bidirectional buffer implementation

--- Quote Start ---

You can specify a pin as either in, out, inout, or buffer. Both inout and buffer are bidirectionals, but buffer offers the option of writing and reading to the same pin at the same time.

--- Quote End ---

Sounds like a basic misunderstanding of VHDL features. An interface object of mode buffer is not bidirectional. It's an output, that can be read back internally. By reading a buffer, you only get the internal driving signal state.