linux-kernel - [PATCH] treewide: Replace zero-length array with flexible-array

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Message-ID: <20200507185301.GA14333@embeddedor>
Date:   Thu, 7 May 2020 13:53:01 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Li Yang <leoyang.li@....com>
Cc:     linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH] treewide: 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>
---
 include/linux/fsl/bestcomm/bestcomm.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/fsl/bestcomm/bestcomm.h b/include/linux/fsl/bestcomm/bestcomm.h
index a0e2e6b19b57..154e541ce57e 100644
--- a/include/linux/fsl/bestcomm/bestcomm.h
+++ b/include/linux/fsl/bestcomm/bestcomm.h
@@ -27,7 +27,7 @@
  */
 struct bcom_bd {
 	u32	status;
-	u32	data[0];	/* variable payload size */
+	u32	data[];	/* variable payload size */
 };

 /* ======================================================================== */