The mask stride is calculated with:

  fbComposeGetStart (pMask, xMask, yMask, CARD8, maskStride, maskLine, 1);

For the purposes of the stride only, this is equivalent to:

  maskStride = pMask->pixels->stride / sizeof(FbBits) * sizeof(FbBits) / sizeof(CARD8);

But performing an integer-divide and then a multiply can lose data!


For example, consider when pMask->pixels->stride == 2. Since sizeof(FbBits) == 4, the division yields 
zero, and maskStride ends up zero--which is clearly incorrect.

I am including a test case. Compile test.c against cairo, and observe that although the mask used is a 
checkerboard pattern, the result is vertical lines. Because the mask stride is zero, each row ends up 
using the same mask.


It is quite possible (likely?) that many places where fbGetDrawable is used have problems similar to this.