Exposing case folding behavior

Christian, let's lock this one in. I will post subsequent changes as delta patches.

Following on from:

https://lore.kernel.org/linux-nfs/20251021-zypressen-bazillus-545a44af57fd@brauner/T/#m0ba197d75b7921d994cf284f3cef3a62abb11aaa

I'm attempting to implement enough support in the Linux VFS to enable file services like NFSD and ksmbd (and user space equivalents) to provide the actual status of case folding support in local file systems. The default behavior for local file systems not explicitly supported in this series is to reflect the usual POSIX behaviors:

case-insensitive = false
case-nonpreserving = false

The case-insensitivity and case-nonpreserving booleans can be consumed immediately by NFSD. These two attributes have been part of the NFSv3 and NFSv4 protocols for decades, in order to support NFS client implementations on non-POSIX systems.

Support for user space file servers is why this series exposes case folding information via a user-space API. I don't know of any other category of user-space application that requires access to case folding info.

The Linux NFS community has a growing interest in supporting NFS clients on Windows and MacOS platforms, where file name behavior does not align with traditional POSIX semantics.

One example of a Windows-based NFS client is [1]. This client implementation explicitly requires servers to report FATTR4_WORD0_CASE_INSENSITIVE = TRUE for proper operation, a hard requirement for Windows client interoperability because Windows applications expect case-insensitive behavior. When an NFS client knows the server is case-insensitive, it can avoid issuing multiple LOOKUP/READDIR requests to search for case variants, and applications like Win32 programs work correctly without manual workarounds or code changes.

Even the Linux client can take advantage of this information. Trond merged patches 4 years ago [2] that introduce support for case insensitivity, in support of the Hammerspace NFS server. In particular, when a client detects a case-insensitive NFS share, negative dentry caching must be disabled (a lookup for "FILE.TXT" failing shouldn't cache a negative entry when "file.txt" exists) and directory change invalidation must clear all cached case-folded file name variants.

Hammerspace servers and several other NFS server implementations operate in multi-protocol environments, where a single file service instance caters to both NFS and SMB clients. In those cases, things work more smoothly for everyone when the NFS client can see and adapt to the case folding behavior that SMB users rely on and expect. NFSD needs to support the case-insensitivity and case-nonpreserving booleans properly in order to participate as a first-class citizen in such environments.

[1] https://github.com/kofemann/ms-nfs41-client

[2] https://patchwork.kernel.org/project/linux-nfs/cover/[email protected]/

---
Changes since v13:
- Address findings from sashiko (gemini-3.1):
- ntfs3: Drop fileattr_get from symlink and special inode ops
- nfsd: Probe nfsd_get_case_info() under kernel creds to avoid spurious NFS4ERR_ACCESS from per-client MAC policy

Changes since v12:
- Address findings from sashiko (gemini-3.1):
- cifs: Restrict case-handling flags to directories per UAPI
- nfs: Clear case caps before PATHCONF so a failed reply does not retain stale bits from the prior probe
- nfsd: Document the parent-resolution corner cases of nfsd_get_case_info() (single-file exports, disconnected dentries, hardlinks) in the v3 and v4 commit messages

Changes since v11:
- isofs: Wire .fileattr_get only on directory inodes, since NFSD and ksmbd query casefolding on directories (Jan Kara)
- xfs, hfsplus: Drop the FS_CASEFOLD_FL fileattr_get mask; admit the bit through fileattr_set's allowlist instead
- Address findings from sashiko(gemini-3) and gpt-5.5:
- cifs: Wire .fileattr_get on cifs_namespace_inode_operations so DFS referral / automount directories report case handling
- fat, ntfs3: Fill FS_IMMUTABLE_FL in fileattr_get
- hfsplus: Hide FS_CASEFOLD_FL from the legacy flags view so chattr round-trips do not hit the setflags whitelist
- nfs: Clear NFS_CAP_CASE_INSENSITIVE and NFS_CAP_CASE_NONPRESERVING before re-OR'ing in the v3 and v4 probe paths so re-probe / TSM does not retain stale caps
- nfsd: Switch nfsd_get_case_info() to errno return so v3 PATHCONF and v4 GETATTR can apply version-appropriate policy on failure
- nfsd: Use dget_parent() in v4 case-attr probe to keep the parent dentry referenced across the query
- isofs: Report FS_XFLAG_CASENONPRESERVING for map=n/map=a

Changes since v10:
- cifs: Source case-handling flags from the server's cached FS_ATTRIBUTE_INFORMATION reply instead of the nocase mount option, with a nocase fallback when the reply is absent
- Address findings from sashiko(gemini-3) and gpt-5.5:
- nfs: Skip pathconf case bits on NFSv4 (set via FATTR4_CASE_* instead)
- xfs: Hide FS_CASEFOLD_FL from the legacy flags view so chattr round-trips do not hit the setflags whitelist
- ext4, f2fs: Drop redundant fileattr_get patches; the FS_CASEFOLD_FL translation in fileattr_fill_flags() already reports FS_XFLAG_CASEFOLD for casefolded directories
- nfsd: Report FATTR4_HOMOGENEOUS = FALSE when the exported filesystem has a Unicode encoding, since per-directory casefold makes the fs-scoped case attributes inhomogeneous
- nfsd: Document in nfsd_get_case_info() why -ENOIOCTLCMD and -ENOTTY are swallowed while other errors propagate
- fat: Honor vfat 'check=strict' when reporting FS_XFLAG_CASEFOLD
- Set FS_CASEFOLD_FL so FS_IOC_GETFLAGS reflects case-insensitive mount
- isofs: Register fileattr_get on regular file and symlink inodes, not just directories
- nfsd: Query NFSv4 FATTR4_CASE_* from the parent directory for non-directory objects, since casefold lives on the directory

Changes since v9:
- nfs: always probe PATHCONF for case caps. Default to case- preserving when the server does not report case_preserving
- nfsd, ksmbd: tolerate -ENOTTY from vfs_fileattr_get() so overlayfs exports on backing filesystems without fileattr_get do not fail the RPC
- xfs: map FS_XFLAG_CASEFOLD inside xfs_ip2xflags() so BULKSTAT and FS_IOC_FSGETXATTR report the flag consistently
- vboxsf: reject a short host reply to SHFL_INFO_VOLUME before trusting volinfo.properties.case_sensitive

Changes since v8:
- Rebase on v7.0-rc1

Changes since v7:
- Split file_attr initialization changes into a separate patch

Changes since v6:
- Remove the memset from vfs_fileattr_get

Changes since v5:
- Finish the conversion to FS_XFLAGs
- NFSv4 GETATTR now clears the attr mask bit if nfsd_get_case_info() fails

Changes since v4:
- Observe the MSDOS "nocase" mount option
- Define new FS_XFLAGs for the user API

Changes since v3:
- Change fa->case_preserving to fa_case_nonpreserving
- VFAT is case preserving
- Make new fields available to user space

Changes since v2:
- Remove unicode labels
- Replace vfs_get_case_info
- Add support for several more local file system implementations
- Add support for in-kernel SMB server

Changes since RFC:
- Use file_getattr instead of statx
- Postpone exposing Unicode version until later
- Support NTFS and ext4 in addition to FAT
- Support NFSv4 fattr4 in addition to NFSv3 PATHCONF

---
Chuck Lever (15):
fs: Move file_kattr initialization to callers
fs: Add case sensitivity flags to file_kattr
fat: Implement fileattr_get for case sensitivity
exfat: Implement fileattr_get for case sensitivity
ntfs3: Implement fileattr_get for case sensitivity
hfs: Implement fileattr_get for case sensitivity
hfsplus: Report case sensitivity in fileattr_get
xfs: Report case sensitivity in fileattr_get
cifs: Implement fileattr_get for case sensitivity
nfs: Implement fileattr_get for case sensitivity
vboxsf: Implement fileattr_get for case sensitivity
isofs: Implement fileattr_get for case sensitivity
nfsd: Report export case-folding via NFSv3 PATHCONF
nfsd: Implement NFSv4 FATTR4_CASE_INSENSITIVE and FATTR4_CASE_PRESERVING
ksmbd: Report filesystem case sensitivity via FS_ATTRIBUTE_INFORMATION

 fs/exfat/exfat_fs.h            |  2 +
 fs/exfat/file.c                | 18 ++++++++-
 fs/exfat/namei.c               |  1 +
 fs/fat/fat.h                   |  3 ++
 fs/fat/file.c                  | 36 +++++++++++++++++
 fs/fat/namei_msdos.c           |  1 +
 fs/fat/namei_vfat.c            |  1 +
 fs/file_attr.c                 | 16 ++++----
 fs/hfs/dir.c                   |  1 +
 fs/hfs/hfs_fs.h                |  2 +
 fs/hfs/inode.c                 | 14 +++++++
 fs/hfsplus/inode.c             | 16 +++++++-
 fs/isofs/dir.c                 | 16 ++++++++
 fs/isofs/isofs.h               |  3 ++
 fs/nfs/client.c                | 21 +++++++---
 fs/nfs/inode.c                 | 15 +++++++
 fs/nfs/internal.h              |  3 ++
 fs/nfs/namespace.c             |  2 +
 fs/nfs/nfs3proc.c              |  2 +
 fs/nfs/nfs3xdr.c               |  7 +++-
 fs/nfs/nfs4proc.c              | 10 +++--
 fs/nfs/proc.c                  |  3 ++
 fs/nfs/symlink.c               |  3 ++
 fs/nfsd/nfs3proc.c             | 36 +++++++++++++----
 fs/nfsd/nfs4xdr.c              | 52 +++++++++++++++++++++++--
 fs/nfsd/vfs.c                  | 88 ++++++++++++++++++++++++++++++++++++++++++
 fs/nfsd/vfs.h                  |  3 ++
 fs/nfsd/xdr3.h                 |  4 +-
 fs/ntfs3/file.c                | 29 ++++++++++++++
 fs/ntfs3/namei.c               |  1 +
 fs/ntfs3/ntfs_fs.h             |  1 +
 fs/smb/client/cifsfs.c         | 53 +++++++++++++++++++++++++
 fs/smb/client/cifsfs.h         |  3 ++
 fs/smb/client/namespace.c      |  1 +
 fs/smb/server/smb2pdu.c        | 30 +++++++++++---
 fs/vboxsf/dir.c                |  1 +
 fs/vboxsf/file.c               |  6 ++-
 fs/vboxsf/super.c              |  7 ++++
 fs/vboxsf/utils.c              | 30 ++++++++++++++
 fs/vboxsf/vfsmod.h             |  6 +++
 fs/xfs/libxfs/xfs_inode_util.c |  2 +
 fs/xfs/xfs_ioctl.c             | 22 +++++++++--
 include/linux/fileattr.h       |  3 +-
 include/linux/nfs_fs_sb.h      |  2 +-
 include/linux/nfs_xdr.h        |  2 +
 include/uapi/linux/fs.h        |  7 ++++
 46 files changed, 536 insertions(+), 49 deletions(-)

---
base-commit: 6596a02b207886e9e00bb0161c7fd59fea53c081
change-id: 20260422-case-sensitivity-5cbffc8f1558

Best regards,

Chuck Lever <[email protected]>