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

[Markus Armbruster]

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.

>                                                          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.

Both are useless when the signal needs to go to the guest.  Signalling
the guest is a device model job.

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.