From: Chuck Lever <chuck.lever@oracle.com>

Upper layers such as NFSD need to query whether a filesystem
is case-sensitive. Add FS_XFLAG_CASEFOLD to xfs_ip2xflags()
when the filesystem is formatted with the ASCIICI feature
flag. This serves both FS_IOC_FSGETXATTR (via xfs_fill_fsxattr()
in xfs_fileattr_get()) and XFS_IOC_BULKSTAT (which populates
bs_xflags directly from xfs_ip2xflags()), so bulkstat consumers
and per-inode queries see a consistent view of the filesystem's
case-folding behavior.

FS_XFLAG_CASEFOLD is read-only: FS_XFLAG_RDONLY_MASK ensures
FS_IOC_FSSETXATTR strips it, and xfs_flags2diflags() has no
clause for CASEFOLD so the on-disk diflags are unaffected.
The legacy FS_IOC_SETFLAGS path in xfs_fileattr_set() also
allows FS_CASEFOLD_FL through its allowlist on ASCIICI
filesystems so that a chattr read-modify-write cycle does
not fail with EOPNOTSUPP.

XFS always preserves case. XFS is case-sensitive by default,
but supports ASCII case-insensitive lookups when formatted
with the ASCIICI feature flag.

Reviewed-by: Roland Mainz <redacted>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/xfs/libxfs/xfs_inode_util.c |  2 ++
 fs/xfs/xfs_ioctl.c             | 20 +++++++++++++++++---
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_inode_util.c b/fs/xfs/libxfs/xfs_inode_util.c
index 551fa51befb6..82be54b6f8d3 100644
--- a/fs/xfs/libxfs/xfs_inode_util.c
+++ b/fs/xfs/libxfs/xfs_inode_util.c

@@ -130,6 +130,8 @@ xfs_ip2xflags(
 
 	if (xfs_inode_has_attr_fork(ip))
 		flags |= FS_XFLAG_HASATTR;
+	if (xfs_has_asciici(ip->i_mount))
+		flags |= FS_XFLAG_CASEFOLD;
 	return flags;
 }
 

diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index ed9b4846c05f..f8216f74679f 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c

@@ -755,9 +755,23 @@ xfs_fileattr_set(
 	trace_xfs_ioctl_setattr(ip);
 
 	if (!fa->fsx_valid) {
-		if (fa->flags & ~(FS_IMMUTABLE_FL | FS_APPEND_FL |
-				  FS_NOATIME_FL | FS_NODUMP_FL |
-				  FS_SYNC_FL | FS_DAX_FL | FS_PROJINHERIT_FL))
+		unsigned int allowed = FS_IMMUTABLE_FL | FS_APPEND_FL |
+				       FS_NOATIME_FL | FS_NODUMP_FL |
+				       FS_SYNC_FL | FS_DAX_FL |
+				       FS_PROJINHERIT_FL;
+
+		/*
+		 * FS_CASEFOLD_FL reflects the ASCIICI superblock feature,
+		 * a read-only property. Accept it as a no-op so chattr's
+		 * RMW round-trip succeeds; reject any attempt to enable
+		 * it on a non-ASCIICI filesystem. xfs_flags2diflags()
+		 * has no clause for CASEFOLD, so the bit is dropped from
+		 * the on-disk diflags regardless.
+		 */
+		if (xfs_has_asciici(mp))
+			allowed |= FS_CASEFOLD_FL;
+
+		if (fa->flags & ~allowed)
 			return -EOPNOTSUPP;
 	}
 

-- 
2.53.0