ovl: initialize ovl_copy_up_ctx.destname inside ovl_do_copy_up()

The ->destname member of struct ovl_copy_up_ctx is initialized inside
ovl_copy_up_one() to ->d_name of the overlayfs dentry being copied up
and then it may be overridden by index name inside ovl_do_copy_up().

ovl_inode_lock() in ovl_copy_up_start() and ovl_copy_up() in ovl_rename()
effectively stabilze ->d_name of the overlayfs dentry being copied up,
but ovl_inode_lock() is not held when ->d_name is being read.

It is not a correctness bug, because if ovl_do_copy_up() races with
ovl_rename() and ctx.destname is freed, we will not end up calling
ovl_do_copy_up() with the dead name reference.

The code becomes much easier to understand and to document if the
initialization of c->destname is always done inside ovl_do_copy_up(),
either to the index entry name, or to the overlay dentry ->d_name.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

fs/overlayfs/copy_up.c

@@ -929,6 +929,13 @@ static int ovl_do_copy_up(struct ovl_copy_up_ctx *c)
 		err = -EIO;
 		goto out_free_fh;
 	} else {
+		/*
+		 * c->dentry->d_name is stabilzed by ovl_copy_up_start(),
+		 * because if we got here, it means that c->dentry has no upper
+		 * alias and changing ->d_name means going through ovl_rename()
+		 * that will call ovl_copy_up() on source and target dentry.
+		 */
+		c->destname = c->dentry->d_name;
 		/*
 		 * Mark parent "impure" because it may now contain non-pure
 		 * upper
@@ -1109,7 +1116,6 @@ static int ovl_copy_up_one(struct dentry *parent, struct dentry *dentry,
 	if (parent) {
 		ovl_path_upper(parent, &parentpath);
 		ctx.destdir = parentpath.dentry;
-		ctx.destname = dentry->d_name;
 
 		err = vfs_getattr(&parentpath, &ctx.pstat,
 				  STATX_ATIME | STATX_MTIME,