Merge patch series "Exposing case folding behavior"

Chuck Lever <cel@kernel.org> says:

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/20211217203658.439352-1-trondmy@kernel.org/

* patches from https://patch.msgid.link/20260507-case-sensitivity-v14-0-e62cc8200435@oracle.com:
  ksmbd: Report filesystem case sensitivity via FS_ATTRIBUTE_INFORMATION
  nfsd: Implement NFSv4 FATTR4_CASE_INSENSITIVE and FATTR4_CASE_PRESERVING
  nfsd: Report export case-folding via NFSv3 PATHCONF
  isofs: Implement fileattr_get for case sensitivity
  vboxsf: Implement fileattr_get for case sensitivity
  nfs: Implement fileattr_get for case sensitivity
  cifs: Implement fileattr_get for case sensitivity
  xfs: Report case sensitivity in fileattr_get
  hfsplus: Report case sensitivity in fileattr_get
  hfs: Implement fileattr_get for case sensitivity
  ntfs3: Implement fileattr_get for case sensitivity
  exfat: Implement fileattr_get for case sensitivity
  fat: Implement fileattr_get for case sensitivity
  fs: Add case sensitivity flags to file_kattr
  fs: Move file_kattr initialization to callers

Link: https://patch.msgid.link/20260507-case-sensitivity-v14-0-e62cc8200435@oracle.com
Signed-off-by: Christian Brauner <brauner@kernel.org>