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SubjectRe: A possible divide by zero bug in alloc_nodes_vectors
Date
On Fri, May 14 2021 at 19:31, Yiyuan guo wrote:

> In kernel/irq/affinity.c, the function alloc_nodes_vectors has the
> following code:
>
> static void alloc_nodes_vectors(unsigned int numvecs,
> cpumask_var_t *node_to_cpumask,
> const struct cpumask *cpu_mask,
> const nodemask_t nodemsk,
> struct cpumask *nmsk,
> struct node_vectors *node_vectors) {
> unsigned n, remaining_ncpus = 0;
> ...
> for_each_node_mask(n, nodemsk) {
> ...
> ncpus = cpumask_weight(nmsk);
>
> if (!ncpus)
> continue;
> remaining_ncpus += ncpus;
> ...
> }
>
> numvecs = min_t(unsigned, remaining_ncpus, numvecs);
> ...
> for (n = 0; n < nr_node_ids; n++) {
> ...
> WARN_ON_ONCE(numvecs == 0);
> ...
> nvectors = max_t(unsigned, 1,
> numvecs * ncpus / remaining_ncpus);
> }
> }
>
> The variable remaining_ncpus may remain 0 if cpumask_weight(nmsk)
> keeps returning 0 in the for loop. However, remaining_ncpus is used as
> a divisor, leading to a potential divide by zero problem.

How so? It's guaranteed that there is at least ONE node which is not
empty. So remaining_ncpus cannot be 0.

Thanks,

tglx



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