Look into new mount api for providing overlayfs layers via fds

[allisonkarlitskaya]

No description provided.

[allisonkarlitskaya]

Using fsconfig_set_fd() for the overlayfs "lowerdir+" with the return value of fsmount() for the erofs is failing with EBADF on the 6.13.0 kernel in rawhide.

The place inside the kernel that this is most likely coming from is trying to get the fd with FMODE_PATH. https://github.com/brauner/man-pages-md/blob/main/fsmount.md says that the returned fd "looks like the one created by open() with O_PATH"...

Not exactly sure what's going wrong here.

[allisonkarlitskaya]

So I read it wrong: the mask being passed through on fget is intended to reject those bits, so FMODE_PATH rejects O_PATH fds. I'm not sure if that's intentional or not.

Trying to pass in an anonymous mount fd that I've reopened via openat(mount_fd, ".", O_RDONLY) gets further but fails on the attempt to actually create the mount with an error about "failed to clone lowerpath".

It's not clear to me if it is intended to use this stuff with raw mounted fds that haven't been associated with a mountpoint yet.

[allisonkarlitskaya]

So this conversation is happening: https://lore.kernel.org/linux-unionfs/[email protected]/T/#m47bc7194333c2ea942e62adb4e8d843c4b494a7d

Looks like we just need to wait a bit longer for this...

[allisonkarlitskaya]

My feeling on if we should make use of this API or not depends on if a kernel with this feature will land in Fedora 42 or not... We already need features that aren't available in 41, so we may as well go all the way, but if it means we need to continue to depend on Rawhide even after Fedora 42 is released, then I don't want to do it.