Re: [PATCH-for-6.0] net: tap: fix crash on hotplug

[Cole Robinson <crobinso@redhat.com>]

On 4/21/21 3:54 PM, Peter Maydell wrote:
> On Wed, 21 Apr 2021 at 16:24, Cole Robinson <crobinso@redhat.com> wrote:
>>
>> Attempting to hotplug a tap nic with libvirt will crash qemu:
>>
>> $ sudo virsh attach-interface f32 network default
>> error: Failed to attach interface
>> error: Unable to read from monitor: Connection reset by peer
>>
>> 0x000055875b7f3a99 in tap_send (opaque=0x55875e39eae0) at ../net/tap.c:206
>> 206             if (!s->nc.peer->do_not_pad) {
>> gdb$ bt
>>
>> s->nc.peer may not be set at this point. This seems to be an
>> expected case, as qemu_send_packet_* explicitly checks for NULL
>> s->nc.peer later.
>>
>> Fix it by checking for s->nc.peer here too. Padding is applied if
>> s->nc.peer is not set.
>>
>> https://bugzilla.redhat.com/show_bug.cgi?id=1949786
>> Fixes: 969e50b61a2
> 
> Is this a regression since 5.2 ? (I guess so given the Fixes tag.)
> 

Yes

> Also, I'm kind of irritated that this was reported to RH on the
> 15th and we only get a patch now after rc4.

Sorry about that, I was slow attempting the reproducer, only gave it a
spin today. I saw Jason had some reverts in rc3 so I guessed that would
fix things, I was surprised to see it still reproduced with rc4.

 I really really don't
> want to have to roll an rc5, so this now has a much higher
> hill to climb to get into 6.0 than if it had been reported
> (eg on the "Planning" wiki page) as a for-6.0 issue before rc4
> was tagged..
I'm not too in tune to rules of the rc releases TBH, I used the subject
prefix just to ensure this got attention. For Fedora's needs it's not a
big deal if this isn't in 6.0.0 GA. But AFAICT most nic hotplug via
libvirt will crash qemu 100% of the time so I imagine every distro will
want to immediately backport this patch.

Thanks,
Cole