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Date: Thu, 7 Jul 2016 11:32:11 +0200
From: Roger Pau Monne <roger.pau@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        <xen-devel@lists.xenproject.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] xen-blkfront: avoid NULL de-reference in CDROM ioctl
 handling
Message-ID: <20160707093211.dn5mrvgmavpd2mul@mac>
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On Thu, Jul 07, 2016 at 01:40:54AM -0600, Jan Beulich wrote:
> The ioctl can be called prior to full device setup having completed.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
>  drivers/block/xen-blkfront.c |    6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> --- 4.7-rc6-xen.orig/drivers/block/xen-blkfront.c
> +++ 4.7-rc6-xen/drivers/block/xen-blkfront.c
> @@ -496,12 +496,10 @@ static int blkif_ioctl(struct block_devi
>  				return -EFAULT;
>  		return 0;
>  
> -	case CDROM_GET_CAPABILITY: {
> -		struct gendisk *gd = info->gd;
> -		if (gd->flags & GENHD_FL_CD)
> +	case CDROM_GET_CAPABILITY:
> +		if (info->gd && (info->gd->flags & GENHD_FL_CD))

I don't follow how blkif_ioctl can be called with a NULL info->gd, because 
the set of file operations is set inside of info->gd->fops. And the disk 
should not be available until add_disk is called, which happens after having 
info->gd already set.

Roger.