fs: prepare for extending file_get/setattr()

We intend to add support for more xflags to selective filesystems and
We cannot rely on copy_struct_from_user() to detect this extension.

In preparation of extending the API, do not allow setting xflags unknown
by this kernel version.

Also do not pass the read-only flags and read-only field fsx_nextents to
filesystem.

These changes should not affect existing chattr programs that use the
ioctl to get fsxattr before setting the new values.

Link: https://lore.kernel.org/linux-fsdevel/20250216164029.20673-4-pali@kernel.org/
Cc: Pali Rohár <pali@kernel.org>
Cc: Andrey Albershteyn <aalbersh@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
Link: https://lore.kernel.org/20250630-xattrat-syscall-v6-5-c4e3bc35227b@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: amir73il

fs/file_attr.c

@@ -100,9 +100,10 @@ EXPORT_SYMBOL(vfs_fileattr_get);
 int copy_fsxattr_to_user(const struct fileattr *fa, struct fsxattr __user *ufa)
 {
 	struct fsxattr xfa;
+	__u32 mask = FS_XFLAGS_MASK;
 
 	memset(&xfa, 0, sizeof(xfa));
-	xfa.fsx_xflags = fa->fsx_xflags;
+	xfa.fsx_xflags = fa->fsx_xflags & mask;
 	xfa.fsx_extsize = fa->fsx_extsize;
 	xfa.fsx_nextents = fa->fsx_nextents;
 	xfa.fsx_projid = fa->fsx_projid;
@@ -119,11 +120,16 @@ static int copy_fsxattr_from_user(struct fileattr *fa,
 				  struct fsxattr __user *ufa)
 {
 	struct fsxattr xfa;
+	__u32 mask = FS_XFLAGS_MASK;
 
 	if (copy_from_user(&xfa, ufa, sizeof(xfa)))
 		return -EFAULT;
 
+	if (xfa.fsx_xflags & ~mask)
+		return -EOPNOTSUPP;
+
 	fileattr_fill_xflags(fa, xfa.fsx_xflags);
+	fa->fsx_xflags &= ~FS_XFLAG_RDONLY_MASK;
 	fa->fsx_extsize = xfa.fsx_extsize;
 	fa->fsx_nextents = xfa.fsx_nextents;
 	fa->fsx_projid = xfa.fsx_projid;

include/linux/fileattr.h

@@ -14,6 +14,26 @@
 	 FS_XFLAG_NODUMP | FS_XFLAG_NOATIME | FS_XFLAG_DAX | \
 	 FS_XFLAG_PROJINHERIT)
 
+/* Read-only inode flags */
+#define FS_XFLAG_RDONLY_MASK \
+	(FS_XFLAG_PREALLOC | FS_XFLAG_HASATTR)
+
+/* Flags to indicate valid value of fsx_ fields */
+#define FS_XFLAG_VALUES_MASK \
+	(FS_XFLAG_EXTSIZE | FS_XFLAG_COWEXTSIZE)
+
+/* Flags for directories */
+#define FS_XFLAG_DIRONLY_MASK \
+	(FS_XFLAG_RTINHERIT | FS_XFLAG_NOSYMLINKS | FS_XFLAG_EXTSZINHERIT)
+
+/* Misc settable flags */
+#define FS_XFLAG_MISC_MASK \
+	(FS_XFLAG_REALTIME | FS_XFLAG_NODEFRAG | FS_XFLAG_FILESTREAM)
+
+#define FS_XFLAGS_MASK \
+	(FS_XFLAG_COMMON | FS_XFLAG_RDONLY_MASK | FS_XFLAG_VALUES_MASK | \
+	 FS_XFLAG_DIRONLY_MASK | FS_XFLAG_MISC_MASK)
+
 /*
  * Merged interface for miscellaneous file attributes.  'flags' originates from
  * ext* and 'fsx_flags' from xfs.  There's some overlap between the two, which