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Date:	Mon, 20 Oct 2014 11:02:08 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Martin Kepplinger <martink@...teo.de>
Cc:	lguest@...ts.ozlabs.org,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: lguest: lguest_user.c is open() called by the user?

Martin Kepplinger <martink@...teo.de> writes:
> hi
>
> Just a question for understanding: open() is not implemented in
> lguest_user.c's miscdevice. The miscdevice core, in this case, does
> _not_ set file->private_data on a user's open() call. Is open() called
> by the user here? and do you here _depend_ on file->private_data being
> NULL after open()? (could you even?)
>
> Would the following force to NULL be necessary if the miscdevice core
> _would_ set private_data?

Hi Martin!

        Yes, the private_data is NULL on a new file.  See
get_empty_filp in fs/file_table.c, which does kmem_cache_zalloc
(zeroing all the contents).

So this isn't necessary here.

Thanks!
Rusty.

> diff --git a/drivers/lguest/lguest_user.c b/drivers/lguest/lguest_user.c
> index 4263f4c..3d472ea 100644
> --- a/drivers/lguest/lguest_user.c
> +++ b/drivers/lguest/lguest_user.c
> @@ -497,6 +497,12 @@ static int close(struct inode *inode, struct file
> *file)
>         return 0;
>  }
>
> +static int open(struct inode *inode, struct file *file)
> +{
> +       /* assuming the miscdevice core sets private_data on open() */
> +       file->private_data = NULL;
> +}
> +
>  /*L:000
>   * Welcome to our journey through the Launcher!
>   *
> @@ -514,6 +520,7 @@ static int close(struct inode *inode, struct file *file)
>   */
>  static const struct file_operations lguest_fops = {
>         .owner   = THIS_MODULE,
> +       .open    = open,
>         .release = close,
>         .write   = write,
>         .read    = read,
> -- 
>
> thanks
>                                martin
> --
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