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

[Daniel P . Berrangé]

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."

Every process will have a separate tempdir, and if there are
multiple instances of this class, 'id(self)' will provide
uniqueness within the process.


With regards,
Daniel
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