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devfs_io() can't fall back to calling vfs_synchronous_io(), if the device driver doesn't support handling requests asynchronously. The presence of the io() hook leads the VFS (do_iterative_fd_io()) to believe that asynchronous handling is supported and set a finished-callback on the request which calls the io() hook to start the next chunk. Thus, instead of iterating through the request in a loop the iteration happens recursively. For sufficiently fragmented requests the stack may overflow (ticket #9900). * Introduce a new vnode operation supports_operation(). It can be called by the VFS to determine whether a present hook is actually currently supported for a given vnode. * devfs: implement the new hook and remove the fallback handling in devfs_io(). * vfs_request_io.cpp: use the new hook to determine whether the io() hook is really supported.