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

[Peter Delevoryas]

On Tue, Jan 10, 2023 at 06:18:29PM -0500, John Snow wrote:
> 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?

Yeah, I think that's it

> 
> --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
> >
> >
>