linux-kernel - Re: [PATCH 0/9] overlay: fix inconsistency of ro file after copy-up

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Date:   Mon, 20 Feb 2017 16:16:04 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Al Viro <viro@...iv.linux.org.uk>
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,
        David Howells <dhowells@...hat.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...chiereds.net>
Subject: Re: [PATCH 0/9] overlay: fix inconsistency of ro file after copy-up

On Sun, Feb 19, 2017 at 10:14 AM, Al Viro <viro@...iv.linux.org.uk> wrote:

> This is one hell of a DoS vector - it's really easy to eat tons of struct
> file that way.  Preparatory parts of that series make sense on their own,
> but your "let's allocate a struct file, call ->open() and schedule that
> struct file for closing upon the exit to userland on each kernel_read()"
> is not.

Found a couple of instances of this pattern (haven't looked very hard,
possibly there's more):

nfsd_read()
cachefiles_write_page()

How come this hasn't been a problem for them?

Would flush_delayed_fput() work here?  I couldn't really find what the
locking issues with synchronous fputs were.

Thanks,
Miklos