[nezirus]

It been long since I have stopped following grsecurity, but I would not be surprised if segment registers are still used (e.g. Pax UDEREF)

https://forums.grsecurity.net/viewtopic.php?f=7&t=3046

https://pax.grsecurity.net/docs/PaXTeam-H2HC12-PaX-kernel-se...

[sweetjuly]

I would be amazed of PaX still used segment registers seeing as they don't really do much in long mode. In fact, they specifically call this out in the description of UDEREF in your second link.