linux-kernel - Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get

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Message-ID: <c6af73a2-f12d-eeef-616e-ae0cdb4f6f2d@huawei.com>
Date:   Tue, 14 Sep 2021 11:11:06 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     <axboe@...nel.dk>, <josef@...icpanda.com>, <hch@...radead.org>,
        <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <nbd@...er.debian.org>, <yi.zhang@...wei.com>
Subject: Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get_req()

On 2021/09/14 9:11, Ming Lei wrote:
> On Thu, Sep 09, 2021 at 10:12:55PM +0800, Yu Kuai wrote:
>> blk_mq_tag_to_rq() can only ensure to return valid request in
>> following situation:
>>
>> 1) client send request message to server first
>> submit_bio
>> ...
>>   blk_mq_get_tag
>>   ...
>>   blk_mq_get_driver_tag
>>   ...
>>   nbd_queue_rq
>>    nbd_handle_cmd
>>     nbd_send_cmd
>>
>> 2) client receive respond message from server
>> recv_work
>>   nbd_read_stat
>>    blk_mq_tag_to_rq
>>
>> If step 1) is missing, blk_mq_tag_to_rq() will return a stale
>> request, which might be freed. Thus convert to use
>> blk_mq_find_and_get_req() to make sure the returned request is not
>> freed.
> 
> But NBD_CMD_INFLIGHT has been added for checking if the reply is
> expected, do we still need blk_mq_find_and_get_req() for covering
> this issue? BTW, request and its payload is pre-allocated, so there
> isn't real use-after-free.

Hi, Ming

Checking NBD_CMD_INFLIGHT relied on the request founded by tag is valid,
not the other way round.

nbd_read_stat
  req = blk_mq_tag_to_rq()
  cmd = blk_mq_rq_to_pdu(req)
  mutex_lock(cmd->lock)
  checking NBD_CMD_INFLIGHT

The checking doesn't have any effect on blk_mq_tag_to_rq().

Thanks,
Kuai