Re: [PATCHSET v5] libfuse: run fuse servers as a contained service

["Darrick J. Wong" <djwong@kernel.org>]

On Sun, Apr 26, 2026 at 06:35:11PM +0200, Bernd Schubert wrote:
> 
> 
> On 4/23/26 01:18, Darrick J. Wong wrote:
> > Hi all,
> > 
> > This patchset defines the necessary communication protocols and library
> > code so that users can mount fuse servers that run in unprivileged
> > systemd service containers.  That in turn allows unprivileged untrusted
> > mounts, because the worst that can happen is that a malicious image
> > crashes the fuse server and the mount dies, instead of corrupting the
> > kernel's memory.
> > 
> > v5: Refactor socket IO into helpers, tighten the security checks in
> >     mount_service.c, always set nosuid/nodev for unprivileged mounts,
> >     use posix_spawnp in mount.fuse, restructure sample programs and hl
> >     library code to avoid the need for unmounting during startup
> > v4.1: fix various cppcheck/codecheck complaints
> > v4: fix a large number of security problems that only matter when the
> >     mount helper is being run as a setuid program; fix protocol
> >     byteswapping problems; add CLOEXEC to all files being traded
> >     back and forth; add an umount command; and strengthen mount socket
> >     protocol checks.
> > v3: refactor the sample code to reduce duplication; fix all the
> >     checkpatch complaints; examples actually build standalone;
> >     fuservicemount handles utab now; cleaned up meson feature detection;
> >     handle MS_ flags that don't translate to MOUNT_ATTR_*
> > v2: cleaned up error code handling and logging; add some example fuse
> >     service; fuservicemount3 can now be a setuid program to allow
> >     unprivileged userspace to fire up a contained filesystem driver.
> >     This could be opening Pandora's box...
> > v1: detach from fuse-iomap series
> > 
> > If you're going to start using this code, I strongly recommend pulling
> > from my git trees, which are linked below.
> > 
> > With a bit of luck, this should all go splendidly.
> > Comments and questions are, as always, welcome.
> > 
> > --D
> > 
> > kernel git tree:
> > https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=fuse-service-container
> 
> Hi Darrick,
> 
> going to look for your previous pull request - kernel tree doesn't help
> me for libfuse ;)

Urrk, that wasn't helpful of me. :(

The following changes since commit ff7aa456d426d89eb19661da7b4c171153bac516:

update kernel FUSE io_uring doc URL (2026-04-20 10:34:32 +0200)

are available in the Git repository at:

https://git.kernel.org/pub/scm/linux/kernel/git/djwong/libfuse.git tags/fuse-service-container_2026-04-22

for you to fetch changes up to 4f47bd86cd84bd511afdeb59fc18994915eb13fa:

nullfs: support fuse systemd service mode (2026-04-22 16:08:25 -0700)

(Sorry for the slow reply, I'm at LinuxFest this weekend.  Hopefully you
could construct the path to the 22 April version from the previous PR.)

--D

> Bernd
> 
> > ---
> > Commits in this patchset:
> >  * Refactor mount code / move common functions to mount_util.c
> >  * mount_service: add systemd socket service mounting helper
> >  * mount_service: create high level fuse helpers
> >  * mount_service: use the new mount api for the mount service
> >  * mount_service: update mtab after a successful mount
> >  * util: hoist the fuse.conf parsing and setuid mode enforcement code
> >  * util: fix checkpatch complaints in fuser_conf.[ch]
> >  * mount_service: enable unprivileged users in a similar manner as fusermount
> >  * mount.fuse3: integrate systemd service startup
> >  * mount_service: allow installation as a setuid program
> >  * example/service_ll: create a sample systemd service fuse server
> >  * example/service: create a sample systemd service for a high-level fuse server
> >  * nullfs: support fuse systemd service mode
> > ---
> >  example/single_file.h                            |  191 ++
> >  include/fuse.h                                   |   34 
> >  include/fuse_service.h                           |  243 +++
> >  include/fuse_service_priv.h                      |  161 ++
> >  lib/fuse_i.h                                     |    3 
> >  lib/mount_common_i.h                             |   22 
> >  lib/mount_util.h                                 |    8 
> >  lib/util.h                                       |   35 
> >  util/fuser_conf.h                                |   62 +
> >  util/mount_service.h                             |   49 +
> >  .github/workflows/install-ubuntu-dependencies.sh |    4 
> >  README.md                                        |    3 
> >  doc/fuservicemount3.8                            |   32 
> >  doc/meson.build                                  |    3 
> >  example/meson.build                              |   26 
> >  example/null.c                                   |   51 +
> >  example/null.socket.in                           |   15 
> >  example/null@.service                            |  102 +
> >  example/service_hl.c                             |  224 ++
> >  example/service_hl.socket.in                     |   15 
> >  example/service_hl@.service                      |  102 +
> >  example/service_ll.c                             |  313 +++
> >  example/service_ll.socket.in                     |   15 
> >  example/service_ll@.service                      |  102 +
> >  example/single_file.c                            |  970 ++++++++++
> >  include/meson.build                              |    4 
> >  lib/fuse_service.c                               | 1220 +++++++++++++
> >  lib/fuse_service_stub.c                          |  106 +
> >  lib/fuse_versionscript                           |   18 
> >  lib/helper.c                                     |  160 ++
> >  lib/meson.build                                  |   17 
> >  lib/mount.c                                      |   72 +
> >  lib/mount_util.c                                 |    9 
> >  meson.build                                      |   53 +
> >  meson_options.txt                                |    9 
> >  test/ci-build.sh                                 |   14 
> >  util/fuser_conf.c                                |  396 ++++
> >  util/fusermount.c                                |  363 ----
> >  util/fuservicemount.c                            |   65 +
> >  util/install_helper.sh                           |    6 
> >  util/meson.build                                 |   24 
> >  util/mount.fuse.c                                |  169 ++
> >  util/mount_service.c                             | 2111 ++++++++++++++++++++++
> >  43 files changed, 7197 insertions(+), 404 deletions(-)
> >  create mode 100644 example/single_file.h
> >  create mode 100644 include/fuse_service.h
> >  create mode 100644 include/fuse_service_priv.h
> >  create mode 100644 lib/mount_common_i.h
> >  create mode 100644 util/fuser_conf.h
> >  create mode 100644 util/mount_service.h
> >  create mode 100644 doc/fuservicemount3.8
> >  create mode 100644 example/null.socket.in
> >  create mode 100644 example/null@.service
> >  create mode 100644 example/service_hl.c
> >  create mode 100644 example/service_hl.socket.in
> >  create mode 100644 example/service_hl@.service
> >  create mode 100644 example/service_ll.c
> >  create mode 100644 example/service_ll.socket.in
> >  create mode 100644 example/service_ll@.service
> >  create mode 100644 example/single_file.c
> >  create mode 100644 lib/fuse_service.c
> >  create mode 100644 lib/fuse_service_stub.c
> >  create mode 100644 util/fuser_conf.c
> >  create mode 100644 util/fuservicemount.c
> >  create mode 100644 util/mount_service.c
> > 
>