Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON()

[Greg Kurz]

On Fri, 09 Sep 2016 11:08:56 +0200
Markus Armbruster <address@hidden> wrote:

> Greg Kurz <address@hidden> writes:
> 
> > 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.  
> 
> 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.
> 

Agreed.

> * 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.
> 

Agreed. FWIW a bunch of FIXMEs are missing in the virtio code then :)

> * Nobody expects you to get things perfectly right in one step.  Just
>   try to move towards the goal.
> 

Sure ! I'm now reading through Stefan's series to address the issue:

https://lists.nongnu.org/archive/html/qemu-devel/2016-04/msg01978.html

Cheers.

--
Greg

> >
> > Cheers.
> >
> > --
> > Greg
> >
> > [1] sending an empty buffer was sufficient before commit 1e7aed70144b4 as 
> > said
> >     in my previous answer