Re: [PATCH 1/2] dm mpath: fix infinite recursion in ioctl when no paths and queue_if_no_path is not set

[Bart Van Assche <bart.vanassche@sandisk.com>]

On 11/17/2015 01:36 AM, Junichi Nomura wrote:
> In multipath_prepare_ioctl(),
>    - pgpath is a path selected from available paths
>    - m->queue_io is true if we cannot send a request immediately to
>      paths, either because:
>        * there is no available path
>        * the path group needs activation (pg_init)
>            - pg_init is not started
>            - pg_init is still running
>    - m->queue_if_no_path is true if the device is configured to queue
>      I/O if there is no available path
>
> If !pgpath && !m->queue_if_no_path, the handler should return -EIO.
> However in the course of refactoring the condition check has broken
> and returns success in that case.  Since bdev points to the dm device
> itself, dm_blk_ioctl() calls __blk_dev_driver_ioctl() for itself and
> recurses until crash.
>
> You could reproduce the problem like this:
>
>    # dmsetup create mp --table '0 1024 multipath 0 0 0 0'
>    # sg_inq /dev/mapper/mp
>    <crash>
>    [  172.648615] BUG: unable to handle kernel paging request at fffffffc81b10268
>    [  172.662843] PGD 19dd067 PUD 0
>    [  172.666269] Thread overran stack, or stack corrupted
>    [  172.671808] Oops: 0000 [#1] SMP
>    ...
>
> This patch fixes the condition check with some clarifications.
>
> Fixes: e56f81e0b01e ("dm: refactor ioctl handling")
> Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Mike Snitzer <snitzer@redhat.com>

Since I was able to reproduce this crash and since I haven't seen that 
crash anymore after I had applied this patch,

Tested-by: Bart Van Assche <bart.vanassche@sandisk.com>