/etc/netns/NSNAME works again!

[ausbin]

On Linux 5.15, I can mount an overlayfs with an upperdir of /etc/netns/NSNAME and a lowerdir of /etc/ at /mnt/ without the cryptic ELOOP error now. (Apparently has been fixed since Linux 5.10: https://unix.stackexchange.com/a/675959/62375)

So if we want to support /etc/netns/NSNAME, I have a few concerns:

1. Is this defined behavior on the overlayfs end? Will a future kernel release break this? (That is, is support right now a fluke?)
   • I should probably email some folks that have touched that code and ask
2. How do we handle folks on kernel versions where this is still "broken"? (I have been trying to find the commit that "fixed" this. I think it's torvalds/linux@0be0bfd2de9d, but I'm not sure, since that first appeared in 5.4, not 5.10 as the link above claims)
   • Hopefully the "fix" is in new enough kernel versions that we can offer our condolences to users of older kernels
3. How do we handle folks that have already put files in /var/ns-etc/netns/?
   • I think we can use one upperdir (/etc/netns/NSNAME/) and two lowerdirs, like -olowerdir=/var/ns-etc/NSNAME:/etc