linux-kernel - RE: [PATCH v2] vfio: Use WARN_ON for low-probability allocation failure issue in vfio_pci_bus

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Message-ID: <DS0PR11MB6373DB70ED49E18C3232973FDC732@DS0PR11MB6373.namprd11.prod.outlook.com>
Date: Tue, 16 Jan 2024 12:08:53 +0000
From: "Wang, Wei W" <wei.w.wang@...el.com>
To: Alex Williamson <alex.williamson@...hat.com>
CC: Kunwu Chan <chentao@...inos.cn>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] vfio: Use WARN_ON for low-probability allocation
 failure issue in vfio_pci_bus_notifier

On Tuesday, January 16, 2024 12:29 AM, Alex Williamson wrote:
> > On Monday, January 15, 2024 2:35 PM, Kunwu Chan wrote:
> > > kasprintf() returns a pointer to dynamically allocated memory which
> > > can be NULL upon failure.
> > >
> > > This is a blocking notifier callback, so errno isn't a proper return
> > > value. Use WARN_ON to small allocation failures.
> > >
> > > Signed-off-by: Kunwu Chan <chentao@...inos.cn>
> > > ---
> > > v2: Use WARN_ON instead of return errno
> > > ---
> > >  drivers/vfio/pci/vfio_pci_core.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/drivers/vfio/pci/vfio_pci_core.c
> > > b/drivers/vfio/pci/vfio_pci_core.c
> > > index 1cbc990d42e0..61aa19666050 100644
> > > --- a/drivers/vfio/pci/vfio_pci_core.c
> > > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > > @@ -2047,6 +2047,7 @@ static int vfio_pci_bus_notifier(struct
> > > notifier_block *nb,
> > >  			 pci_name(pdev));
> > >  		pdev->driver_override = kasprintf(GFP_KERNEL, "%s",
> > >  						  vdev->vdev.ops->name);
> > > +		WARN_ON(!pdev->driver_override);
> >
> > Saw Alex's comments on v1. Curious why not return "NOTIFY_BAD" on
> > errors though less likely? Similar examples could be found in
> kvm_pm_notifier_call, kasan_mem_notifier etc.
> 
> If the statement is that there are notifier call chains that return NOTIFY_BAD, I
> would absolutely agree, but the return value needs to be examined from the
> context of the caller.  BUS_NOTIFY_ADD_DEVICE is notified via bus_notify() in
> device_add().  What does it accomplish to return NOTIFY_BAD in a chain that
> ignores the return value?  At best we're preventing callbacks further down the
> chain from being called.
> That doesn't seem obviously beneficial either.

OK, thanks for the clarification. My curiosity came from the statement "This is a
blocking notifier callback, so errno isn't a proper return value". Probably the
commit log needs some rewording.