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

[Daniel P . Berrangé]

On Wed, Jan 11, 2023 at 10:06:49AM -0500, John Snow wrote:
> On Wed, Jan 11, 2023 at 4:07 AM Daniel P. Berrangé <berrange@redhat.com> 
> wrote:
> >
> > 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.
> 
> Right. The only small thing is if a caller passes the same directory
> to multiple instances across multiple processes, you could
> *theoretically* get a collision, and we don't guard against it. It's
> not a super likely occurrence so I'm fine with ignoring it, but I
> think it's technically possible.

If they want to be insane, then they can also pass a 'name' explicitly
to override the default socket path choice.


With regards,
Daniel
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