How do you avoid having to copy buffers from the kernel into WebAssembly? As far as I know WebAssembly does not provide a way to access memory outside the linea

How do you avoid having to copy buffers from the kernel into WebAssembly? As far as I know WebAssembly does not provide a way to access memory outside the linear memory block, but maybe in this specific case (nginx) it is possible to have all syscalls write to a buffer allocated by WebAssembly.