linux-kernel - [PATCH] sched/fair: Replace zero-length array with flexible-array

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Message-ID: <20200507192141.GA16183@embeddedor>
Date:   Thu, 7 May 2020 14:21:41 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] sched/fair: Replace zero-length array with flexible-array

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 kernel/sched/fair.c  |    2 +-
 kernel/sched/sched.h |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 02f323b85b6d..b34549708ef4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1094,7 +1094,7 @@ struct numa_group {
 	 * more by CPU use than by memory faults.
 	 */
 	unsigned long *faults_cpu;
-	unsigned long faults[0];
+	unsigned long faults[];
 };

 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index db3a57675ccf..f504f3ac3967 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1461,7 +1461,7 @@ struct sched_group {
 	 * by attaching extra space to the end of the structure,
 	 * depending on how many CPUs the kernel has booted up with)
 	 */
-	unsigned long		cpumask[0];
+	unsigned long		cpumask[];
 };

 static inline struct cpumask *sched_group_span(struct sched_group *sg)