[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit ins
From: |
Greg Kurz |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON() |
Date: |
Fri, 9 Sep 2016 09:30:21 +0200 |
On Fri, 09 Sep 2016 08:38:13 +0200
Markus Armbruster <address@hidden> wrote:
> Greg Kurz <address@hidden> writes:
>
> > On Thu, 8 Sep 2016 18:19:27 +0300
> > "Michael S. Tsirkin" <address@hidden> 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" <address@hidden> 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 <address@hidden> wrote:
> >> > > >
> >> > > > > On Wed, 07 Sep 2016 19:19:24 +0200
> >> > > > > Greg Kurz <address@hidden> 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 <address@hidden>
> >> > > > > > ---
> >> > > > > > 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.
Cheers.
--
Greg
[1] sending an empty buffer was sufficient before commit 1e7aed70144b4 as said
in my previous answer
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), (continued)
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Cornelia Huck, 2016/09/08
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Michael S. Tsirkin, 2016/09/08
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Greg Kurz, 2016/09/08
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Michael S. Tsirkin, 2016/09/08
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Cornelia Huck, 2016/09/09
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Greg Kurz, 2016/09/09
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Cornelia Huck, 2016/09/09
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Greg Kurz, 2016/09/09
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Greg Kurz, 2016/09/09
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Markus Armbruster, 2016/09/09
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(),
Greg Kurz <=
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Markus Armbruster, 2016/09/09
- Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON(), Greg Kurz, 2016/09/09
[Qemu-devel] [PATCH 2/2] virtio-pci: error out when both legacy and modern modes are disabled, Greg Kurz, 2016/09/07