I have tried several attempts but without success. AMDGPU-PRO 17.40, 17.50, Oibaf PPA and something more, but nothing makes clinfo gives right results and xmr-stak working.
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The AMD drivers are indeed frustrating on Linux, but these are required by XMR-Stak for AMD cards. You will almost certainly need to download the official drivers from AMD and try the various releases until you find one that works with your specific card. For me, this was only amdgpu-pro-17.40.2712-510357, but it depends which card you have. There are several minor point releases and finding one that works is honestly trial and error unfortunatley.
The other issue you may face is that you are on Ubuntu 18.04 - and none of the currently released drivers state supporting Ubuntu 18.04, you might get lucky though.
With the latest Radeon Software for Linux driver, the 18.50 version, I was able to make xmr-stak working without issues.
You need the 18.20 drivers for Ubuntu 18.04 https://support.amd.com/en-us/kb-articles/Pages/Radeon-Software-for-Linux-18.20-Early-Preview-Release-Notes.aspx
18.20 drivers didnt work for me... I could mine cryptonight lite and heavy but no way cryptonight v7. Downgrading drivers back to 17.* solved this issue but still i belive xmr-stak worked better with ubuntu 16.* and 17.* series. I belive except drivers issue there is also some issue related with 4.15/4.16 kernels. With 4.15.* my cards randomly crash (one day one card, another day another one... withouth any pattern) and with 4.16.* it is even worse.