linux-kernel - Re: [PATCH] PCI: Check value of resource alignment before using _

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Date:   Thu, 22 Apr 2021 13:41:51 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Amey Narkhede <ameynarkhede03@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: Check value of resource alignment before using __ffs

On Thu, Apr 22, 2021 at 03:13:23PM +0530, Amey Narkhede wrote:
> On 21/04/22 09:43AM, Leon Romanovsky wrote:
> > On Thu, Apr 22, 2021 at 12:17:47AM +0530, Amey Narkhede wrote:
> > > Return value of __ffs is undefined if no set bit exists in
> > > its argument. This indicates that the associated BAR has
> > > invalid alignment.
> > >
> > > Signed-off-by: Amey Narkhede <ameynarkhede03@...il.com>
> > > ---
> > >  drivers/pci/setup-bus.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> > > index 2ce636937c6e..44e8449418ae 100644
> > > --- a/drivers/pci/setup-bus.c
> > > +++ b/drivers/pci/setup-bus.c
> > > @@ -1044,6 +1044,11 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
> > >  			 * resources.
> > >  			 */
> > >  			align = pci_resource_alignment(dev, r);
> > > +			if (!align) {
> > > +				pci_warn(dev, "BAR %d: %pR has bogus alignment\n",
> > > +					 i, r);
> > > +				continue;
> > > +			}
> >
> > I see that you copied it from pdev_sort_resources(), but it is
> > incorrect change, see how negative order is handled and later
> > ARRAY_SIZE() check.
> >
> > Thanks
> >
> Is it guaranteed that it will return value which will result
> in negative value or >= ARRAY_SIZE? Comment on __ffs says value
> is undefined for 0 that means it could be anything or am I missing
> something?

diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
index 2ce636937c6e..ce5380bdd2fd 100644
--- a/drivers/pci/setup-bus.c
+++ b/drivers/pci/setup-bus.c
@@ -1044,10 +1044,11 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
                         * resources.
                         */
                        align = pci_resource_alignment(dev, r);
-                       order = __ffs(align) - 20;
-                       if (order < 0)
-                               order = 0;
-                       if (order >= ARRAY_SIZE(aligns)) {
+                       if (align) {
+                               order = __ffs(align) - 20;
+                               order = (order < 0) ? 0 : order;
+                       }
+                       if (!align || order >= ARRAY_SIZE(aligns)) {
                                pci_warn(dev, "disabling BAR %d: %pR (bad alignment %#llx)\n",
                                         i, r, (unsigned long long) align);
                                r->flags = 0;


> 
> Thanks,
> Amey