue(&Theirs), ...);
    }

    if (!NdisIsNblQueueEmpty(&Mine)) {
        MyReturnNbls(Filter, NdisGetNblChainFromNblQueue(&Mine));
    }
}

ULONG_PTR IsMyNbl(PVOID Filter, NET_BUFFER_LIST *Nbl)
{
    return (Nbl->SourceHandle == Filter);
}

Compare that to what a typical LWF might do, when writing the same algorithm by hand:

#include <ndis.h>

VOID FilterSendNetBufferListsComplete(NDIS_HANDLE FilterContext, NET_BUFFER_LIST *NblChain, ...)
{
    MY_FILTER *Filter = (MY_FILTER*)FilterContext;

    NET_BUFFER_LIST *Current = NblChain;
    NET_BUFFER_LIST *TheirFirst = NULL;
    NET_BUFFER_LIST *TheirLast = NULL;
    NET_BUFFER_LIST *MyFirst = NULL;
    NET_BUFFER_LIST *MyLast = NULL;

    while (Current) {
        NET_BUFFER_LIST *Next = Current->Next;

        if (Current->SourceHandle == Filter) {
            //
            // Append to My chain
            //
            if (MyFirst == NULL) {
                MyFirst = Current;
            } else {
                MyLast->Next = Current;
            }
            MyLast = Current;
        } else {
            //
            // Append to Their chain
            //
            if (TheirFirst == NULL) {
                TheirFirst = Current;
            } else {
                TheirLast->Next = Current;
            }
            TheirLast = Current;
        }

        Current = Next;
    }

    if (TheirFirst != NULL) {
        NdisFSendNetBufferListsComplete(pFilter->FilterHandle, TheirFirst, SendCompleteFlags);
    }

    if (MyFirst != NULL) {
        MyReturnNbls(Filter, MyFirst);
    }
}

I think you'll agree that the first version is easier to read, and definitely easier to spot any logic bugs in. Happily, NdisClassifyNblChain2 is even faster than the hand-written code below it, since it uses fewer branches, prefetches list entries, and is careful to avoid redundant writes to memory. So you don't have to feel guilty about a performance tradeoff here: the simpler code is also the faster code.

Quick start

All you have to do is place the src/include directory into your compiler's include path. You can either make this repository a git submodule of your own driver's repository, or just grab the files and copy them directly into your code — whichever is most convenient for you.

From there, you only need to include the header file(s) you want to use.

Header overview

#include <ndis/ndl/nblchain.h>

nblchain.h has a few basic helper routines for dealing with a linked list of NBLs. For example:

• NdisNumNblsInNblChain counts the NBLs in an NBL chain
• NdisNumDataBytesInNblChain counts the number of bytes of packet payload in the NBL chain
• NdisSetStatusInNblChain sets an NDIS_STATUS code in each NBL in the chain

You get the idea. It's really simple stuff — you could have easily written it yourself. But now you don't have to.

#include <ndis/ndl/nblqueue.h>

nblqueue.h introduces the NBL_QUEUE, which is a fancy version of an NBL chain. While an ordinary NBL chain is just a singly-linked list, the NBL_QUEUE tracks the end of the list too, so it supports fast O(1) append.

If you're starting out with an NBL chain, you can convert it into a queue:

void ExampleNblQueue(NET_BUFFER_LIST *NblChain)
{
    NBL_QUEUE NblQueue;
    NdisInitializeNblQueue(&NblQueue);

    //
    // Convert the NBL queue into an NBL chain.
    //
    // This operation is O(n), but any future append operations on the queue
    // will be fast O(1).
    //
    NdisAppendNblChainToNblQueue(&NblQueue, NblChain);

    . . . use the NblQueue . . .
}

From there, you can efficiently append two NBL queues with NdisAppendNblQueueToNblQueueFast or append 1 loose NBL to the queue with NdisAppendSingleNblToNblQueue.

The header also offers a variation on the above in NBL_COUNTED_QUEUE. As with an ordinary NBL_QUEUE, the NBL_COUNTED_QUEUE supports fast O(1) append operations, but it also keeps a running count of the number of NBLs in the queue. Every time you append, push, or pop from an NBL_COUNTED_QUEUE, its count is automatically updated appropriately. This is useful for the receive path, which needs to carry along a NumberOfNetBufferLists parameter.

#include <ndis/ndl/nblclassify.h>

nblclassify.h has routines for demuxing NBL chains. This means taking a chain of NBLs, and splitting it into 2 or more queues, based on any criterion that you provide. We used this in our example LWF to pull our own NBLs out of the send-complete path.

Rather than providing one highly-generic classification routine, this library has a family of very similar routines. This allows each routine to be micro-optimized for performance. So you only pay for what you actually need.

The simplest classification routine is NdisClassifyNblChain2, which splits 1 NBL chain into 2 NBL queues. You provide a function that takes an NBL and returns 0 or 1 to select which queue it should go into.

If you want to bucket NBLs into several queues, use NdisClassifyNblChainByIndex. Your classification function returns a number from 0 to N-1 which selects which queue the NBL should go into.

But sometimes, you can't exactly predict in advance how many queues there will be. For example, suppose you're classifying packets by destination IP address. You don't know how many distinct IP addresses appear in the NBL chain — you just know that if 2 NBLs have different destination IP addresses, they should go into different queues. This is where NdisClassifyNblChainByValue shines: it accumulates similar NBLs, allowing you to process them in batches.

If batching is really important to your driver, you can use NdisClassifyNblChainByValueLookahead. This is a drop-in replacement for NdisClassifyNblChainByValue, but it tries harder to group similar NBLs together. For example, if you have a pathological NBL chain of [A, B, A, B, A, B]; then NdisClassifyNblChainByValue will not recover any batching; each NBL will be processed separately. But NdisClassifyNblChainByValueLookahead looks ahead enough to rearrange the NBL chain into [A, A, A, B, B, B], so it can process the whole thing in 2 big batches.

Sometimes it's inconvenient to process NBL queues in a callback function. As an alternative, NdisPartialClassifyNblChainByValue allows you to keep your processing logic inline. It accumulates as many similar NBLs as it can, then returns back to you to let you process them. Typical usage might look like this:

VOID ExamplePartialClassify(NET_BUFFER_LIST *NblChain)
{
    while (NblChain != NULL)
    {
        NBL_QUEUE Nbls;
        ULONG_PTR IPv4Address;
        NdisPartialClassifyNblChainByValue(&NblChain, GetIPv4Address, NULL, &IPv4Address);

        //
        // Every NBL in the queue has the same IP address, so we can process
        // them all in one batch.
        //

        if (FirewallAllowDestinationIpAddress(IPv4Address)) {
            IndicateNbls(&Nbls);
        } else {
            DropNbls(&Nbls);
        }
    }
}

By way of comparison, here's how the same logic would work with NdisClassifyNblChainByValue:

NDIS_NBL_FLUSH_CALLBACK ProcessNbls;

VOID ExampleCallbackClassify(NET_BUFFER_LIST *NblChain)
{
    NdisClassifyNblChainByValue(&NblChain, GetIPv4Address, NULL, ProcessNbls, NULL);
}

VOID
ProcessNbls(PVOID FlushContext, ULONG_PTR IPv4Address, NBL_QUEUE *Queue)
{
    if (FirewallAllowDestinationIpAddress(IPv4Address)) {
        IndicateNbls(Queue);
    } else {
        DropNbls(Queue);
    }
}

NdisPartialClassifyNblChainByValue keeps everything in the same function, which is convenient and efficient if you have a lot of local state. NdisClassifyNblChainByValue might be a tiny bit more efficient if the classification routine is expensive, since it avoids redundant classifications. Use whichever one fits your code the best.

Versioning

Current version: 1.0.0

This project uses semantic versioning (semver.org).

We may make any breaking changes to these header files, for example: renaming routines, removing routines, adding/removing parameters, or changing the size of structures. We commit to two limitations:

• If make any breaking change, we'll change the major version number.
• We will not make a breaking change that causes silent bad behavior. For example, if v1.0 permits a NULL parameter, we wouldn't change v2.0 to crash if given NULL — we would rename the routine to draw your attention to the change in behavior.

Because of this, we recommend that you take a dependency on a specific version of these header files, and not just build against the rolling HEAD of the repository.

Contributions

We gladly accept issues here on GitHub. File an issue if you spot a bug, need clarification on documentation, or have an idea for a new feature that would be a good fit for this library.

For legal reasons, we can only accept pull requests from Microsoft employees.

Code of Conduct

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

Security

Please do not report security vulnerabilities through public GitHub issues.

Instead, please report them to the Microsoft Security Response Center (MSRC) at https://msrc.microsoft.com/create-report.

If you prefer to submit without logging in, send email to secure@microsoft.com. If possible, encrypt your message with our PGP key; please download it from the the Microsoft Security Response Center PGP Key page.

You should receive a response within 24 hours. If for some reason you do not, please follow up via email to ensure we received your original message. Additional information can be found at microsoft.com/msrc.

Legal

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