On Wed, Oct 20, 2021 at 09:29:54AM +0100, Lorenz Bauer wrote:

Add support for renameat2(RENAME_EXCHANGE) on bpffs. This is useful
for atomic upgrades of our sk_lookup control plane.

* Create a temporary directory on bpffs
* Migrate maps and pin them into temporary directory
* Load new sk_lookup BPF, attach it and pin the link into temp dir
* renameat2(temp dir, state dir, RENAME_EXCHANGE)
* rmdir temp dir

Due to the sk_lookup semantics this means we can never end up in a
situation where an upgrade breaks the existing control plane.

That is a cool idea.
The selftest doesn't exercise all of this logic.
It's only testing that RENAME_EXCHANGE works on directories within bpffs.
Do we need a test for other types of paths?