Date: Tue, 21 Apr 2015 10:07:00 -0400 (EDT)
From: Ulrich Obergfell <uobergfe@redhat.com>
To: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, Don Zickus <dzickus@redhat.com>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrew Jones <drjones@redhat.com>,
        chai wen <chaiw.fnst@cn.fujitsu.com>,
        Fabian Frederick <fabf@skynet.be>, Aaron Tomlin <atomlin@redhat.com>,
        Ben Zhang <benzh@chromium.org>, Christoph Lameter <cl@linux.com>,
        Gilad Ben-Yossef <gilad@benyossef.com>,
        Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
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In-Reply-To: <1429295838-6328-2-git-send-email-cmetcalf@ezchip.com>
References: <20150416152808.GA16270@lerouge> <1429295838-6328-1-git-send-email-cmetcalf@ezchip.com> <1429295838-6328-2-git-send-email-cmetcalf@ezchip.com>
Subject: Re: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist
 nohz
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Thread-Topic: watchdog: add watchdog_cpumask sysctl to assist nohz
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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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