Re: [PATCH v4 1/1] python/machine: Fix AF_UNIX path too long on macOS

[John Snow]

On Tue, Jan 10, 2023 at 3:34 AM Peter Delevoryas <peter@pjd.dev> wrote:
>
> On macOS, private $TMPDIR's are the default. These $TMPDIR's are
> generated from a user's unix UID and UUID [1], which can create a
> relatively long path:
>
>     /var/folders/d7/rz20f6hd709c1ty8f6_6y_z40000gn/T/
>
> QEMU's avocado tests create a temporary directory prefixed by
> "avo_qemu_sock_", and create QMP sockets within _that_ as well.
> The QMP socket is unnecessarily long, because a temporary directory
> is created for every QEMUMachine object.
>
>     /avo_qemu_sock_uh3w_dgc/qemu-37331-10bacf110-monitor.sock
>
> The path limit for unix sockets on macOS is 104: [2]
>
>     /*
>      * [XSI] Definitions for UNIX IPC domain.
>      */
>     struct  sockaddr_un {
>         unsigned char   sun_len;        /* sockaddr len including null */
>         sa_family_t     sun_family;     /* [XSI] AF_UNIX */
>         char            sun_path[104];  /* [XSI] path name (gag) */
>     };
>
> This results in avocado tests failing on macOS because the QMP unix
> socket can't be created, because the path is too long:
>
>     ERROR| Failed to establish connection: OSError: AF_UNIX path too long
>
> This change resolves by reducing the size of the socket directory prefix
> and the suffix on the QMP and console socket names.
>
> The result is paths like this:
>
>     pdel@pdel-mbp:/var/folders/d7/rz20f6hd709c1ty8f6_6y_z40000gn/T
>     $ tree qemu*
>     qemu_df4evjeq
>     qemu_jbxel3gy
>     qemu_ml9s_gg7
>     qemu_oc7h7f3u
>     qemu_oqb1yf97
>     ├── 10a004050.con
>     └── 10a004050.qmp
>
> [1] 
> https://apple.stackexchange.com/questions/353832/why-is-mac-osx-temp-directory-in-weird-path
> [2] 
> /Library/Developer/CommandLineTools/SDKs/MacOSX12.3.sdk/usr/include/sys/un.h
>
> Signed-off-by: Peter Delevoryas <peter@pjd.dev>

I'm tentatively staging this with a benefit-of-the-doubt [1] -- my
tests are still running -- but I do have a question:

> ---
>  python/qemu/machine/machine.py         | 6 +++---
>  tests/avocado/avocado_qemu/__init__.py | 2 +-
>  2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/python/qemu/machine/machine.py b/python/qemu/machine/machine.py
> index 748a0d807c9d..d70977378305 100644
> --- a/python/qemu/machine/machine.py
> +++ b/python/qemu/machine/machine.py
> @@ -157,7 +157,7 @@ def __init__(self,
>          self._wrapper = wrapper
>          self._qmp_timer = qmp_timer
>
> -        self._name = name or f"qemu-{os.getpid()}-{id(self):02x}"
> +        self._name = name or f"{id(self):x}"

Why is it safe to not differentiate based on the process ID?

... I suppose the thinking is: by default, in machine.py, this is a
temp dir created by tempfile.mkdtemp which will be unique per-process.
I suppose there's no protection against a caller supplying the same
tempdir (or sockdir) to multiple instances, but I suppose in those
cases we get to argue that "Well, don't do that, then."

Does that sound about right?

--js

[1] staged @ https://gitlab.com/jsnow/qemu/-/commits/python


>          self._temp_dir: Optional[str] = None
>          self._base_temp_dir = base_temp_dir
>          self._sock_dir = sock_dir
> @@ -167,7 +167,7 @@ def __init__(self,
>              self._monitor_address = monitor_address
>          else:
>              self._monitor_address = os.path.join(
> -                self.sock_dir, f"{self._name}-monitor.sock"
> +                self.sock_dir, f"{self._name}.qmp"
>              )
>
>          self._console_log_path = console_log
> @@ -192,7 +192,7 @@ def __init__(self,
>          self._console_set = False
>          self._console_device_type: Optional[str] = None
>          self._console_address = os.path.join(
> -            self.sock_dir, f"{self._name}-console.sock"
> +            self.sock_dir, f"{self._name}.con"
>          )
>          self._console_socket: Optional[socket.socket] = None
>          self._remove_files: List[str] = []
> diff --git a/tests/avocado/avocado_qemu/__init__.py 
> b/tests/avocado/avocado_qemu/__init__.py
> index 910f3ba1eab8..25a546842fab 100644
> --- a/tests/avocado/avocado_qemu/__init__.py
> +++ b/tests/avocado/avocado_qemu/__init__.py
> @@ -306,7 +306,7 @@ def require_netdev(self, netdevname):
>              self.cancel('no support for user networking')
>
>      def _new_vm(self, name, *args):
> -        self._sd = tempfile.TemporaryDirectory(prefix="avo_qemu_sock_")
> +        self._sd = tempfile.TemporaryDirectory(prefix="qemu_")
>          vm = QEMUMachine(self.qemu_bin, base_temp_dir=self.workdir,
>                           sock_dir=self._sd.name, log_dir=self.logdir)
>          self.log.debug('QEMUMachine "%s" created', name)
> --
> 2.39.0
>
>