libvirt users - Mar 2019 - Re: Obtaining the PID of a domain's QEMU process from C

Hello all,

I'm currently writing a C program that uses the libvirt API and I need a 
way to obtain the pid of a given domain's QEMU process.

Specifically, I'm writing an ivshmem server that uses SO_PEERCRED to get 
the pid of clients that connect to it, and I would like to use that pid 
to look up the domain in libvirt to determine the proper domain ID to 
return to the client.

As far as I can tell, libvirt doesn't expose this information in an easy 
to access manner. Of course it is possible to call `ps` and grep for the 
information I'm looking for, but I was hoping for a cleaner solution.

If anybody knows how to do this, advice would be greatly appreciated.

Thanks in advance,
Shawn

On 2/28/19 8:31 PM, Shawn Anastasio wrote:
> Hello all,
> 
> I'm currently writing a C program that uses the libvirt API and I need
a
> way to obtain the pid of a given domain's QEMU process.
> 
> Specifically, I'm writing an ivshmem server that uses SO_PEERCRED to
get
> the pid of clients that connect to it, and I would like to use that pid
> to look up the domain in libvirt to determine the proper domain ID to
> return to the client.
> 
> As far as I can tell, libvirt doesn't expose this information in an
easy
> to access manner. Of course it is possible to call `ps` and grep for the
> information I'm looking for, but I was hoping for a cleaner solution.
> 
> If anybody knows how to do this, advice would be greatly appreciated.
> 
> Thanks in advance,
There isn't any libvirt API for getting the qemu pid, so I think going
outside libvirt is your only option

- Cole

On 3/1/19 2:31 AM, Shawn Anastasio wrote:
> Hello all,
> 
> I'm currently writing a C program that uses the libvirt API and I need
a
> way to obtain the pid of a given domain's QEMU process.
> 
> Specifically, I'm writing an ivshmem server that uses SO_PEERCRED to
get
> the pid of clients that connect to it, and I would like to use that pid
> to look up the domain in libvirt to determine the proper domain ID to
> return to the client.
> 
> As far as I can tell, libvirt doesn't expose this information in an
easy
> to access manner. Of course it is possible to call `ps` and grep for the
> information I'm looking for, but I was hoping for a cleaner solution.
> 
> If anybody knows how to do this, advice would be greatly appreciated.
There isn't an API for that as we don't want users to fiddle with qemu
behid its back. Anyway, apart from ps there might be a solution. Libvirt
of course know the PID and stores it - in the status XML. This is a file
that libvirt uses to store runtime information for the domain in case
the daemon restarts. It's located under
/var/run/libvirt/qemu/$domain.xml. But you'll need root priviledges to
read it.

Also, it is very wise to follow the comment at the beginning of the file
and not modify it ;-) And just like with any libvirt internals, we make
no guarantees they won't change in the long run.

Michal

Thanks for the response. /var/run/libvirt/qemu/ seems to be exactly what 
I'm looking for. It would, however, be nice to be able to access this 
file without root privileges. Is there any way to access the file using 
the credentials from a connected virConnectPtr?

If not, I can stick to my current workaround of walking /proc and 
scanning each PID's cmdline for the VM's UUID string.

Thanks,
Shawn

On 3/9/19 9:32 AM, Michal Prívozník wrote:
> On 3/1/19 2:31 AM, Shawn Anastasio wrote:
>> Hello all,
>>
>> I'm currently writing a C program that uses the libvirt API and I
need a
>> way to obtain the pid of a given domain's QEMU process.
>>
>> Specifically, I'm writing an ivshmem server that uses SO_PEERCRED
to get
>> the pid of clients that connect to it, and I would like to use that pid
>> to look up the domain in libvirt to determine the proper domain ID to
>> return to the client.
>>
>> As far as I can tell, libvirt doesn't expose this information in an
easy
>> to access manner. Of course it is possible to call `ps` and grep for
the
>> information I'm looking for, but I was hoping for a cleaner
solution.
>>
>> If anybody knows how to do this, advice would be greatly appreciated.
> 
> There isn't an API for that as we don't want users to fiddle with
qemu
> behid its back. Anyway, apart from ps there might be a solution. Libvirt
> of course know the PID and stores it - in the status XML. This is a file
> that libvirt uses to store runtime information for the domain in case
> the daemon restarts. It's located under
> /var/run/libvirt/qemu/$domain.xml. But you'll need root priviledges to
> read it.
> 
> Also, it is very wise to follow the comment at the beginning of the file
> and not modify it ;-) And just like with any libvirt internals, we make
> no guarantees they won't change in the long run.
> 
> Michal
>

On Sat, Mar 09, 2019 at 04:32:00PM +0100, Michal Prívozník
wrote:
> On 3/1/19 2:31 AM, Shawn Anastasio wrote:
> > Hello all,
> >
> > I'm currently writing a C program that uses the libvirt API and I
need a
> > way to obtain the pid of a given domain's QEMU process.
> >
> > Specifically, I'm writing an ivshmem server that uses SO_PEERCRED
to get
> > the pid of clients that connect to it, and I would like to use that
pid
> > to look up the domain in libvirt to determine the proper domain ID to
> > return to the client.
> >
> > As far as I can tell, libvirt doesn't expose this information in
an easy
> > to access manner. Of course it is possible to call `ps` and grep for
the
> > information I'm looking for, but I was hoping for a cleaner
solution.
> >
> > If anybody knows how to do this, advice would be greatly appreciated.
>
> There isn't an API for that as we don't want users to fiddle with
qemu
That's right, and it should stay that way. On the other hand, the same way
we
can't prevent anyone from editing /etc/libvirt/qemu/<domain>.xml or
/var/run/libvirt/qemu/<domain>.xml or run the 'ps' command or
whatever we might
as well report the PID as part of virConnectListAllDomains data. I don't
have
a problem with reporting PIDs in principle, provided it's used for
informatory
purposes.
Having said that, there's the question of why libvirt should report
something
that it doesn't need to consume, IOW we report machine ID which we can use
to
control the machine, we also report UUID which we can consume, but we'd do
absolutely nothing with the PID besides reporting it.
Another thing is that reporting PIDs of machines running on a remote host is
quite useless for locally running clients.

Erik
> behid its back. Anyway, apart from ps there might be a solution. Libvirt
> of course know the PID and stores it - in the status XML. This is a file
> that libvirt uses to store runtime information for the domain in case
> the daemon restarts. It's located under
> /var/run/libvirt/qemu/$domain.xml. But you'll need root priviledges to
> read it.
>
> Also, it is very wise to follow the comment at the beginning of the file
> and not modify it ;-) And just like with any libvirt internals, we make
> no guarantees they won't change in the long run.
>
> Michal
>
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