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From:   zhuguangqing83@gmail.com
To:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, gustavoars@kernel.org
Cc:     linux-kernel@vger.kernel.org,
        zhuguangqing <zhuguangqing@xiaomi.com>
Subject: [PATCH] sched: Replace zero-length array with flexible-array
Date:   Wed, 14 Oct 2020 22:02:20 +0800
Message-Id: <20201014140220.11384-1-zhuguangqing83@gmail.com>
Precedence: bulk

From: zhuguangqing <zhuguangqing@xiaomi.com>

In commit 04f5c362ec6, a zero-length array cpumask[0] has been
replaced with cpumask[]. But there is still a cpumask[0] in
struct sched_group_capacity{} which maybe missed.

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: zhuguangqing <zhuguangqing@xiaomi.com>
---
 kernel/sched/sched.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 28709f6b0975..648f02363ff9 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1471,7 +1471,7 @@ struct sched_group_capacity {
 	int			id;
 #endif
 
-	unsigned long		cpumask[0];		/* Balance mask */
+	unsigned long		cpumask[];		/* Balance mask */
 };
 
 struct sched_group {
-- 
2.17.1