Re: [PATCH 4/9] vfs: intercept reads to overlay files
From: Miklos Szeredi
Date: Sun Feb 19 2017 - 04:24:51 EST
On Sun, Feb 19, 2017 at 10:05 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 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.
This is needed if overlay is built in or a module. Couldn't figure
out how makefile could handle that.
>> +#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...
No, I hadn't realized that. Could we force freeing file here?
>> +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
It's mean to be a micro-optimization for the CONFIG_OVERLAYFS=n case.
> 2) what that check should include is overlay_file_consistent(), with
> no method call in overlay_read_iter().
This is again a micro-optimization for the case when this is not an
overlay file. Which is the very very likely case.
What's the problem with putting that test in the non-inline function?
> 3) anything that does a plenty of calls of kernel_read() is going to be
> very unpleasantly surprised by the effects of that thing.
Why is that?