linux-kernel - [PATCH] powercap: idle_inject: Replace zero-length array with flexible-array member

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Message-ID: <20200227190721.GA19083@embeddedor>
Date:   Thu, 27 Feb 2020 13:07:21 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] powercap: idle_inject: Replace zero-length array with
 flexible-array member

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]

Lastly, fix the following checkpatch warning:
WARNING: Prefer 'unsigned long' over 'unsigned long int' as the int is unnecessary
+	unsigned long int cpumask[];

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 <gustavo@...eddedor.com>
---
 drivers/powercap/idle_inject.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/powercap/idle_inject.c b/drivers/powercap/idle_inject.c
index cd1270614cc6..e9bbd3c42eef 100644
--- a/drivers/powercap/idle_inject.c
+++ b/drivers/powercap/idle_inject.c
@@ -67,7 +67,7 @@ struct idle_inject_device {
 	struct hrtimer timer;
 	unsigned int idle_duration_us;
 	unsigned int run_duration_us;
-	unsigned long int cpumask[0];
+	unsigned long cpumask[];
 };
 
 static DEFINE_PER_CPU(struct idle_inject_thread, idle_inject_thread);
-- 
2.25.0