Re: [PATCH 1/3] block/nbd: only detach existing iochannel from aio_context

[Vladimir Sementsov-Ogievskiy]

28.01.2021 23:14, Roman Kagan wrote:
> When the reconnect in NBD client is in progress, the iochannel used for
> NBD connection doesn't exist.  Therefore an attempt to detach it from
> the aio_context of the parent BlockDriverState results in a NULL pointer
> dereference.
>
> The problem is triggerable, in particular, when an outgoing migration is
> about to finish, and stopping the dataplane tries to move the
> BlockDriverState from the iothread aio_context to the main loop.  If the
> NBD connection is lost before this point, and the NBD client has entered
> the reconnect procedure, QEMU crashes:
>
>      at /build/qemu-gYtjVn/qemu-5.0.1/io/channel.c:452
>      at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6151
>      new_context=new_context@entry=0x562a260c9580,
>      ignore=ignore@entry=0x7feeadc9b780)
>      at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6230
>      (bs=bs@entry=0x562a268d6a00, ctx=0x562a260c9580,
>      ignore_child=<optimized out>, errp=<optimized out>)
>      at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6332
>      new_context=0x562a260c9580,
>      update_root_node=update_root_node@entry=true, errp=errp@entry=0x0)
>      at /build/qemu-gYtjVn/qemu-5.0.1/block/block-backend.c:1989
>      new_context=<optimized out>, errp=errp@entry=0x0)
>      at /build/qemu-gYtjVn/qemu-5.0.1/block/block-backend.c:2010
>      out>)
>      at /build/qemu-gYtjVn/qemu-5.0.1/hw/block/dataplane/virtio-blk.c:292
>      at /build/qemu-gYtjVn/qemu-5.0.1/hw/virtio/virtio-bus.c:245
>      running=0, state=<optimized out>)
>      at /build/qemu-gYtjVn/qemu-5.0.1/hw/virtio/virtio.c:3220
>      state=state@entry=RUN_STATE_FINISH_MIGRATE)
>      at /build/qemu-gYtjVn/qemu-5.0.1/softmmu/vl.c:1275
>      send_stop=<optimized out>)
>      at /build/qemu-gYtjVn/qemu-5.0.1/cpus.c:1032
>      at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:2914
>      at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:3275
>      at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:3439
>      at /build/qemu-gYtjVn/qemu-5.0.1/util/qemu-thread-posix.c:519
>      at pthread_create.c:333
>      oldval=0x562a2452b138, oldlenp=0x0, newval=0x562a2452c5e0
>      <__func__.28102>, newlen=0)
>      at ../sysdeps/unix/sysv/linux/sysctl.c:30

function names are somehow lost :(

> Fix it by checking that the iochannel is non-null before trying to
> detach it from the aio_context.  If it is null, no detaching is needed,
> and it will get reattached in the proper aio_context once the connection
> is reestablished.
>
> Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>

Thanks!

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

> ---
>   block/nbd.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/block/nbd.c b/block/nbd.c
> index 42e10c7c93..bcd6641e90 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -235,7 +235,14 @@ static void nbd_client_detach_aio_context(BlockDriverState 
> *bs)
>   
>       /* Timer is deleted in nbd_client_co_drain_begin() */
>       assert(!s->reconnect_delay_timer);
> -    qio_channel_detach_aio_context(QIO_CHANNEL(s->ioc));
> +    /*
> +     * If reconnect is in progress we may have no ->ioc.  It will be
> +     * re-instantiated in the proper aio context once the connection is
> +     * reestablished.
> +     */
> +    if (s->ioc) {
> +        qio_channel_detach_aio_context(QIO_CHANNEL(s->ioc));
> +    }
>   }
>   
>   static void nbd_client_attach_aio_context_bh(void *opaque)


--
Best regards,
Vladimir