fat: add a vfat_rename2() and make existing .rename callback a helper

Patch series "fat: add support for the renameat2 RENAME_EXCHANGE flag", v6.

The series adds support for the renameat2 system call RENAME_EXCHANGE flag
(which allows to atomically replace two paths) to the vfat filesystem
code.

There are many use cases for this, but we are particularly interested in
making possible for vfat filesystems to be part of OSTree [0] deployments.

Currently OSTree relies on symbolic links to make the deployment updates
an atomic transactional operation.  But RENAME_EXCHANGE could be used [1]
to achieve a similar level of robustness when using a vfat filesystem.

Patch #1 is just a preparatory patch to introduce the RENAME_EXCHANGE
support, patch #2 moves some code blocks in vfat_rename() to a set of
helper functions, that can be reused by tvfat_rename_exchange() that's
added by patch #3 and finally patch #4 adds some kselftests to test it.


This patch (of 4):

Currently vfat only supports the RENAME_NOREPLACE flag which is handled by
the virtual file system layer but doesn't support the RENAME_EXCHANGE
flag.

Add a vfat_rename2() function to be used as the .rename callback and move
the current vfat_rename() handler to a helper.  This is in preparation for
implementing the RENAME_NOREPLACE flag using a different helper function.

Link: https://lkml.kernel.org/r/20220610075721.1182745-1-javierm@redhat.com
Link: https://lkml.kernel.org/r/20220610075721.1182745-2-javierm@redhat.comSigned-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Christian Kellner <ckellner@redhat.com>
Cc: Peter Jones <pjones@redhat.com>
Cc: Chung-Chiang Cheng <cccheng@synology.com>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Alexander Larsson <alexl@redhat.com>
Cc: Colin Walters <walters@verbum.org>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/fat/namei_vfat.c

@@ -889,9 +889,8 @@ static int vfat_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
 	return err;
 }
 
-static int vfat_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
-		       struct dentry *old_dentry, struct inode *new_dir,
-		       struct dentry *new_dentry, unsigned int flags)
+static int vfat_rename(struct inode *old_dir, struct dentry *old_dentry,
+		       struct inode *new_dir, struct dentry *new_dentry)
 {
 	struct buffer_head *dotdot_bh;
 	struct msdos_dir_entry *dotdot_de;
@@ -902,9 +901,6 @@ static int vfat_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
 	int err, is_dir, update_dotdot, corrupt = 0;
 	struct super_block *sb = old_dir->i_sb;
 
-	if (flags & ~RENAME_NOREPLACE)
-		return -EINVAL;
-
 	old_sinfo.bh = sinfo.bh = dotdot_bh = NULL;
 	old_inode = d_inode(old_dentry);
 	new_inode = d_inode(new_dentry);
@@ -1021,13 +1017,24 @@ static int vfat_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
 	goto out;
 }
 
+static int vfat_rename2(struct user_namespace *mnt_userns, struct inode *old_dir,
+			struct dentry *old_dentry, struct inode *new_dir,
+			struct dentry *new_dentry, unsigned int flags)
+{
+	if (flags & ~RENAME_NOREPLACE)
+		return -EINVAL;
+
+	/* VFS already handled RENAME_NOREPLACE, handle it as a normal rename */
+	return vfat_rename(old_dir, old_dentry, new_dir, new_dentry);
+}
+
 static const struct inode_operations vfat_dir_inode_operations = {
 	.create		= vfat_create,
 	.lookup		= vfat_lookup,
 	.unlink		= vfat_unlink,
 	.mkdir		= vfat_mkdir,
 	.rmdir		= vfat_rmdir,
-	.rename		= vfat_rename,
+	.rename		= vfat_rename2,
 	.setattr	= fat_setattr,
 	.getattr	= fat_getattr,
 	.update_time	= fat_update_time,