Re: [PATCH] docs: show how to spawn qemu-storage-daemon with fd passing

[Daniel P . Berrangé]

On Mon, Mar 01, 2021 at 03:31:59PM +0000, Stefan Hajnoczi wrote:
> The QMP monitor, NBD server, and vhost-user-blk export all support file
> descriptor passing. This is a useful technique because it allows the
> parent process to spawn and wait for qemu-storage-daemon without busy
> waiting, which may delay startup due to arbitrary sleep() calls.
> 
> This Python example is inspired by the test case written for libnbd by
> Richard W.M. Jones <rjones@redhat.com>:
> https://gitlab.com/nbdkit/libnbd/-/commit/89113f484effb0e6c322314ba75c1cbe07a04543
> 
> Thanks to Daniel P. Berrangé <berrange@redhat.com> for suggestions on
> how to get this working. Now let's document it!
> 
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  docs/tools/qemu-storage-daemon.rst | 38 ++++++++++++++++++++++++++++--
>  1 file changed, 36 insertions(+), 2 deletions(-)
> 
> diff --git a/docs/tools/qemu-storage-daemon.rst 
> b/docs/tools/qemu-storage-daemon.rst
> index f63627eaf6..45854c131e 100644
> --- a/docs/tools/qemu-storage-daemon.rst
> +++ b/docs/tools/qemu-storage-daemon.rst
> @@ -101,10 +101,12 @@ Standard options:
>  
>  .. option:: --nbd-server 
> addr.type=inet,addr.host=<host>,addr.port=<port>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
>    --nbd-server 
> addr.type=unix,addr.path=<path>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
> +  --nbd-server 
> addr.type=fd,addr.str=<fd>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
>  
>    is a server for NBD exports. Both TCP and UNIX domain sockets are 
> supported.
> -  TLS encryption can be configured using ``--object`` tls-creds-* and authz-*
> -  secrets (see below).
> +  A listen socket can be provided via file descriptor passing (see Examples
> +  below). TLS encryption can be configured using ``--object`` tls-creds-* and
> +  authz-* secrets (see below).
>  
>    To configure an NBD server on UNIX domain socket path ``/tmp/nbd.sock``::
>  
> @@ -127,6 +129,38 @@ QMP commands::
>        --chardev socket,path=qmp.sock,server,nowait,id=char1 \
>        --monitor chardev=char1
>  
> +Launch the daemon from Python with a QMP monitor socket using file descriptor
> +passing so there is no need to busy wait for the QMP monitor to become
> +available::
> +
> +  #!/usr/bin/env python3
> +  import os
> +  import subprocess
> +  import socket
> +
> +  sock_path = '/tmp/qmp-{}.sock'.format(os.getpid())

Example code inevitably gets cut+paste into real world apps, and this
example is a tmpfile CVE flaw. At least put it in $CWD instead.

> +
> +  with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as listen_sock:
> +      listen_sock.bind(sock_path)
> +      listen_sock.listen()
> +
> +      fd = listen_sock.fileno()
> +
> +      subprocess.Popen(
> +          ['qemu-storage-daemon',
> +           '--chardev', f'socket,fd={fd},server=on,id=char1',
> +           '--monitor', 'chardev=char1'],
> +          pass_fds=[fd],
> +      )
> +
> +  qmp_sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
> +  qmp_sock.connect(sock_path)
> +  ...QMP interaction...
> +
> +The same socket spawning approach also works with the ``--nbd-server
> +addr.type=fd,addr.str=<fd>`` and ``--export
> +type=vhost-user-blk,addr.type=fd,addr.str=<fd>`` options.
> +
>  Export raw image file ``disk.img`` over NBD UNIX domain socket ``nbd.sock``::
>  
>    $ qemu-storage-daemon \
> -- 
> 2.29.2
> 

Regards,
Daniel
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