linux-kernel - Re: [patch 2/3] kcmp: Add KCMP_EPOLL_TFD mode to compare epoll target files

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Date:   Mon, 13 Mar 2017 21:37:58 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Andrei Vagin <avagin@...tuozzo.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, viro@...iv.linux.org.uk,
        akpm@...uxfoundation.org, xemul@...tuozzo.com,
        mtk.manpages@...il.com, kir@...nvz.org,
        Andrey Vagin <avagin@...nvz.org>,
        Jason Baron <jbaron@...mai.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [patch 2/3] kcmp: Add KCMP_EPOLL_TFD mode to compare epoll
 target files

On Mon, Mar 13, 2017 at 10:37:24AM -0700, Andrei Vagin wrote:
> 
> Here is one question inline.
> 
> Acked-by: Andrei Vagin <avagin@...tuozzo.com>

Thanks!

> > +
> > +	files = get_files_struct(task2);
> > +	if (!files)
> > +		return -EBADF;
> > +
> > +	spin_lock(&files->file_lock);
> > +	filp_epoll = fcheck_files(files, slot.efd);
> > +	if (filp_epoll)
> > +		get_file(filp_epoll);
> > +	else
> > +		filp_tgt = ERR_PTR(-EBADF);
> > +	spin_unlock(&files->file_lock);
> > +	put_files_struct(files);
> 
> Why can we not use fget here ^^^^ ?

Because it's not @current's file table but foreigner process.
So we make sure there is no expand_fdtable in progress and
we test for file descriptor which is being freed.