[Qemu-devel] Monitor file path length limitation

[Qemu-devel] Monitor file path length limitation

[Simon]

Hello,

It seems that the monitor file path length is limited to about 100 
characters (107 to be precise).

Currently I'm using the '-monitor' parameter to create the monitor file 
in a directory which gather all the files related to a given VM (monitor 
file, PID file, disk image, etc.):

-monitor 
'unix:/home/user/vmtools/test/out/foobar.iso/foobar.iso_A/foobar.iso_B/foobar.iso_C/foobar.iso_D/foobar.iso_E/monitor.sock,server,nowait'

With this command Qemu does not produce any warning or error message but 
create the socket file "mon" instead of the expected "monitor.sock".

Things go even worse if I add a further nesting:

-monitor 
'unix:/home/user/vmtools/test/out/foobar.iso/foobar.iso_A/foobar.iso_B/foobar.iso_C/foobar.iso_D/foobar.iso_E/foobar.iso_F/monitor.sock,server,nowait'

Now, still without any warning, Qemu creates a file named "foo" in the 
parent directory ('foobar.iso_E/') instead of the expected directory 
('foobar.iso_E/').

Is there a clean way to work around this limitation? For instance, is 
there an alternative option I can use to generate this monitor file? Or 
should I just assume that absolute paths are not supported in the 
'-monitor' option and change the working directory before launching Qemu 
so I can pass "./monitor.sock" as monitor file path (I really prefer to 
use absolute path whenever possible though).

As a side-note, is there any technical reason for such a limitation?

Regards,
Simon.

Re: [Qemu-devel] Monitor file path length limitation

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 1235 bytes --]

On 05/24/2017 10:16 AM, Simon wrote:
> Hello,
> 
> It seems that the monitor file path length is limited to about 100
> characters (107 to be precise).

Welcome to the joy of Unix socket (AF_UNIX) files.  The kernel imposes a
hard length limit on sockaddr_un.sun_path[] (see 'man 7 unix') - and it
is indeed 107 characters plus a NUL terminator.

> -monitor
> 'unix:/home/user/vmtools/test/out/foobar.iso/foobar.iso_A/foobar.iso_B/foobar.iso_C/foobar.iso_D/foobar.iso_E/monitor.sock,server,nowait'
> 
> 
> With this command Qemu does not produce any warning or error message but
> create the socket file "mon" instead of the expected "monitor.sock".

Ideally, qemu should be telling you your path is too long to be created
as a unix socket, and failing up front, rather than silently truncating
and perhaps doing the wrong thing.

> Is there a clean way to work around this limitation?

Don't use longer path names than the kernel supports.

> 
> As a side-note, is there any technical reason for such a limitation?

Yes, but you'll have to ask kernel folks for the reason.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 604 bytes --]

Re: [Qemu-devel] Monitor file path length limitation

[Simon]

Thank you for your answer Eric, I've learned a new thing today :) !

Re: [Qemu-devel] Monitor file path length limitation

[Daniel P. Berrange]

On Wed, May 24, 2017 at 10:26:58AM -0500, Eric Blake wrote:
> On 05/24/2017 10:16 AM, Simon wrote:
> > Hello,
> > 
> > It seems that the monitor file path length is limited to about 100
> > characters (107 to be precise).
> 
> Welcome to the joy of Unix socket (AF_UNIX) files.  The kernel imposes a
> hard length limit on sockaddr_un.sun_path[] (see 'man 7 unix') - and it
> is indeed 107 characters plus a NUL terminator.
> 
> > -monitor
> > 'unix:/home/user/vmtools/test/out/foobar.iso/foobar.iso_A/foobar.iso_B/foobar.iso_C/foobar.iso_D/foobar.iso_E/monitor.sock,server,nowait'
> > 
> > 
> > With this command Qemu does not produce any warning or error message but
> > create the socket file "mon" instead of the expected "monitor.sock".
> 
> Ideally, qemu should be telling you your path is too long to be created
> as a unix socket, and failing up front, rather than silently truncating
> and perhaps doing the wrong thing.

I just copied you on a patch todo that :-)



Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|