allow bind mount on root directory

[giuseppe]

currently crun fails with the following error with a bind mount on the root directory:

➜ sudo crun run foo
2025-05-20T13:51:57.136297Z: mount `/tmp/foo/bar/root` to ``: Invalid argument 

[lsm5]

@giuseppe I'm seeing failures with crun checkpoint... on podman system tests with the latest crun.

For example:

# [17:45:37.534795671] # /usr/bin/podman container checkpoint -a
# [17:45:37.587734488] unable to resolve external bind mount `/dev/shm` under rootfs: No such file or directory
# Error: `/usr/bin/crun checkpoint --image-path /var/lib/containers/storage/overlay-containers/e20d4b45e2c325d1238a1ed9c6530acf9fdadc91282b70d63cef276107122f55/userdata/checkpoint --work-path /var/lib/containers/storage/overlay-containers/e20d4b45e2c325d1238a1ed9c6530acf9fdadc91282b70d63cef276107122f55/userdata e20d4b45e2c325d1238a1ed9c6530acf9fdadc91282b70d63cef276107122f55` failed: exit status 1
# [17:45:37.591341592] [ rc=125 (** EXPECTED 0 **) ]

Can be found here.

Is that related to this issue?

[giuseppe]

that is unlikely, crun never supported mounts on the root directory (at least since we moved to openat2 few years ago).

Is that a regression?

[lsm5]

The initial TMT enablement PR on podman had these tests passing https://artifacts.dev.testing-farm.io/9dcfd34f-ff46-4c9a-be8d-86818431f181/ . Last run was 2 weeks ago. The PR was merged today and now those tests are failing everywhere today.