Re: [PATCH v2] irqchip/sifive-plic: Fix insufficient irq_groups allocation

[Thomas Gleixner <tglx@kernel.org>]

On Wed, Jan 21 2026 at 13:45, Yangyu Chen wrote:
> Since the first irq source is 1 instead of 0, when the number of
> irqs is multiple of 32, the last irq group will be ignored during
> allocation, saving, and restoring. This lead to memory corruption
> when accessing enable_save beyond allocated memory after commit
> 14ff9e54dd14 ("irqchip/sifive-plic: Cache the interrupt enable state")
> which will access enable_save for all sources during plic_probe.
> Thus, we should allocate irq_groups based on (nr_irqs + 1) instead of
> nr_irqs to avoid this issue.
>
> This is an long standing bug since Linux v6.4 but since the last irq
> source is rarely used, it may not be triggered in practice until commit
> 14ff9e54dd14 ("irqchip/sifive-plic: Cache the interrupt enable state").

I'm absolutely not convinced that this is the right fix. The handling of
nr_irqs in this driver is completely inconsistent:

1)

   static int plic_irq_suspend(void *data)
	/* irq ID 0 is reserved */
	for (unsigned int i = 1; i < priv->nr_irqs; i++) {

2)
   static void plic_irq_resume(void *data)
	/* irq ID 0 is reserved */
	for (unsigned int i = 1; i < priv->nr_irqs; i++) {
        ...
	for (i = 0; i < DIV_ROUND_UP(priv->nr_irqs, 32); i++) {

3)

   static irq_hw_number_t cp100_get_hwirq(struct plic_handler *handler, void __iomem *claim)
	int nr_irq_groups = DIV_ROUND_UP(handler->priv->nr_irqs, 32);

        for (i = 0; i < nr_irq_groups; i++) {
4)

   static int plic_probe(struct fwnode_handle *fwnode)
  
	priv->prio_save = bitmap_zalloc(nr_irqs, GFP_KERNEL);
        ...
		for (int j = 0; j <= nr_irqs / 32; j++)
			writel(0, enable_base + j);
        ...                        
                handler->enable_save = kcalloc(DIV_ROUND_UP(nr_irqs, 32),...
	...
	for (hwirq = 1; hwirq <= nr_irqs; hwirq++) {
        ...
	priv->irqdomain = irq_domain_create_linear(fwnode, nr_irqs + 1,

So can the SIFIVE people please clarify once and forever what's the
actual meaning of nr_irqs:

   A) The actual number of hardware interrupts available for drivers
      starting from hwirq=1

   B) The total number of hardware interrupts, where hwirq=0 is reserved
      and therefore the number of interrupts available to drivers is
      nr_irqs - 1.

That needs to be clarified first and then subsequently every single
instance which deals with nr_irqs has to be updated accordingly.

A random fix which cures the symptom of an out of bounds memory access
is not helpful at all to cure the underlying root cause of
inconsistency.

Thanks,

        tglx



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