Linux 7.2 To Better Communicate File-System Casefolding For Helping Windows NFS & More

Newly-merged code for the in-development Linux 7.2 kernel will now expose the case-folding (case insensitive) behavior of local file-systems so that Linux file servers and others can properly report the actual behavior rather than guessing if case-folding is actually used/supported.

Linus Torvalds today merged many of the VFS related pull requests for Linux 7.2. Among the notable VFS material is now being able to nicely report case-folding behavior of local file-systems for the likes of NFSD, KSMBD, and user-space servers. The motivating factor in getting this functionality in place via new file_kattr flags is for addressing the needs of Microsoft Windows NFS clients.

Christian Brauner explained of this case-folding work for Linux 7.2:

"This exposes the case folding behavior of local filesystems so that file servers - nfsd, ksmbd, and user space file servers - can report the actual behavior to clients instead of guessing.

Filesystems report case-insensitive and case-nonpreserving behavior via new file_kattr flags in their fileattr_get implementations. fat, exfat, ntfs3, hfs, hfsplus, xfs, cifs, nfs, vboxsf, and isofs are wired up; local filesystems not explicitly handled default to the usual POSIX behavior of case-sensitive and case-preserving. nfsd uses this to report case folding via NFSv3 PATHCONF and to implement the NFSv4 FATTR4_CASE_INSENSITIVE and FATTR4_CASE_PRESERVING attributes - both have been part of the NFS protocols for decades to support clients on non-POSIX systems - and ksmbd reports it via FS_ATTRIBUTE_INFORMATION. Exposing the information through the fileattr uapi covers user space file servers.

The immediate motivation is interoperability: Windows NFS clients hard-require servers to report case-insensitivity for Win32 applications to work correctly, and a client that knows the server is case-insensitive can avoid issuing multiple LOOKUP/READDIR requests searching for case variants. The Linux NFS client already grew support for case-insensitive shares years ago in support of the Hammerspace NFS server - negative dentry caching must be disabled (a lookup for "FILE.TXT" failing must not cache a negative entry when "file.txt" exists) and directory change invalidation must drop cached case-folded name variants. Such servers often operate in multi-protocol environments where a single file service instance caters to both NFS and SMB clients, and nfsd needs to report case folding properly to participate as a first-class citizen there."

This pull is now merged for all this work around case sensitivity reporting.