[RFC PATCH 1/2] fs: prepare for extending [gs]etfsxattrat()

[Amir Goldstein <amir73il@gmail.com>]

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

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>
---
 fs/inode.c               |  4 +++-
 fs/ioctl.c               | 19 +++++++++++++------
 include/linux/fileattr.h | 22 +++++++++++++++++++++-
 3 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 3cfcb1b9865ea..6c4d08bd53052 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -3049,7 +3049,9 @@ SYSCALL_DEFINE5(setfsxattrat, int, dfd, const char __user *, filename,
 	if (error)
 		return error;
 
-	fsxattr_to_fileattr(&fsx, &fa);
+	error = fsxattr_to_fileattr(&fsx, &fa);
+	if (error)
+		return error;
 
 	name = getname_maybe_null(filename, at_flags);
 	if (!name) {
diff --git a/fs/ioctl.c b/fs/ioctl.c
index 840283d8c4066..b19858db4c432 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -540,8 +540,10 @@ EXPORT_SYMBOL(vfs_fileattr_get);
 
 void fileattr_to_fsxattr(const struct fileattr *fa, struct fsxattr *fsx)
 {
+	__u32 mask = FS_XFALGS_MASK;
+
 	memset(fsx, 0, sizeof(struct fsxattr));
-	fsx->fsx_xflags = fa->fsx_xflags;
+	fsx->fsx_xflags = fa->fsx_xflags & mask;
 	fsx->fsx_extsize = fa->fsx_extsize;
 	fsx->fsx_nextents = fa->fsx_nextents;
 	fsx->fsx_projid = fa->fsx_projid;
@@ -568,13 +570,20 @@ int copy_fsxattr_to_user(const struct fileattr *fa, struct fsxattr __user *ufa)
 }
 EXPORT_SYMBOL(copy_fsxattr_to_user);
 
-void fsxattr_to_fileattr(const struct fsxattr *fsx, struct fileattr *fa)
+int fsxattr_to_fileattr(const struct fsxattr *fsx, struct fileattr *fa)
 {
+	__u32 mask = FS_XFALGS_MASK;
+
+	if (fsx->fsx_xflags & ~mask)
+		return -EINVAL;
+
 	fileattr_fill_xflags(fa, fsx->fsx_xflags);
+	fa->fsx_xflags &= ~FS_XFLAG_RDONLY_MASK;
 	fa->fsx_extsize = fsx->fsx_extsize;
-	fa->fsx_nextents = fsx->fsx_nextents;
 	fa->fsx_projid = fsx->fsx_projid;
 	fa->fsx_cowextsize = fsx->fsx_cowextsize;
+
+	return 0;
 }
 
 static int copy_fsxattr_from_user(struct fileattr *fa,
@@ -585,9 +594,7 @@ static int copy_fsxattr_from_user(struct fileattr *fa,
 	if (copy_from_user(&xfa, ufa, sizeof(xfa)))
 		return -EFAULT;
 
-	fsxattr_to_fileattr(&xfa, fa);
-
-	return 0;
+	return fsxattr_to_fileattr(&xfa, fa);
 }
 
 /*
diff --git a/include/linux/fileattr.h b/include/linux/fileattr.h
index 31888fa2edf10..f682bfc7749dd 100644
--- a/include/linux/fileattr.h
+++ b/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_XFALGS_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
@@ -35,7 +55,7 @@ struct fileattr {
 
 void fileattr_to_fsxattr(const struct fileattr *fa, struct fsxattr *fsx);
 int copy_fsxattr_to_user(const struct fileattr *fa, struct fsxattr __user *ufa);
-void fsxattr_to_fileattr(const struct fsxattr *fsx, struct fileattr *fa);
+int fsxattr_to_fileattr(const struct fsxattr *fsx, struct fileattr *fa);
 
 void fileattr_fill_xflags(struct fileattr *fa, u32 xflags);
 void fileattr_fill_flags(struct fileattr *fa, u32 flags);
-- 
2.34.1