[libvirt] [PATCH] dnsmasq: Fix parsing of the version number

[Laine Stump]

On 12/06/2012 07:34 AM, Daniel P. Berrange wrote:
> On Thu, Dec 06, 2012 at 07:29:12AM -0500, Laine Stump wrote:
>> On 12/06/2012 06:34 AM, Michal Privoznik wrote:
>>> If the debugging is enabled, the virCommand subsystem catches debug
>>> messages in the command output as well. In that case, we can't assume
>>> the string corresponding to command's stdout will start with specific
>>> prefix. But the prefix can be moved deeper in the string. This bug
>>> shows itself when parsing dnsmasq output:
>>>
>>> 2012-12-06 11:18:11.445+0000: 18491: error :
>>> dnsmasqCapsSetFromBuffer:664 : internal error cannot parse
>>> /usr/sbin/dnsmasq version number in '2012-12-06 11:11:02.232+0000:
>>> 18492: debug : virFileClose:72 : Closed fd 22'
>>>
>>> We can clearly see that the output of dnsmasq --version
>>> doesn't start with expected "Dnsmasq version " string but a libvirt
>>> debug output.
>> This is a bug in virCommand, and should also affect
>> qemuCapsParseHelpStr(). Is there no way to fix it at the source, rather
>> than working around it like this?
> Well the debug logging messages should be going to stderr,
> while the code should be parsing stdout. It sounds like
> something caused the logging messages on stderr to get
> mixed together with stdout. Ensuring that mix does not
> happen is likely the correct fix.

It turns out that fix is correct and works in this case (I had
redirected both because, not having access to every version of dnsmasq,
I couldn't know for certain that all versions sent the output of help
and version to stdout). But the fact remains that there is libvirtd
stderr leaking into the stderr output of the application being called,
and that's wrong. This behavior will cause problems if, for example,
libvirtd execs some program that does have useful output on stderr -
libvirtd's debug output will be mixed in. (Note that "qemu -device
virtio-net-pci,?", for example, sends its useful output to stderr.)

Aside from that, didn't we previously determine that any kind of logging
in the child process prior to exec'ing the other binary was dangerous?