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

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Date:   Mon, 20 Feb 2017 09:52:27 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     "zhangyi (F)" <yi.zhang@...wei.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        "linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>, miaoxie@...wei.com
Subject: Re: [PATCH 4/9] vfs: intercept reads to overlay files

On Mon, Feb 20, 2017 at 8:47 AM, zhangyi (F) <yi.zhang@...wei.com> wrote:
> On 2017/2/18 00:10, Miklos Szeredi wrote:
>> diff --git a/fs/open.c b/fs/open.c
>> index 9921f70bc5ca..4916ccff29f5 100644
>> --- a/fs/open.c
>> +++ b/fs/open.c
>> @@ -762,6 +762,8 @@ static int do_dentry_open(struct file *f,
>>       if ((f->f_mode & FMODE_WRITE) &&
>>            likely(f->f_op->write || f->f_op->write_iter))
>>               f->f_mode |= FMODE_CAN_WRITE;
>> +     if (unlikely(d_inode(f->f_path.dentry) != inode))
>> +             f->f_mode |= FMODE_OVERLAY;
>
> Can we just add flag to the "readonly && not copied" file, not all overlayfs files?
> Beacuse we just want to check the ro file before copied-up.

Can't do it without adding new infrastructure.  Likely not worth it.

>
>>       f->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
>>
>> diff --git a/fs/overlay_util.c b/fs/overlay_util.c new file mode 100644 index 000000000000..0daff19bad0b
>> --- /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)
>> +
>> +#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);
>> +
>> +     return ret;
>> +}
>> +EXPORT_SYMBOL(overlay_read_iter);
>
> Can we try to replace the old file with the new file, and then clear the f_mode flag we added?
> If so, we can avoid opening file for each reading call and avoid copied file consistency check.

Could probably try replacing file in fd table.  But no point in doing
so without having an actual real life use case that would benefit from
that.  AFAIK there's no such thing.

And there are other pointers to file that can't be replaced, so this
fallback would need to be kept around.

Thanks,
Miklos