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

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Date:   Tue, 14 Jan 2020 19:38:55 +0800
From:   Zhiqiang Liu <liuzhiqiang26@...wei.com>
To:     Ming Lei <ming.lei@...hat.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 2020/1/14 17:45, Ming Lei wrote:
> 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 for your patient replyI will resend the v3 patch as your suggestion.
Changes in v3:
	1)clear .minors when running out of consecutive minor space in brd_alloc.
	2)remove limit of rd_nr