Re: [PATCH 4/9] vfs: intercept reads to overlay files

Al Viro
2017-02-19T09:06:24Z

On Fri, Feb 17, 2017 at 05:09:33PM +0100, Miklos Szeredi wrote:
> ...in order to handle the corner case when the file is copyied up after
> being opened read-only.

> --- /dev/null
> +++ b/fs/overlay_util.c
> @@ -0,0 +1,39 @@
> +/*
> + * Copyright (C) 2017 Red Hat, Inc.
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License version 2 as published by
> + * the Free Software Foundation.
> + */
> +#if IS_ENABLED(CONFIG_OVERLAY_FS)

This is crap - it should be handled in fs/Makefile, not with IS_ENABLED.

> +#include <linux/overlay_util.h>
> +#include <linux/fs.h>
> +#include <linux/file.h>
> +#include "internal.h"
> +
> +static bool overlay_file_consistent(struct file *file)
> +{
> + return d_real_inode(file->f_path.dentry) == file_inode(file);
> +}
> +
> +ssize_t overlay_read_iter(struct file *file, struct kiocb *kio,
> + struct iov_iter *iter)
> +{
> + ssize_t ret;
> +
> + if (likely(overlay_file_consistent(file)))
> + return file->f_op->read_iter(kio, iter);
> +
> + file = filp_clone_open(file);
> + if (IS_ERR(file))
> + return PTR_ERR(file);
> +
> + ret = vfs_iter_read(file, iter, &kio->ki_pos);
> + fput(file);

You do realize that a bunch of such calls will breed arseloads of struct file,
right? Freeing is delayed...

> +static inline bool is_overlay_file(struct file *file)
> +{
> + return IS_ENABLED(CONFIG_OVERLAY_FS) && file->f_mode & FMODE_OVERLAY;
> +}
> +
> static inline ssize_t call_read_iter(struct file *file, struct kiocb *kio,
> struct iov_iter *iter)
> {
> + if (unlikely(is_overlay_file(file)))
> + return overlay_read_iter(file, kio, iter);
> +
> return file->f_op->read_iter(kio, iter);
> }

1) that IS_ENABLED is fairly pointless and it's not obvious that nobody
else will use that flag

2) what that check should include is overlay_file_consistent(), with
no method call in overlay_read_iter().

3) anything that does a plenty of calls of kernel_read() is going to be
very unpleasantly surprised by the effects of that thing.