x86/smpboot: Prevent false positive out of bounds cpumask access warning

prefill_possible_map() reinitializes the cpu_possible_map by setting the
possible cpu bits and clearing all other bits up to NR_CPUS.

This is technically always correct because cpu_possible_map is statically
allocated and sized NR_CPUS. With CPUMASK_OFFSTACK and DEBUG_PER_CPU_MAPS
enabled the bounds check of cpu masks happens on nr_cpu_ids. nr_cpu_ids is
initialized to NR_CPUS and only limited after the set/clear bit loops have
been executed.

But if the system was booted with "nr_cpus=N" on the command line, where N
is < NR_CPUS then nr_cpu_ids is limited in the parameter parsing function
before prefill_possible_map() is invoked. As a consequence the cpumask
bounds check triggers when clearing the bits past nr_cpu_ids.

Add a helper which allows to reset cpu_possible_map w/o the bounds check
and then set only the possible bits which are well inside bounds.

Reported-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: 0x7f454c46@gmail.com
Cc: Jan Beulich <JBeulich@novell.com>
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1612131836050.3415@nanos
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index e09aa58a7603fb433b9d760ffae35bb762e3473a..46732dc3b73cd67e874dfe99b0eeb807f816e06a 100644 (file)
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1463,15 +1463,15 @@ __init void prefill_possible_map(void)
                possible = i;
        }
 
+       nr_cpu_ids = possible;
+
        pr_info("Allowing %d CPUs, %d hotplug CPUs\n",
                possible, max_t(int, possible - num_processors, 0));
 
+       reset_cpu_possible_mask();
+
        for (i = 0; i < possible; i++)
                set_cpu_possible(i, true);
-       for (; i < NR_CPUS; i++)
-               set_cpu_possible(i, false);
-
-       nr_cpu_ids = possible;
 }
 
 #ifdef CONFIG_HOTPLUG_CPU

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index da7fbf1cdd5613cb709c1a50fa44d2e2e7159d91..c717f5ea88cb7e7f04471f34ac14bcf9bc75a4d1 100644 (file)
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -722,6 +722,11 @@ void init_cpu_present(const struct cpumask *src);
 void init_cpu_possible(const struct cpumask *src);
 void init_cpu_online(const struct cpumask *src);
 
+static inline void reset_cpu_possible_mask(void)
+{
+       bitmap_zero(cpumask_bits(&__cpu_possible_mask), NR_CPUS);
+}
+
 static inline void
 set_cpu_possible(unsigned int cpu, bool possible)
 {