A hash-based DOS attack on Btrfs

I believe the ext* family uses salting (the "Directory Hash Seed" field you can see in dumpe2fs). This should be enough to make the hash unpredictable as long as the attacker does not know the seed, and the only way to know the seed should be having raw block access to the partition and reading the superblock.

But if the hash output is visible via telldir, the attacker can still find out the salt unless you are using strong crypto, which would not be very good for performance. I think the hash output is not visible via telldir on the ext* family.

You could play games like having a per-directory randomly chosen salt, but if you are getting to that point IMHO it is better to work on making the algorithms robust against degenerate edge cases.