Greg Kurz <gr...@kaod.org> writes: > On Fri, 09 Sep 2016 08:38:13 +0200 > Markus Armbruster <arm...@redhat.com> wrote: > >> Greg Kurz <gr...@kaod.org> writes: >> >> > On Thu, 8 Sep 2016 18:19:27 +0300 >> > "Michael S. Tsirkin" <m...@redhat.com> wrote: >> > >> >> On Thu, Sep 08, 2016 at 05:04:47PM +0200, Cornelia Huck wrote: >> >> > On Thu, 8 Sep 2016 18:00:28 +0300 >> >> > "Michael S. Tsirkin" <m...@redhat.com> wrote: >> >> > >> >> > > On Thu, Sep 08, 2016 at 11:12:16AM +0200, Greg Kurz wrote: >> >> > > > On Thu, 8 Sep 2016 10:59:26 +0200 >> >> > > > Cornelia Huck <cornelia.h...@de.ibm.com> wrote: >> >> > > > >> >> > > > > On Wed, 07 Sep 2016 19:19:24 +0200 >> >> > > > > Greg Kurz <gr...@kaod.org> wrote: >> >> > > > > >> >> > > > > > Calling assert() really makes sense when hitting a genuine bug, >> >> > > > > > which calls >> >> > > > > > for a fix in QEMU. However, when something goes wrong because >> >> > > > > > the guest >> >> > > > > > sends a malformed message, it is better to write down a more >> >> > > > > > meaningul >> >> > > > > > error message and exit. >> >> > > > > > >> >> > > > > > Signed-off-by: Greg Kurz <gr...@kaod.org> >> >> > > > > > --- >> >> > > > > > hw/9pfs/virtio-9p-device.c | 20 ++++++++++++++++++-- >> >> > > > > > 1 file changed, 18 insertions(+), 2 deletions(-) >> >> > > > > >> >> > > > > While this is an improvement over the current state, I don't >> >> > > > > think the >> >> > > > > guest should be able to kill qemu just by doing something stupid. >> >> > > > > >> >> > > > >> >> > > > Hi Connie, >> >> > > > >> >> > > > I'm glad you're pointing this out... this was also my impression, >> >> > > > but >> >> > > > since there are a bunch of sanity checks in the virtio code that >> >> > > > cause >> >> > > > QEMU to exit (even recently added like 1e7aed70144b), I did not dare >> >> > > > stand up :) >> >> > > >> >> > > It's true that it's broken in many places but we should just >> >> > > fix them all. >> >> > > >> >> > > >> >> > > A separate question is how to log such hardware/guest bugs generally. >> >> > > People already complained about disk filling up because of us printing >> >> > > errors on each such bug. Maybe print each message only N times, and >> >> > > then set a flag to skip the log until management tells us to restart >> >> > > logging again. >> >> > >> >> > I'd expect to get the message just once per device if we set the device >> >> > to broken (unless the guess continuously resets it again...) >> >> >> >> Which it can do, so we should limit that anyway. >> >> >> >> > Do we have >> >> > a generic print/log ratelimit infrastructure in qemu? >> >> >> >> There are actually two kinds of errors >> >> host side ones and ones triggered by guests. >> >> >> >> We should distinguish between them API-wise, then >> >> we will be able to limit the logging of those >> >> that guest can trigger. >> >> >> > >> > FWIW it makes sense to use error_report() if QEMU exits. >> >> exit(STATUS) with STATUS != 0 without printing a message is always >> wrong. >> > > I fully agree. > >> > If it continues >> > execution, this means we're expecting the guest or the host to do something >> > to fix the error condition. This requires QEMU to emit an event of some >> > sort, but not necessarily to log an error message in a file. I guess this >> > depends if QEMU is run by some tooling, or by a human. >> >> error_report() normally goes to stderr. Tooling or humans can of course >> make it go to a file instead. >> >> error_report() is indeed a sub-par way to send an "attention" signal to >> the host, because recognizing such a signal reliably is unnecessary hard >> for management applications. QMP events are much easier. >> > > My wording was poor but yes, that was my point. :) > >> Both are useless when the signal needs to go to the guest. Signalling >> the guest is a device model job. >> > > I also agree with that. In the case of virtio, this is explained in section > 2.1.2 of the spec. > >> error_report() without exit() has its uses. Error conditions in need of >> fixing aren't the only reason to call error_report(). But when you add >> a call, ask yourself whether management application or guest would like >> to respond to it. > > In the case of the present patch, we currently have BUG_ON() which generates > a cryptic and unusable message. > > It turns out that the first one (elem->out_num == 0 || elem->in_num == 0) is > correct since it is now [1] impossible to hit this according to the code (see > virtqueue_pop() and virtqueue_map_desc()). > > The second one (len != sizeof out) though matches a potential guest originated > error. If I do as suggested by Connie, then the error_report() isn't needed > anymore.
I dive into the details of your analysis right now, only make high-level recommendations: * Issues common to all virtio devices should be addressed in the virtio core. If that's not feasible, they should be addressed in all devices consistently. * Guest misbehavior should put the device in a guest-observable error state. It should not crash QEMU, it should not spam stderr. Code handling it in other ways should be marked FIXME. * Nobody expects you to get things perfectly right in one step. Just try to move towards the goal. > > Cheers. > > -- > Greg > > [1] sending an empty buffer was sufficient before commit 1e7aed70144b4 as said > in my previous answer
