Re: [dpdk-dev] [PATCH 2/2] virtio: fix PCI config err handling

[Tiwei Bie <tiwei.bie@intel.com>]

On Mon, Aug 20, 2018 at 05:45:35PM +0100, Luca Boccassi wrote:
> On Mon, 2018-08-20 at 16:18 +0800, Tiwei Bie wrote:
> > On Thu, Aug 16, 2018 at 07:49:43PM +0100, Luca Boccassi wrote:
> > > On Thu, 2018-08-16 at 19:47 +0100, Luca Boccassi wrote:
> > > > From: Brian Russell <brussell@brocade.com>
> > > > 
> > > > In virtio_read_caps and vtpci_msix_detect, rte_pci_read_config
> > > > returns
> > > > the number of bytes read from PCI config or < 0 on error.
> > > > If less than the expected number of bytes are read then log the
> > > > failure and return rather than carrying on with garbage.
> > > > 
> > > > Fixes: 6ba1f63b5ab0 ("virtio: support specification 1.0")
> > > > 
> > > > Signed-off-by: Brian Russell <brussell@brocade.com>
> > > > Signed-off-by: Luca Boccassi <bluca@debian.org>
> > > > ---
> > > > v2: handle additional rte_pci_read_config incomplete reads
> > > > 
> > > >  drivers/net/virtio/virtio_pci.c | 35 +++++++++++++++++++++----
> > > > ----
> > > > ----
> > > >  1 file changed, 22 insertions(+), 13 deletions(-)
> > > > 
> > > > diff --git a/drivers/net/virtio/virtio_pci.c
> > > > b/drivers/net/virtio/virtio_pci.c
> > > > index 6bd22e54a6..ff6f96f361 100644
> > > > --- a/drivers/net/virtio/virtio_pci.c
> > > > +++ b/drivers/net/virtio/virtio_pci.c
> > > 
> > > ...
> > > > @@ -610,9 +613,13 @@ virtio_read_caps(struct rte_pci_device *dev,
> > > > struct virtio_hw *hw)
> > > >  			hw->common_cfg = get_cfg_addr(dev,
> > > > &cap);
> > > >  			break;
> > > >  		case VIRTIO_PCI_CAP_NOTIFY_CFG:
> > > > -			rte_pci_read_config(dev, &hw-
> > > > > notify_off_multiplier,
> > > > 
> > > > -					4, pos + sizeof(cap));
> > > > -			hw->notify_base = get_cfg_addr(dev,
> > > > &cap);
> > > > +			/* Avoid half-reads into hw */
> > > > +			ret = rte_pci_read_config(dev,
> > > > &multiplier,
> > > > 4,
> > > > +					pos + sizeof(cap));
> > > > +			if (ret == 4) {
> > > > +				hw->notify_off_multiplier =
> > > > multiplier;
> > > > +				hw->notify_base =
> > > > get_cfg_addr(dev,
> > > > &cap);
> > > > +			}
> > > >  			break;
> > > >  		case VIRTIO_PCI_CAP_DEVICE_CFG:
> > > >  			hw->dev_cfg = get_cfg_addr(dev, &cap);
> > > 
> > > Tiwei: not 100% sure what's the best way to handle a failure here,
> > > this
> > > will avoid writing to *hw in case of errors. Let me know if this is
> > > OK.
> > 
> > My concern is about reading the virtio capability directly.
> > With this patch, it will give up reading other capabilities
> > when failed to read a full virtio PCI capability structure
> > (the returned bytes are less than the expected bytes) even
> > though when the capability it's reading isn't a virtio vendor
> > specific capability. I'm not quite sure whether it will bring
> > any bad consequence. How about just reading the first two
> > bytes first? Something like this:
> > 
> > https://github.com/freebsd/freebsd/blob/72445a41b3ff/sys/dev/pci/pci.
> > c#L1491-L1497
> 
> I'm not sure, to be honest. That bit didn't give me trouble at all, so
> at this point I'd much rather leave it alone so that the maintainers
> can take care of it how they see fit, if necessary :-)
> 
> I've sent a v3 that removes that individual change.

My concern isn't about the above change (which handled the
errors when reading multiplier). In fact, above change looks
good to me! :-)  I mean the below changes in this patch:

 	while (pos) {
 		ret = rte_pci_read_config(dev, &cap, sizeof(cap), pos);
-		if (ret < 0) {
-			PMD_INIT_LOG(ERR,
-				"failed to read pci cap at pos: %x", pos);
+		if (ret != sizeof(cap)) {
+			PMD_INIT_LOG(DEBUG,
+				     "failed to read pci cap at pos: %x ret %d",
+				     pos, ret);
 			break;
 		}

With this change, it will give up reading other capabilities
when failed to read a full virtio PCI capability structure
(the returned bytes are less than the expected bytes) even
though when the capability it's reading isn't a virtio vendor
specific capability. Maybe it would be better to read the
first two bytes first and check whether it's the capability
we want to parse (i.e. vendor capability and MSIX capability).
Something like this:

https://github.com/freebsd/freebsd/blob/72445a41b3ff/sys/dev/pci/pci.c#L1491-L1497

How do you think?

Thanks