Re: [Qemu-devel] [Qemu-ppc] [RFC PATCH] tests: Add a device_add/del HMP test

[Eduardo Habkost]

On Wed, Sep 06, 2017 at 08:59:32AM +0200, Markus Armbruster wrote:
> Thomas Huth <address@hidden> writes:
> 
> > On 05.09.2017 18:48, Dr. David Alan Gilbert wrote:
> >> * Markus Armbruster (address@hidden) wrote:
> >>> Thomas Huth <address@hidden> writes:
> >>>
> >>>> People tend to forget to mark internal devices with "user_creatable = 
> >>>> false
> >>>> or hotpluggable = false, and these devices can crash QEMU if added via 
> >>>> the
> >>>> HMP monitor. So let's add a test to run through all devices and that 
> >>>> tries
> >>>> to add them blindly (without arguments) to see whether this could crash 
> >>>> the
> >>>> QEMU instance.
> >>>>
> >>>> Signed-off-by: Thomas Huth <address@hidden>
> >>>> ---
> >>>>  I've marked the patch as RFC since not all of the required device bug
> >>>>  fixes have been merged yet (so this patch can not be included yet 
> >>>> without
> >>>>  breaking "make check"). It's also sad that "macio-oldworld" currently
> >>>>  has to be blacklisted - I tried to find a fix for that device,  but I 
> >>>> was
> >>>>  not able to spot the exact problem so far. So help for fixing that 
> >>>> device
> >>>>  is very very welcome! The crash can be reproduced like this:
> >>>>
> >>>>  $ qemu-system-ppc64 -nographic -S -monitor stdio -serial none
> >>>>  QEMU 2.10.50 monitor - type 'help' for more information
> >>>>  (qemu) device_add macio-oldworld,id=x
> >>>>  (qemu) device_del x
> >>>>  (qemu) **
> >>>>  ERROR:qom/object.c:1611:object_get_canonical_path_component:
> >>>>   assertion failed: (obj->parent != NULL)
> >>>>  Aborted (core dumped)
> >>>>
> >>>>  tests/test-hmp.c | 60 
> >>>> +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> >>>>  1 file changed, 59 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/tests/test-hmp.c b/tests/test-hmp.c
> >>>> index 7a38cdc..e8a25c4 100644
> >>>> --- a/tests/test-hmp.c
> >>>> +++ b/tests/test-hmp.c
> >>>> @@ -28,7 +28,6 @@ static const char *hmp_cmds[] = {
> >>>>      "commit all",
> >>>>      "cpu-add 1",
> >>>>      "cpu 0",
> >>>> -    "device_add ?",
> >>>>      "device_add usb-mouse,id=mouse1",
> >>>>      "mouse_button 7",
> >>>>      "mouse_move 10 10",
> >>>> @@ -116,6 +115,64 @@ static void test_info_commands(void)
> >>>>      g_free(info_buf);
> >>>>  }
> >>>>  
> >>>> +/*
> >>>> + * People tend to forget to mark internal devices with "user_creatable 
> >>>> = false
> >>>> + * and these devices can crash QEMU if added via the HMP monitor. So 
> >>>> let's run
> >>>> + * through all devices and try to add them blindly (without arguments) 
> >>>> to see
> >>>> + * whether this could crash the QEMU instance.
> >>>> + */
> >>>> +static void test_device_add_commands(void)
> >>>> +{
> >>>> +    char *resp, *devices, *devices_buf, *endp;
> >>>> +
> >>>> +    devices = devices_buf = hmp("device_add help");
> >>>> +
> >>>> +    while (*devices) {
> >>>> +        /* Ignore header lines etc. */
> >>>> +        if (strncmp(devices, "name \"", 6)) {
> >>>> +            devices = strchr(devices, '\n');
> >>>> +            if (!devices) {
> >>>> +                break;
> >>>> +            }
> >>>> +            devices += 1;
> >>>> +            continue;
> >>>> +        }
> >>>> +        /* Extract the device name, ignore parameters and description */
> >>>> +        devices += 6;
> >>>> +        endp = strchr(devices, '"');
> >>>> +        g_assert(endp != NULL);
> >>>> +        *endp = '\0';
> >>>> +        /* Check whether it is blacklisted... */
> >>>> +        if (g_str_equal("macio-oldworld", devices)) {
> >>>> +            devices = strchr(endp + 1, '\n');
> >>>> +            if (!devices) {
> >>>> +                break;
> >>>> +            }
> >>>> +            devices += 1;
> >>>> +            continue;
> >>>> +        }
> >>>> +        /* Now run the device_add + device_del commands */
> >>>> +        if (verbose) {
> >>>> +            fprintf(stderr, "\tdevice_add %s,id=%s\n", devices, 
> >>>> devices);
> >>>> +        }
> >>>> +        resp = hmp("device_add %s,id=%s", devices, devices);
> >>>> +        g_free(resp);
> >>>> +        if (verbose) {
> >>>> +            fprintf(stderr, "\tdevice_del %s\n", devices);
> >>>> +        }
> >>>> +        resp = hmp("device_del %s", devices);
> >>>> +        g_free(resp);
> >>>> +        /* And move forward to the next line */
> >>>> +        devices = strchr(endp + 1, '\n');
> >>>> +        if (!devices) {
> >>>> +            break;
> >>>> +        }
> >>>> +        devices += 1;
> >>>> +    }
> >>>> +
> >>>> +    g_free(devices_buf);
> >>>> +}
> >>>> +
> >>>>  static void test_machine(gconstpointer data)
> >>>>  {
> >>>>      const char *machine = data;
> >>>> @@ -125,6 +182,7 @@ static void test_machine(gconstpointer data)
> >>>>      qtest_start(args);
> >>>>  
> >>>>      test_info_commands();
> >>>> +    test_device_add_commands();
> >>>>      test_commands();
> >>>>  
> >>>>      qtest_end();
> >>>
> >>> This finds devices by parsing output of HMP help.  I think you should
> >>> use introspection instead, like device-introspect-test does.  You may
> >>> want to extract common utility code from there.
> >
> > Well, I wrote a HMP test, to simulate what users can do at the HMP
> > prompt. But ok, it's likely to crash QEMU also when using the QMP
> > interface instead ... but then the code should also go into a different
> > .c file, I think.
> 
> HMP device_add is a trivial wrapper around QMP, as is proper:
> 
>     void hmp_device_add(Monitor *mon, const QDict *qdict)
>     {
>         Error *err = NULL;
> 
>         qmp_device_add((QDict *)qdict, NULL, &err);
>         hmp_handle_error(mon, &err);
>     }
> 
> If anything ever breaks in this wrapper, it won't be specific to HMP
> device_add.
> 
> >>> The actual device_add and device_del also use HMP.  Failures are
> >>> ignored.  A few device_add failures I'd expect:
> >>>
> >>> * There is no suitable bus.
> >>>
> >>> * There are suitable buses, but the default one is full.
> >
> > You can partly work around the above two problems by looping a couple of
> > times through the "device_add"s first, before doing the "device_del"s.
> > Then the first iteration adds additional buses which get populated in
> > the second iteration. I can get additional QEMU crashes if I modify my
> > test that way... but currently I lack time for debugging all these
> > crashes, so I don't want to include that in this patch here yet.
> 
> Kind of a random walk.
> 
> Eduardo has been working on "socket introspection", i.e. means to find
> available "sockets" for devices to plug into.  Could be used to for a
> more directed walk.  Eduardo, any thoughts?

My query-device-slots work in progress included a Python script
that tried to validate some of the query-device-slots data.  It
could include one additional check where it tries to plug devices
to those slots and see if something breaks.

But considering the way the device hotplug code works today, test
code that tries random device-types/arguments that are _not_
reported by query-device-slots would be useful too.

> 
> >>> * Mandatory parameters are missing, such as device backend.
> >
> > That's quite hard to handle even with QMP, isn't it? How should a
> > generic test know which parameter have to be populated with which value?
> 
> It could perhaps populate sufficiently generic parameters such as common
> backends.
> 
> Sadly, device-list-properties is weak, and unless we improve or replace
> it, this would involve somewhat ugly hardcoding of descriptions.  For
> instance, we'd have to recognize from
> 
>     {"name": "netdev", "description": "ID of a netdev to use as a backend", 
> "type": "str"}
> 
> that property netdev is a network backend, and very likely mandatory.
> 
> >>> * The bus doesn't support hot plug (some other bus might).
> >
> > That should not be a problem - at least QEMU should not crash in this
> > situation.
> 
> Yes, and testing that has some value.  It doesn't test the device,
> though, only the qdev core.
> 
> I don't mean to suggest your test is useless.  I'm merely pointing at
> some gaping coverage holes :)
> 
> >>> * The device supports only cold plug with -device, not hot plug with
> >>>   device_add.
> >
> > We've got Eduardo's scripts/device-crash-test script for that already,
> > so no need to cover that here.
> 
> Point taken.  So this test is really about hot plug / unplug.  Suggest
> to clarify the commit message: s/add them blindly/hotplug and unplug
> them blindly/.

We could extend device-crash-test to test device_add too, as it
already has extra code to deal with known crashes and testing
multiple machine-types.  Also, any additional code we write to
ensure we add mandatory arguments or plug only to valid buses
would apply to both -device and device_add.  I also think Python
test code is easier to maintain and extend, but that's just my
personal preference.

> 
> >>> I'm afraid the test only tests one of these common failure modes for
> >>> many devices.
> >>>
> >>> device_del failures I'd expect:
> >>>
> >>> * The device doesn't exist, because it hasn't completed hot plug, yet.
> >>>   In some cases such as ACPI PCI hot plug, hot plug may require guest
> >>>   cooperation, which may take unbounded time.  device_add merely kicks
> >>>   off the hot plug then.  I can't remember how to poll for completion.
> >>>   I also can't remember why we don't send a QMP event.
> >>>
> >>>   The hot plug usually completes quickly, but it may take its own sweet
> >>>   time, or not complete at all, say because the guest doesn't support
> >>>   ACPI, or just doesn't feel like plugging right now.
> >>>
> >>>   The test needs to set up a guest that cooperates.  I guess that
> >>>   involves libqos; yet another thing I've forgotten.
> >
> > That was certainly not my scope when I wrote this test. I just want to
> > get rid of these devices that can easily crash QEMU if you just try to
> > add or remove them at the monitor prompt. A more detailed hotplug test
> > should IMHO be done by the individual device tests instead, like it is
> > done in many tests/virtio*.c and tests/usb*.c files already.
> 
> I'm not trying to talk you into widening the scope of your test.  I am
> pointing out that your testing of unplug is racy, and may therefore miss
> failures randomly: if hotplug is slow, device_del won't actually do
> anything interesting.
> 
> >>> * Same for device_del.  You should wait for the DEVICE_DELETED event
> >>>   with a suitable timeout.
> >> 
> >> Yes, I think that's an interesting problem - although the test
> >> might still be worth it just to see how many issues it finds;   I'm
> >> curious how many devices actually work with no parameters though,
> >> most seem to fail.
> >
> > My test already helped to find lots of problems:
> >
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=f58f25599b72c7479e6a1
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=469f3da42ef4af347fa78
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=574ee06de9c4fe63c90be
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=84ebd3e8c7d4fe955b359
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=0d4fa4996fc5ee56ea7d0
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=1f98e55385d11da1dc0de
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=8ccccff9dd7ba24c7a788
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=0479097859372a760843a
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=a808c0865b720e22ca292
> > https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg04618.html
> > https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg04164.html
> > https://lists.gnu.org/archive/html/qemu-arm/2017-08/msg00306.html
> > https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg04622.html
> > https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg04635.html
> > https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg05390.html
> >
> > ... so does it now sound at least a little bit usable?
> 
> I don't doubt it is, but its limitations need to be understood and
> either relaxed or documented.
> 
> >> If I'm reading the code right it's creating the device with the same
> >> name as the device;  I wonder if that always works?
> >
> > Why not? The id is just an arbitrary string, isn't it?
> 
> Since you're using HMP, you get to quote ',', which occurs in some
> device names[*].  Enjoy!  ;-P
> 
> Picking IDs that aren't anti-social may be easier.
> 
> >> But still,  it should mean that if the previous hotplug hasn't really
> >> happened then you can move onto the next add.
> >> 
> >>> We could improve device_add to cold plug before the machine starts,
> >>> i.e. between start with -S and the first cont.  We could similarly
> >>> improve device_del to cold plug.  Together, that would let you sidestep
> >>> the hot plug complications.
> >>>
> >>> I guess this test is a case of "if it was easy, we would've done it
> >>> already"...
> >> 
> >> Still maybe it's worth it as a start.
> >
> > Agreed that we should finally move to a smarter, QMP-based test. But
> > until someone wrote that, maybe we could include this as a temporary
> > guard to avoid that problems like the ones above creep in again?
> 
> I think its limitations need to be understood to a useful degree, and
> either relaxed or documented.
> 
> 
> [*] Blame IEEE 1275 device trees.

-- 
Eduardo