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

[Eric Blake <eblake@redhat.com>]

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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


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