Making a magic deduplicating tar using the FICLONE ioctl (asg2023)

A walkthrough of an interesting use case for the `FICLONE` ioctl: cloning file data into a tar archive, and cloning files out of it again. "Free" archiving and unarchiving at zero-copy speeds! Topics: - Copy-on-write and the `FICLONE` ioctl - The ancient `tar` format - A trick for adding arbitrary padding to the `tar` format in order to force file system page alignment - How to avoid symlink attacks and other TOCTOU issues, using the fairly recently introduced (linux 5.6) `openat2` system call. - An interesting bug in GNU tar At the end you'll receive a free autographed copy of [deduptar](https://git.sr.ht/~nullenenenen/deduptar/tree/master/item/README.md) to use for party tricks. 🥳 about this event: https://cfp.all-systems-go.io/all-systems-go-2023/talk/99PZDY/