XFS parent pointers [LWN.net]

XFS parent pointers

Posted May 8, 2018 22:29 UTC (Tue) by nevyn (guest, #33129)
In reply to: XFS parent pointers by Paf
Parent article: XFS parent pointers

Answering backwards:

> And come to think of it, why aren’t those allowed today?

Because it's impossible to stop loops. Eg. /tmp/p/c and then you hardlink p into c. gg.
Lots of bad things happen if you do that.

> Wait, how?

Because, in theory, if you have a single "parent" for each directory entry you don't have loops. I'm less sure this solves all the problems though. At best it seems like another way to do mount --bind, at worst you'd have all the same problems.

XFS parent pointers

Posted May 14, 2018 12:55 UTC (Mon) by cortana (subscriber, #24596) [Link] (2 responses)

I believe Mac OS X supports directory hardlinks on HFS+... does anyone know how software that isn't prepared for this behaves if you set up an infinite loop in this way?

XFS parent pointers

Posted May 16, 2018 1:15 UTC (Wed) by foom (subscriber, #14868) [Link]

HFS+'s directory hardlink support doesn't let you make loops. It has a bunch of restrictions which make that impossible, although they also prohibit making some directory hardlinks which _don't_ create a loop. (That's okay, it wasn't intended to be a generally-used feature, only really for Time Machine backups' use.)

See the comment starting "Source parent and" here, for what it actually checks: https://opensource.apple.com/source/hfs/hfs-407.30.1/core...

XFS parent pointers

Posted Jun 8, 2018 23:58 UTC (Fri) by JanC_ (subscriber, #34940) [Link]

You can make directory hardlinks on NTFS too, by doing direct NT kernel API calls, but it's not a good idea to actually do that (I don't think anything in Windows userspace can actually handle that properly).