linux-kernel - [PATCH] mfd: pm8xxx: Replace zero-length array with flexible-array member

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Message-ID: <20200212235911.GA20179@embeddedor.com>
Date:   Wed, 12 Feb 2020 17:59:11 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Lee Jones <lee.jones@...aro.org>
Cc:     linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] mfd: pm8xxx: 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]

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/mfd/qcom-pm8xxx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mfd/qcom-pm8xxx.c b/drivers/mfd/qcom-pm8xxx.c
index 29133326c6fd..acd172ddcbd6 100644
--- a/drivers/mfd/qcom-pm8xxx.c
+++ b/drivers/mfd/qcom-pm8xxx.c
@@ -76,7 +76,7 @@ struct pm_irq_chip {
 	unsigned int		num_masters;
 	const struct pm_irq_data *pm_irq_data;
 	/* MUST BE AT THE END OF THIS STRUCT */
-	u8			config[0];
+	u8			config[];
 };
 
 static int pm8xxx_read_block_irq(struct pm_irq_chip *chip, unsigned int bp,
-- 
2.23.0