fstat is supposed to work on all FDs, but we currently only support it on file-backed FDs.

I am not sure what exactly it should do for other FDs; figuring that out would be the first step in working on this issue. So to work on this you should be willing to do some research and explain the results so that we can then together figure out how to best implement this in Miri. However, I don't expect this to be complicated from the Miri side, so prior experience with Miri is not required.