Re: acpi_pcihp_eject_slot() bug if passed 'slots == 0'

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 26 Mar 2020 09:31:09 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Thu, Mar 26, 2020 at 09:28:27AM -0400, Michael S. Tsirkin wrote:
> > On Thu, Mar 26, 2020 at 02:23:17PM +0100, Igor Mammedov wrote:  
> > > On Thu, 26 Mar 2020 11:52:36 +0000
> > > Peter Maydell <peter.maydell@linaro.org> wrote:
> > >   
> > > > Hi; Coverity spots that if hw/acpi/pcihp.c:acpi_pcihp_eject_slot()
> > > > is passed a zero 'slots' argument then ctz32(slots) will return 32,
> > > > and then the code that does '1U << slot' is C undefined behaviour
> > > > because it's an oversized shift. (This is CID 1421896.)
> > > > 
> > > > Since the pci_write() function in this file can call
> > > > acpi_pcihp_eject_slot() with an arbitrary value from the guest,
> > > > I think we need to handle 'slots == 0' safely. But what should
> > > > the behaviour be?  
> > > 
> > > it also uncovers a bug, where we are not able to eject slot 0 on bridge,  
> > 
> > 
> > And that is by design. A standard PCI SHPC register can support up to 31
> > hotpluggable slots. So we chose slot 0 as non hotpluggable.
> > It's consistent across SHPC, PCI-E, so I made ACPI match.  
> 
> Sorry I was confused. It's a PCI thing, PCI-E does not have
> slot numbers for downstream ports at all.

Scratch that, it was mistake on my part where I tests with a change
that masks 0 and wrongly at that.

slot 0 on bridge can be removed just fine 

> 
> > You can't hot-add it either.
> >   
> > > can be reproduced with:
> > > 
> > >  -enable-kvm -m 4G -device pci-bridge,chassis_nr=1 -global PIIX4_PM.acpi-pci-hotplug-with-bridge-support=on -device virtio-net-pci,bus=pci.1,addr=0,id=netdev12
> > > 
> > > (monitor) device_del netdev12
> > > (monitor) qtree # still shows the device
> > > 
> > > reason is that acpi_pcihp_eject_slot()
> > >    if (PCI_SLOT(dev->devfn) == slot) { # doesn't  match (0 != 32)
> > > 
> > > so device is not deleted  
> > 
> > We should probably teach QEMU that some slots aren't hotpluggable
> > even if device in it is hotpluggable in theory. But that is
> > a separate issue.
> >   
> > > > thanks
> > > > -- PMM
> > > >   
>