There are many methods to root your phone sm-g955f but this method works 100% and easy way also,
there are little bit risk but before do this, take backup of your phone.
Method of rooting your phone S8 plus:
Just flash with the following firmware which i give you,
this file is rar format just extract it and flash with odin or any other software, odin is also
included with firmware which will be you downloaded. It takes 2 to 3 minutes and your phone will be
rooted after this procedure.
Root file and flasher download from here: Download