Re: [PATCH 1/2] ns16550: reject IRQ above nr_irqs

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Mar 10, 2022 at 04:21:50PM +0000, Julien Grall wrote:
> Hi,
> 
> On 10/03/2022 16:12, Roger Pau Monné wrote:
> > On Thu, Mar 10, 2022 at 05:08:07PM +0100, Jan Beulich wrote:
> > > On 10.03.2022 16:47, Roger Pau Monné wrote:
> > > > On Thu, Mar 10, 2022 at 04:23:00PM +0100, Jan Beulich wrote:
> > > > > On 10.03.2022 15:34, Marek Marczykowski-Górecki wrote:
> > > > > > --- a/xen/drivers/char/ns16550.c
> > > > > > +++ b/xen/drivers/char/ns16550.c
> > > > > > @@ -1221,6 +1221,9 @@ pci_uart_config(struct ns16550 *uart, bool_t 
> > > > > > skip_amt, unsigned int idx)
> > > > > >                               pci_conf_read8(PCI_SBDF(0, b, d, f),
> > > > > >                                              PCI_INTERRUPT_LINE) : 
> > > > > > 0;
> > > > > > +                if (uart->irq >= nr_irqs)
> > > > > > +                    uart->irq = 0;
> > > > > 
> > > > > Don't you mean nr_irqs_gsi here? Also (nit) please add the missing 
> > > > > blanks
> > > > > immediately inside the parentheses.
> > > > 
> > > > If we use nr_irqs_gsi we will need to make the check x86 only AFAICT.
> > > 
> > > Down the road (when Arm wants to select HAS_PCI) - yes. Not necessarily
> > > right away. After all Arm wants to have an equivalent check here then,
> > > not merely checking against nr_irqs instead. So putting a conditional
> > > here right away would hide the need for putting in place an Arm-specific
> > > alternative.
> > 
> > Oh, I always forget Arm doesn't have CONFIG_HAS_PCI enabled just yet.
> The PCI code in ns16550.c is gated by CONFIG_HAS_PCI and CONFIG_X86. I am
> not sure we will ever see a support for PCI UART card in Xen on Arm.
> 
> However, if it evers happens then neither nr_irqs or nr_irqs_gsi would help
> here because from the interrupt controller PoV 0xff may be a valid (GICv2
> supports up to 1024 interrupts).
> 
> Is there any reason we can't explicitely check 0xff?

That's what my v0.1 did, but Roger suggested nr_irqs. And I agree,
because the value is later used (on x86) to access irq_desc array (via
irq_to_desc), which has nr_irqs size.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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