linux-kernel - RE: revert bab65e48cb064 PCI/MSI Sanitize MSI-X checks

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Date:   Thu, 6 Apr 2023 15:36:36 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Bjorn Helgaas' <helgaas@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Jason Gunthorpe <jgg@...dia.com>,
        "Bjorn Helgaas" <bhelgaas@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "regressions@...ts.linux.dev" <regressions@...ts.linux.dev>
Subject: RE: revert bab65e48cb064 PCI/MSI Sanitize MSI-X checks

From: Bjorn Helgaas
> Sent: 06 April 2023 16:08
> 
> [+cc linux-pci, regressions]
> 
> On Thu, Apr 06, 2023 at 11:05:14AM +0000, David Laight wrote:
> > The change in bab65e48cb064 breaks pci_enable_msix_range().
> > The intent is to optimise the sanity checks, but it is
> > somewhat overenthusiastic.
> >
> > The interface allows you to ask for a lot of vectors and
> > returns the number that were allocated.
> > However, after the change, you can't request a vector
> > that is higher than the largest the hardware supports.
> > Which makes that rather pointless.
> >
> > So code like:
> > 	for (i = 0; i < 16; i++)
> > 		msix_tbl[i].entry = i;
> > 	nvec = pci_enable_msix_range(dev, msix_tbl, 1, 16);
> > Now returns -22 if the hardware only supports 8 interrupts.
> >
> > Previously it returned 8.
> >
> > I can fix my driver, but I suspect that any code that relies
> > on a smaller number of vectors being returned is now broken.
> 
> Thanks for the report!  bab65e48cb06 ("PCI/MSI: Sanitize MSI-X
> checks") appeared in v6.2-rc1, so this is a recent regression and it
> would be good to fix it for v6.3.

I do try to test every release at around rc3.

> bab65e48cb06 only touches drivers/pci/msi/msi.c, but since it didn't
> go through the PCI tree, I'll let Thomas handle any revert (or better,
> an improvement to pci_msix_validate_entries()) since he wrote and
> applied the original.

Looking it:

static bool pci_msix_validate_entries(struct pci_dev *dev, struct msix_entry *entries,
				      int nvec, int hwsize)
{
	bool nogap;
	int i, j;

	if (!entries)
		return true;

	nogap = pci_msi_domain_supports(dev, MSI_FLAG_MSIX_CONTIGUOUS, DENY_LEGACY);

	for (i = 0; i < nvec; i++) {
		/* Entry within hardware limit? */
		if (entries[i].entry >= hwsize)
			return false;

		/* Check for duplicate entries */
		for (j = i + 1; j < nvec; j++) {
			if (entries[i].entry == entries[j].entry)
				return false;
		}
		/* Check for unsupported gaps */
		if (nogap && entries[i].entry != i)
			return false;
	}
	return true;
}

It probably needs to return an updated 'nvec'.
The gap/duplicate check is also a bit horrid, why not:
		if (nogap) {
			if (entries[i].entry != i)
				return false;
			continue;
		}

		if (!i || entries[i].entry > entries[i - 1].entry)
			continue;

		horrid, expensive loop...

	David

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