Commit 427d77a3 authored Dec 13, 2016 by Thomas Gleixner

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>

parent 31dcfec1

arch/x86/kernel/smpboot.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -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

include/linux/cpumask.h

+5 −0

Original line number	Diff line number	Diff line
		@@ -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)
		{