Add a new file descriptor sentinel FD_FAILFS_ROOT following
FD_PIDFS_ROOT and FD_NSFS_ROOT and teach fchdir() to accept it. A
process calling fchdir(FD_FAILFS_ROOT) moves its working directory
into failfs. Every AT_FDCWD-relative lookup afterwards fails with
EOPNOTSUPP including "." and ".." and getcwd() reports the working
directory as unreachable from the process root by returning a path
prefixed with "(unreachable)". Lookups relative to explicit directory
file descriptors are unaffected.
The sentinel is the only way in. No privilege or gating is required.
Setting the working directory to a directory in which every operation
fails grants nothing and loses nothing that closing file descriptors
couldn't lose. An unlinked working directory behaves the same way today
modulo errno. The working directory also plays no role in confining ".."
resolution so no boundary is weakened.
Link: https://patch.msgid.link/20260724-work-failfs-v2-2-485dabbae185@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Pidfd file handles are exhaustive meaning they don't require a handle on
another pidfd to pass to open_by_handle_at() so it can derive the
filesystem to decode in. Instead it can be derived from the file
handle itself. The same is possible for namespace file handles.
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Christian Brauner