linux-kernel - [PATCH] sched, cpumask: don't leak impossible cpus via for_each_cpu

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Message-Id: <20220802214041.2656586-1-neelnatu@google.com>
Date:   Tue,  2 Aug 2022 14:40:41 -0700
From:   Neel Natu <neelnatu@...gle.com>
To:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Cc:     Neel Natu <neelnatu@...gle.com>
Subject: [PATCH] sched, cpumask: don't leak impossible cpus via for_each_cpu_wrap().

The value of 'nr_cpumask_bits' is dependent on CONFIG_CPUMASK_OFFSTACK.
This in turn can change the set of cpus visited by for_each_cpu_wrap()
with a mask that has bits set in the range [nr_cpu_ids, NR_CPUS).

Specifically on !CONFIG_CPUMASK_OFFSTACK kernels the API can iterate
over cpus outside the 'cpu_possible_mask'.

Fix this to make its behavior match for_each_cpu() which always limits
the iteration to the range [0, nr_cpu_ids).

Signed-off-by: Neel Natu <neelnatu@...gle.com>
---
 include/linux/cpumask.h | 2 +-
 lib/cpumask.c           | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index fe29ac7cc469..2a308cfc43da 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -303,7 +303,7 @@ extern int cpumask_next_wrap(int n, const struct cpumask *mask, int start, bool
  */
 #define for_each_cpu_wrap(cpu, mask, start)					\
 	for ((cpu) = cpumask_next_wrap((start)-1, (mask), (start), false);	\
-	     (cpu) < nr_cpumask_bits;						\
+	     (cpu) < nr_cpu_ids;						\
 	     (cpu) = cpumask_next_wrap((cpu), (mask), (start), true))
 
 /**
diff --git a/lib/cpumask.c b/lib/cpumask.c
index a971a82d2f43..d47937fb49eb 100644
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -82,9 +82,9 @@ int cpumask_next_wrap(int n, const struct cpumask *mask, int start, bool wrap)
 	next = cpumask_next(n, mask);
 
 	if (wrap && n < start && next >= start) {
-		return nr_cpumask_bits;
+		return nr_cpu_ids;
 
-	} else if (next >= nr_cpumask_bits) {
+	} else if (next >= nr_cpu_ids) {
 		wrap = true;
 		n = -1;
 		goto again;
-- 
2.37.1.455.g008518b4e5-goog