Processing Data

Microsoft DirectShow 9.0

Processing Data

Parsing Media Data

If your filter parses media data, do not trust headers or other self-describing data in the content. For example, do not trust size values that appear in AVI RIFF chunks or MPEG packets. Common examples of this kind of error include:

• Reading N bytes of data, where the value of N came from the content, without checking N against the actual size of your buffer.
• Jumping to a byte offset within a buffer, without verifying that the offset falls within the buffer.

Another common class of errors involves not validating format descriptions that are found in the content. For example:

• A BITMAPINFOHEADER structure can be followed by a color table. The BITMAPINFO structure is defined as a BITMAPINFOHEADER structure followed by an array of RGBQUAD values that make up the color table. The size of the array is determined by the value of biClrUsed. Never copy a color table into a BITMAPINFO without first checking the size of the buffer that was allocated for the BITMAPINFO structure.
• A WAVEFORMATEX structure might have extra format information appended to the structure. The cbSize member specifies the size of the extra information.

During pin connection, a filter should verify that all format structures are well-formed and contain reasonable values. If not, reject the connection. In the code that validates the format structure, be especially careful about arithmetic overflow. For example, in a BITMAPINFOHEADER, the width and height are 32-bit long values but the image size (which is a function of the product of the two) is only a DWORD value.

If format data from the source is larger than your allocated buffer, do not truncate the source data in order to copy it into your buffer. Doing so can create a structure whose implicit size is larger than its actual size. For example, a bitmap header might specify a palette table which no longer exists. Instead, reallocate the buffer or simply fail the connection.

Errors During Streaming

When the graph is running, if your filter receives malformed content, it should terminate streaming. Do the following:

• Return an error code from Receive.
• Call IPin::EndOfStream on the downstream filter.
• Call CBaseFilter::NotifyEvent to post an EC_ERRORABORT event.

Format Changes

Several mechanisms exist for filters to change formats mid-stream. Each of them involves more than one step, which creates the potential for false acceptances. If your filter gets a request for a dynamic format change, then it must either reject the request, or else honor the new format when it arrives. Similarly, never switch formats unless the other filter agrees. For more information, see Dynamic Format Changes.