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SubjectRe: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

Chris,

I think it would also be nice to check the plausibility of the user input.

+int proc_watchdog_cpumask(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ int err;
+
+ mutex_lock(&watchdog_proc_mutex);
+ err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
+ if (!err && write) {
+ /* Remove impossible cpus to keep sysctl output cleaner. */
+ cpumask_and(watchdog_cpumask, watchdog_cpumask,
+ cpu_possible_mask);
+
+ if (watchdog_enabled && watchdog_thresh)
+ smpboot_update_cpumask_percpu_thread(&watchdog_threads,
+ watchdog_cpumask);
+ }
+ mutex_unlock(&watchdog_proc_mutex);
+ return err;
+}

I think the user should only be allowed to specify a mask that is a subset of
tick_nohz_full_mask as only those CPUs don't have a watchdog thread by default.
In other words, the user should not be able to interfere with housekeeping CPUs.

For example, add a plausibility check like so:

save watchdog_cpumask because proc_do_large_bitmap() is going to change it

proc_do_large_bitmap()

// return an error if the user-specified mask includes a housekeeping CPU
if (watchdog_cpumask and 'negated tick_nohz_full_mask') {
restore saved watchdog_cpumask
return -EINVAL
}


Regards,

Uli


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