Re: [PATCH v2 04/18] nitro_enclaves: Init PCI device driver

[Alexander Graf]

Hey Greg,

On 22.05.20 09:04, Greg KH wrote:
> On Fri, May 22, 2020 at 09:29:32AM +0300, Andra Paraschiv wrote:
> > +/**
> > + * ne_setup_msix - Setup MSI-X vectors for the PCI device.
> > + *
> > + * @pdev: PCI device to setup the MSI-X for.
> > + *
> > + * @returns: 0 on success, negative return value on failure.
> > + */
> > +static int ne_setup_msix(struct pci_dev *pdev)
> > +{
> > +     struct ne_pci_dev *ne_pci_dev = NULL;
> > +     int nr_vecs = 0;
> > +     int rc = -EINVAL;
> > +
> > +     if (WARN_ON(!pdev))
> > +             return -EINVAL;
>
> How can this ever happen?  If it can not, don't test for it.  If it can,
> don't warn for it as that will crash systems that do panic-on-warn, just
> test and return an error.

I think the point here is to catch situations that should never happen, 
but keep a sanity check in in case they do happen. This would've usually 
been a BUG_ON, but people tend to dislike those these days because they 
can bring down your system ...

So in this particular case here I agree that it's a bit silly to check 
whether pdev is != NULL. In other device code internal APIs though it's 
not quite as clear of a cut. I by far prefer code that tells me it's 
broken over reverse engineering stray pointer accesses ...

> > +
> > +     ne_pci_dev = pci_get_drvdata(pdev);
> > +     if (WARN_ON(!ne_pci_dev))
> > +             return -EINVAL;
>
> Same here, don't use WARN_ON if at all possible.
>
> > +
> > +     nr_vecs = pci_msix_vec_count(pdev);
> > +     if (nr_vecs < 0) {
> > +             rc = nr_vecs;
> > +
> > +             dev_err_ratelimited(&pdev->dev,
> > +                                 NE "Error in getting vec count [rc=%d]\n",
> > +                                 rc);
> > +
>
> Why ratelimited, can this happen over and over and over?

In this particular function, no, so here it really should just be 
dev_err. Other functions are implicitly callable from user space through 
an ioctl, which means they really need to stay rate limited.

Thanks a lot for looking through the code and pointing all those bits out :)


Alex



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