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

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Date:   Tue, 14 Sep 2021 14:44:48 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     "yukuai (C)" <yukuai3@...wei.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 Tue, Sep 14, 2021 at 11:11:06AM +0800, yukuai (C) wrote:
> 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

Request and its payload is pre-allocated, and either req->ref or cmd->lock can
serve the same purpose here. Once cmd->lock is held, you can check if the cmd is
inflight or not. If it isn't inflight, just return -ENOENT. Is there any
problem to handle in this way?


Thanks,
Ming