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@ -566,7 +566,7 @@ umount: Device or resource busy
\label{lst:unshare-umount}
\end{listing}
A feature called lazy unmounting or \texttt{MNT\_DETACH} exists for situations where a busy mount still needs to be unmounted. Supplying the \texttt{MNT\_DETACH} flag to \texttt{umount2(2)} causes the mount to be immediately detached from the unified hierarchy, while remaining mounted internally until the last user has finished with it. Whilst this initially seems like a good solution, this system call is incredibly dangerous when combined with shared subtrees. This behaviour is shown in Listing \ref{lst:unshare-umount-lazy}, where a lazy (and hence recursive) unmount is combined with a shared subtree to disastrous effect.
Lazy unmounting or \texttt{MNT\_DETACH} exists for situations where a busy mount still needs to be unmounted. Supplying the \texttt{MNT\_DETACH} flag to \texttt{umount2(2)} causes the mount to be immediately detached from the unified hierarchy, while remaining mounted internally until the last user has finished with it. Whilst this initially seems like a good solution, this system call is incredibly dangerous when combined with shared subtrees. This behaviour is shown in Listing \ref{lst:unshare-umount-lazy}, where a lazy (and hence recursive) unmount is combined with a shared subtree to disastrous effect.
\begin{listing}
\begin{minipage}{.49\textwidth}
@ -612,11 +612,11 @@ doesn't exist).}
This logic held even in the presence of namespaces, with the initial patchset in February 2001 \citep{viro_patchcft_2001}, as mounts were not initially shared but duplicated between namespaces. However, when shared subtrees were added in January 2005 \citep{viro_rfc_2005}, this logic stopped holding.
When setting up a container environment, one calls \texttt{pivot\_root(2)} to replace the old root with a new root for the container. Only then may the old root may be unmounted. Oftentimes the solution is to exec a binary in the new root first, meaning that the old root is no longer in use and may be unmounted. This works, as old root is only a reference in this namespace, and hence may be unmounted with children - the \texttt{vfsmount} in this namespace is not busy, contradicting an assertion in the quotation.
When setting up a container environment, \texttt{pivot\_root(2)} replaces the old root with a new root for the container. Only then may the old root may be unmounted. Oftentimes the solution is to exec a binary in the new root first, meaning that the old root is no longer in use and may be unmounted. This works, as old root is only a reference in this namespace, and hence may be unmounted with children - the \texttt{vfsmount} in this namespace is not busy, contradicting an assertion in the quotation.
If one wishes to continue running the existing binary, this is possible with lazy unmounting. However, the kernel only exposes a recursive lazy unmount to user-space. With shared subtrees, this results in destroying the parent tree. While this is avoidable by removing the shared propagation from the subtree before unmounting, the choice to have \texttt{MNT\_DETACH} aggressively cross shared subtrees can be highly confusing, and perhaps undesired behaviour in a world with shared subtrees by default.
To continue running the existing binary one must unmount lazily. However, the kernel only exposes a recursive lazy unmount. With shared subtrees, this results in destroying the parent tree. While this is avoidable by removing the shared propagation from the subtree before unmounting, the choice to have \texttt{MNT\_DETACH} aggressively cross shared subtrees can be highly confusing, and perhaps undesired behaviour in a world with shared subtrees by default.
Void processes mount an empty \texttt{tmpfs} file system in a new namespace, which doesn't propagate to the parent, and use the \texttt{pivot\_root(8)} command to make this the new root. By pivoting to the \texttt{tmpfs}, the old root exists as the only reference in the otherwise empty \texttt{tmpfs}. Finally, after ensuring the old root is set to \texttt{MNT\_PRIVATE} to avoid propagation, the old root can be lazily detached. This allows the binary from the parent namespace to continue running correctly. Any new processes only have access to the materials in the empty \texttt{tmpfs}. This new \texttt{tmpfs} never appears in the parent namespace, separating the void process effectively from the parent namespace.
Void processes mount an empty \texttt{tmpfs} file system in a new namespace, which doesn't propagate to the parent, and use the \texttt{pivot\_root(8)} command to make this the new root. By pivoting to the \texttt{tmpfs}, the old root exists as the only reference in the otherwise empty \texttt{tmpfs}. Finally, after ensuring the old root is set to \texttt{MNT\_PRIVATE} to avoid propagation, the old root can be lazily detached. This allows the binary from the parent namespace to continue running correctly. Any new processes only have access to the materials in the empty \texttt{tmpfs}.
\section{User namespaces}
\label{sec:voiding-user}