[PATCH RFC 0/3] ovl: specify layers via file descriptors

[Christian Brauner <brauner@kernel.org>]

Hey,

Currently overlayfs only allows specifying layers through path names.
This is inconvenient for users such as systemd that want to assemble an
overlayfs mount purely based on file descriptors.

When porting overlayfs to the new mount api I already provided patches
for this but we decided to keep this work separate. This is a revamp of
the patchset as the use-case has become more urgent.

This introduces the new mount options:

lowerdir_fd+
datadir_fd+
upperdir_fd
workdir_fd

which can be used as follows:

fsconfig(fd_overlay, FSCONFIG_SET_FD, "upperdir_fd+", NULL, fd_upper);
fsconfig(fd_overlay, FSCONFIG_SET_FD, "workdir_fd+", NULL, fd_work);
fsconfig(fd_overlay, FSCONFIG_SET_FD, "lowerdir_fd+", NULL, fd_lower1);
fsconfig(fd_overlay, FSCONFIG_SET_FD, "lowerdir_fd+", NULL, fd_lower2);

The selftest contains an example for this.

The mount api doesn't allow overloading of mount option parameters
(except for strings and flags). Making this work for arbitrary
parameters would be quite ugly or file descriptors would have to be
special cased. Neither is very appealing. I do prefer the *_fd mount
options because they aren't ambiguous.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Christian Brauner (3):
      ovl: specify layers via file descriptors
      selftests: use shared header
      selftests: add overlayfs fd mounting selftests

 fs/overlayfs/params.c                              | 132 +++++++++++++++++----
 .../selftests/filesystems/overlayfs/.gitignore     |   1 +
 .../selftests/filesystems/overlayfs/Makefile       |   2 +-
 .../selftests/filesystems/overlayfs/dev_in_maps.c  |  27 +----
 .../filesystems/overlayfs/set_layers_via_fds.c     | 122 +++++++++++++++++++
 .../selftests/filesystems/overlayfs/wrappers.h     |  47 ++++++++
 6 files changed, 281 insertions(+), 50 deletions(-)
---
base-commit: 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b
change-id: 20241011-work-overlayfs-dbcfa9223e87