Add mount namespaces to linux sandbox

[cd-work]

This patch adds optional mount namespaces to the linux sandbox to
allow for filesystem isolation on systems without landlock support.

Filesystem isolation now requires either landlock OR namespace creation
to be successful in order for the sandbox creation to be successful.

Landlock will be layered on top of the mount namespace if both are
available.

While landlock automatically resolves symlink access, mount namespaces
do not. So to allow access to /usr/lib through /lib, it is now
necessary to allow both /lib AND /usr/lib.

[cd-work]

Landlock will be layered on top of the mount namespace if both are
available.

This will probably have implications that are more complicated than one might anticipate. The mount namespace resolves symlinks for example and bind mounts the actual directories instead, this would mean that with landlock you'd have to now give permissions for the symlink directory rather than the symlink target (since it's not a symlink anymore).

While I don't think this is a big issue for our usecase, it definitely feels somewhat concerning.

I've also confirmed that we cannot enforce landlock before the mount namespace without further changes. But it's probably worth testing if this works when allowing read/write access to our /tmp/birdcage-root root location (and what implications this has on access permissions).

Worst-case we certainly could enforce an "either-or" policy, where mount namespaces are only used when landlock fails. Though this would somewhat invalidate the purpose of their introduction.

[l0kod]

Landlock will be layered on top of the mount namespace if both are
available.

This will probably have implications that are more complicated than one might anticipate. The mount namespace resolves symlinks for example and bind mounts the actual directories instead, this would mean that with landlock you'd have to now give permissions for the symlink directory rather than the symlink target (since it's not a symlink anymore).

FYI, Landlock (kernel code and interface) only handles file descriptors. Dereferencing symlink or not is the choice of user space, and in this case I guess the rust-landlock's PathFd helper. It would be easy to pass a file descriptor to the symlink instead.

I've also confirmed that we cannot enforce landlock before the mount namespace without further changes. But it's probably worth testing if this works when allowing read/write access to our /tmp/birdcage-root root location (and what implications this has on access permissions).

Once sandboxed, a process cannot change the filesystem topology (which would enable it to escape the sandbox).