[PATCH v2 00/21] fuse: extend passthrough to inode operations

[Joanne Koong <joannelkoong@gmail.com>]

This series extends fuse passthrough to support inode operations (getattr +
setattr) and directory readdir.

The existing FUSE_PASSTHROUGH mode attaches a backing file to a fuse inode
only when a passthrough file is open. This series introduces
FUSE_PASSTHROUGH_INO, a stricter mode requiring one-to-one inode number
mapping, which allows attaching a backing file for the lifetime of the fuse
inode.

This is based off Amir's work. Patches 1-7 are from his git tree in [1].

For testing/debugging, this was done using the passthrough_hp server with the
changes in [2] applied.

This series was run through fstests using passthrough_hp in [2]. There were no
regressions seen compared to passthrough_hp --nopassthrough and an improvement
in generic 683, 684, and 685 (dropping suid/sgid) now passing. Compared to
passthrough_hp without inode operations passed through, generic tests 080 and
215 (mapped write timestamp updates) now pass.

This is on top of commit ea2ff881bb8e ("fuse: reduce attributes invalidated on
directory change") + a locally applied patch [3], in Miklos's fuse tree.

[1] https://github.com/amir73il/linux/commits/fuse-backing-inode-wip/
[2] https://github.com/joannekoong/libfuse/commits/extended_passthrough_v2/
[3] https://lore.kernel.org/fuse-devel/20260513231437.1806724-1-joannelkoong@gmail.com/T/#u

Changelog
---------
v1: https://lore.kernel.org/fuse-devel/CAOQ4uxjBZL6CY0mJEiHrw8aR28MHg5pU+Pfr9fUqGdKPDOdwcw@mail.gmail.com/T/
v1 -> v2:
- Address Amir's feedback (especially about not mixing cached io mode with
  passthrough mode, dropping the passthrough open patch, modeling some of the
  logic after overlayfs)
- Diff between v2 and v1 for Amir's patches 1-7:
  https://gist.github.com/joannekoong/451eb02e716fa6321a622a61a95aea04

Amir Goldstein (7):
  fuse: introduce FUSE_PASSTHROUGH_INO mode
  fuse: prepare for passthrough of inode operations
  fuse: prepare for readdir passthrough on directories
  fuse: implement passthrough for readdir
  fuse: prepare for long lived reference on backing file
  fuse: implement passthrough for getattr/statx
  fuse: prepare to setup backing inode passthrough on lookup

Joanne Koong (14):
  fuse: handle zero ops_mask in FUSE_DEV_IOC_BACKING_OPEN
  fuse: handle partial io passthrough for read/write, splice, and mmap
  fuse: prepare to cache statx attributes from entry replies
  fuse: clean up fuse_dentry_revalidate()
  fuse: add struct fuse_entry2_out and helpers for extended entry
    replies
  fuse: add passthrough lookup
  fuse: add passthrough support for entry creation
  fuse: add passthrough support for create+open
  fuse: allow backing_id=0 in open to inherit inode's backing file
  backing-inode: add backing_inode_copyattr()
  backing-inode: add backing_inode_setattr()
  fuse: add passthrough setattr
  fuse: use passthrough getattr in setattr suid/sgid handling
  docs: fuse: document extended passthrough (FUSE_PASSTHROUGH_INO)

 .../filesystems/fuse/fuse-passthrough.rst     | 126 ++++++
 fs/Makefile                                   |   2 +-
 fs/backing-inode.c                            |  97 +++++
 fs/fuse/backing.c                             |  64 ++-
 fs/fuse/cuse.c                                |   2 +-
 fs/fuse/dir.c                                 | 391 +++++++++++++-----
 fs/fuse/file.c                                |  37 +-
 fs/fuse/fuse_i.h                              |  83 +++-
 fs/fuse/inode.c                               |  27 +-
 fs/fuse/iomode.c                              | 122 +++++-
 fs/fuse/passthrough.c                         | 122 +++++-
 fs/fuse/readdir.c                             |   5 +-
 fs/overlayfs/inode.c                          |  66 +--
 fs/overlayfs/util.c                           |  33 +-
 include/linux/backing-inode.h                 |  18 +
 include/uapi/linux/fuse.h                     |  32 +-
 16 files changed, 969 insertions(+), 258 deletions(-)
 create mode 100644 fs/backing-inode.c
 create mode 100644 include/linux/backing-inode.h

-- 
2.52.0