Re: [PATCH RFC 7/7] Documentation: add failfs documentation

[Miquel Sabaté Solà]

Christian Brauner @ 2026-07-23 13:30 +02:

> Document the failfs semantics, the FD_FAILFS_ROOT sentinel, the
> fchroot() entry requirements, and the ways back out.
>
> Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
> ---
>  Documentation/filesystems/failfs.rst | 64 ++++++++++++++++++++++++++++++++++++
>  Documentation/filesystems/index.rst  |  1 +
>  2 files changed, 65 insertions(+)
>
> diff --git a/Documentation/filesystems/failfs.rst b/Documentation/filesystems/failfs.rst
> new file mode 100644
> index 000000000000..46d91525916b
> --- /dev/null
> +++ b/Documentation/filesystems/failfs.rst
> @@ -0,0 +1,64 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +======
> +failfs
> +======
> +
> +failfs is a kernel-internal filesystem that fails every operation
> +reaching it with ``EOPNOTSUPP``. It is the counterpart to nullfs. Where
> +nullfs is a permanently empty failfs means "nothing is supported here". It
> +cannot be mounted from userspace, nothing can be mounted on top of it. It
> +cannot be cloned.

A small change here makes the wording more clear: from 'Where nullfs is
a permanently empty failfs means "nothing is supported here"' to 'Where
nullfs is permanently empty, failfs means "nothing is supported here"'.

> +
> +The only way into it is the ``FD_FAILFS_ROOT`` file descriptor sentinel which
> +is understood by ``fchdir(2)`` and ``fchroot(2)``.
> +
> +Semantics
> +=========
> +
> +Every path walk of a component through failfs fails with
> +``EOPNOTSUPP`` before that component is parsed, including ``.``.
> +
> +The root itself cannot be opened at all not even with ``O_PATH``.
> +
> +A process with its working directory in failfs fails every
> +``AT_FDCWD``-relative lookup. As with any working directory that is
> +unreachable from the process root, the ``getcwd(2)`` system call returns
> +a path prefixed with ``(unreachable)``.
> +
> +A process with its root directory in failfs fails every absolute path
> +lookup including absolute symlinks and the interpreter of dynamically
> +linked binaries. In other words, this fails exec.
> +
> +Lookups anchored at explicit directory file descriptors keep working. It
> +is the ``fs_struct`` equivalent of ``RESOLVE_BENEATH``. The process must
> +anchor every lookup at a file descriptor it explicitly holds.
> +
> +Entering
> +========
> +
> +``fchroot(FD_FAILFS_ROOT, 0)`` requires ``CAP_SYS_CHROOT`` in the
> +caller's user namespace, mirroring ``chroot(2)``. Unprivileged callers
> +may enter if both of the following hold:
> +
> +* ``no_new_privs`` is set: setuid binaries on regular mounts remain
> +  reachable via inherited directory file descriptors and executing them
> +  with an unusable root directory is the classic confused deputy.
> +
> +* The caller is not already chrooted: the root directory is what
> +  confines ``..`` resolution and the failfs root can never be reached by
> +  walking up a real mount tree, so moving the root of a chrooted task to
> +  failfs would allow it to escape its chroot via ``openat(fd, "..")``.
> +
> +Leaving
> +=======
> +
> +A process that entered failfs counts as chrooted. It cannot create user
> +namespaces to regain ``CAP_SYS_CHROOT``, and ``chroot(2)`` or
> +``fchroot(2)`` back out require ``CAP_SYS_CHROOT``. The only other exit
> +is ``setns(2)`` with a mount namespace file descriptor, which requires
> +``CAP_SYS_ADMIN`` over the target mount namespace as well as
> +``CAP_SYS_CHROOT`` and ``CAP_SYS_ADMIN`` in the caller's user namespace
> +and resets both root and working directory. A process that holds no such
> +file descriptor and restricts ``*chdir()``/``*chroot()``/``setns()`` via
> +seccomp has thrown away the key.
> diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
> index 1f71cf159547..734a45e51667 100644
> --- a/Documentation/filesystems/index.rst
> +++ b/Documentation/filesystems/index.rst
> @@ -91,6 +91,7 @@ Documentation for filesystem implementations.
>     ext3
>     ext4/index
>     f2fs
> +   failfs
>     gfs2/index
>     hfs
>     hfsplus

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