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

[Ulrich Obergfell <uobergfe@redhat.com>]

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