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:

quoted

On Thu, Sep 09, 2021 at 10:12:55PM +0800, Yu Kuai wrote:

quoted

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