linux-kernel - Re: [PATCH V2] brd: check parameter validation before register

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Date:   Tue, 14 Jan 2020 17:45:50 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Zhiqiang Liu <liuzhiqiang26@...wei.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mingfangsen <mingfangsen@...wei.com>, Guiyao <guiyao@...wei.com>,
        zhangsaisai <zhangsaisai@...wei.com>,
        "wubo (T)" <wubo40@...wei.com>
Subject: Re: [PATCH V2] brd: check parameter validation before
 register_blkdev func

On Tue, Jan 14, 2020 at 05:16:57PM +0800, Ming Lei wrote:
> On Mon, Jan 13, 2020 at 09:43:23PM +0800, Zhiqiang Liu wrote:
> > In brd_init func, rd_nr num of brd_device are firstly allocated
> > and add in brd_devices, then brd_devices are traversed to add each
> > brd_device by calling add_disk func. When allocating brd_device,
> > the disk->first_minor is set to i * max_part, if rd_nr * max_part
> > is larger than MINORMASK, two different brd_device may have the same
> > devt, then only one of them can be successfully added.
> 
> It is just because disk->first_minor is >= 0x100000, then same dev_t
> can be allocated in blk_alloc_devt().
> 
> 	MKDEV(disk->major, disk->first_minor + part->partno)
> 
> But block layer does support extended dynamic devt allocation, and brd
> sets flag of GENHD_FL_EXT_DEVT too.
> 
> So I think the correct fix is to fallback to extended dynamic allocation
> when running out of consecutive minor space.
> 
> How about the following approach?
> 
> And of course, ext devt allocation may fail too, but that is another
> generic un-solved issue: error handling isn't done for adding disk.
> 
> diff --git a/drivers/block/brd.c b/drivers/block/brd.c
> index a8730cc4db10..9aa7ce7c9abf 100644
> --- a/drivers/block/brd.c
> +++ b/drivers/block/brd.c
> @@ -398,7 +398,16 @@ static struct brd_device *brd_alloc(int i)
>  	if (!disk)
>  		goto out_free_queue;
>  	disk->major		= RAMDISK_MAJOR;
> -	disk->first_minor	= i * max_part;
> +
> +	/*
> +	 * Clear .minors when running out of consecutive minor space since
> +	 * GENHD_FL_EXT_DEVT is set, and we can allocate from extended devt
> +	 */
> +	if ((i * disk->minors) & ~MINORMASK)
> +		disk->minors = 0;
> +	else
> +		disk->first_minor	= i * disk->minors;
> +
>  	disk->fops		= &brd_fops;
>  	disk->private_data	= brd;
>  	disk->flags		= GENHD_FL_EXT_DEVT;

But still suggest to limit 'max_part' <= 256, and the name is actually
misleading, which just reserves consecutive minors.

However, I don't think it is a good idea to add limit on device number.


Thanks,
Ming