[libvirt PATCH] qemu: fix potential resource leak

[Peter Krempa]

On Thu, Oct 22, 2020 at 23:20:20 -0400, Laine Stump wrote:
> On 10/22/20 3:01 AM, Peter Krempa wrote:
> > On Wed, Oct 21, 2020 at 22:31:09 -0400, Laine Stump wrote:
> > > On 10/21/20 5:50 PM, Jonathon Jongsma wrote:
> > > > Coverity reported a potential resource leak. While it's probably not
> > > > a real-world scenario, the code could technically jump to cleanup
> > > > between the time that vdpafd is opened and when it is used. Ensure that
> > > > it gets cleaned up in that case.
> > > > 
> > > > Signed-off-by: Jonathon Jongsma <jjongsma at redhat.com>
> > > > ---
> > > >    src/qemu/qemu_command.c | 3 +++
> > > >    1 file changed, 3 insertions(+)
> > > > 
> > > > diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> > > > index 5c4e37bd9e..cbe7a6e331 100644
> > > > --- a/src/qemu/qemu_command.c
> > > > +++ b/src/qemu/qemu_command.c
> > > > @@ -8135,6 +8135,7 @@ qemuBuildInterfaceCommandLine(virQEMUDriverPtr driver,
> > > >            addfdarg = g_strdup_printf("%s,opaque=%s", fdset,
> > > >                                       net->data.vdpa.devicepath);
> > > >            virCommandAddArgList(cmd, "-add-fd", addfdarg, NULL);
> > > > +        vdpafd = -1;
> > > >        }
> > > >        if (chardev)
> > > > @@ -8204,6 +8205,8 @@ qemuBuildInterfaceCommandLine(virQEMUDriverPtr driver,
> > > >        VIR_FREE(tapfdName);
> > > >        VIR_FREE(vhostfd);
> > > >        VIR_FREE(tapfd);
> > > > +    if (vdpafd >= 0)
> > > > +        VIR_FORCE_CLOSE(vdpafd);
> > > 
> > > VIR_FORCE_CLOSE() ==>virForceCloseHelper() ==> virFileClose()
> > > 
> > > and virFileClose() is a NOP if fd < 0, so this doesn't need the conditional.
> > > 
> > > 
> > > I *was* going to say "With that change,
> > > 
> > > 
> > > Reviewed-by: Laine Stump <laine at redhat.com>
> > > 
> > > "
> > > 
> > > 
> > > but this got me looking at the code of the entire function rather than just
> > > the diffs in the patch, and I've got a question - is there any reason that
> > > you only ope n the vdpa device inside the switch, and save everything else
> > > related to it until later (at the "if (vdpafd)")? You could then device
> > I'd like to point out that opening anything in the command line
> > formatters is IMO bad practice.
> 
> 
> Well... I agree that it is an ugly design, but that's pretty much what's in
> place for almost everything.

Sure, but it shouldn't be used as an argument to not use a better
approach.


> >   The command line formatter should format
> > the commandline and nothing more.
> 
> 
> It would be nice if that was the case, but it already isn't. :-/

No. Please don't put it this way. My message is that while the old code
isn't compliant, it shouldn't be an excuse to make it worse!

In this instace the code is commited. We don't need to change it, but
any further change should be encouraged to use the better approach.

> >   This is visible by the necessity to
> > have a lot of mock override functions which prevent the command line
> > formatter from touching resources on the host in the first place.
> > 
> > Better approach is to open resources in 'qemuProcessPrepareHost' and
> > store them in private data for command line formatting. Fake data for
> > tests are added in 'testCompareXMLToArgvCreateArgs'.
> 
> 
> That's nice in the fact that it eliminates the need for mock overrides
> (would be even *nicer* if that function had even a single line of
> documentation included that described its purpose, and what code in the qemu
> driver it should be mimicking, amirite?).

I agree, documentation is lacking in many parts. This is the
not-so-obvious techincal debt.

> But it's bad because the code in qemuProcessPrepareHost() won't be tested
> (can't be tested if there are no mocks for the system functions it calls).

IMO this statement is misrepresenting what's happening. Sure
qemuProcessPrepareHost can't be tested by unit testing. It's replaced by
another function which fakes inputs. But that is EXACTLY what the mock
functions preloaded into our tests are doing!

With qemuProcessPrepareHost() you at least have one place and one
function where all the code is aggregated rather than spreading it
trhough the command line generator and it being very hard to audit
afterwards.

> Basically you're trading the extra work of mocking system-level functions
> for the extra work of filling in stuff in the privateData (and the
> maintenance of that code), and eliminating testing of the code that's been
> moved (pretty *lame* testing, I'll admit, since it's getting back canned
> results from the fake system calls).

As noted before the extent of testing is exactly identical. The
difference is that all the code which is touching the host is aggregated
in one place and replaced by another function vs scattered accross the
whole file and LD_PRELOAD-ed.


> So it's not really the panacea your advocacy implies :-)
> 
> 
> (Don't get me wrong! I've always disliked the mixing of device/file/whatever
> init with the commandline generating functions.)
> 
> 
> (actually a couple months ago I looked into putting the network interface
> "prepare" stuff into privateData similar to what's done with the slirp stuff
> now. In the end I gave up because it didn't provide the result I wanted - I
> was trying to keep track of what device prep actions had been done for which
> devices during domain startup so that the shutdown in case of startup
> failure would only shutdown those things that had actually been setup; it
> ended up being too complicated to make it work correctly both in the case of
> an aborted startup and a normal shutdown, once you took into account the
> possibility of libvirtd being restarted as part of a libvirt package update.
> 
> 
> I'll point out that during all my searches through the code during the
> experiment referenced in the previous paragraph, I never ran across
> testCompareXMLToArgvCreateArgs(), and didn't know of its existence (or at

Yup, sorry I've extracted it recently. Previously we've just piled up
all the "init" code into testCompareXMLToArgv without thinking twice.

> least didn't remember it, if I had known about it before). Is this
> documented somewhere? Or is it expected to be learned by reading every patch
> coming across the mailing list (I unfortunately fail at that in a major
> way)?

You see, I've failed the same way pointing out that the approach used
was outdated.

Also given that the documentation of the new approach would be applied
via a patch, you could have missed it the same way as the patch
implementing the new approach to initialize host-specific data (or it
would be added at the same time). I doubt that any of us is re-reading
contribution guidelines just for fun.

> > I'm aware though that there's a lot of "prior art" in this area though.
> 
> 
> ... and nothing in the code or the coding practices to warn against it,
> point people in the other direction.

Patches are welcome! :P

> This sounds like another "saga" in the making - split all commandline
> generating functions into separate "prepare device" and "generate
> commandline" parts. I don't know that we should require Jonathon to change
> his code that much just to fix a memory leak though ... (too bad I hadn't
> kept up with the latest cool stuff so I would have pointed it out in review
> of the original patch).

As said, I'm fine with prior art. We should encourage any further
contributions to avoid this pattern though. Obviously if Jonathon wants
to fix it I'm all for it. The fd leak fix is fine though.