linux-kernel - [PATCH][next] jffs2: summary.h: Replace zero-length array with flexible-array member

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Message-ID: <20200309181904.GA4669@embeddedor>
Date:   Mon, 9 Mar 2020 13:19:04 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Richard Weinberger <richard@....at>
Cc:     linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] jffs2: summary.h: 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>
---
 fs/jffs2/summary.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/jffs2/summary.h b/fs/jffs2/summary.h
index 60207a2ae952..e4131cb1f1d4 100644
--- a/fs/jffs2/summary.h
+++ b/fs/jffs2/summary.h
@@ -61,7 +61,7 @@ struct jffs2_sum_dirent_flash
 	jint32_t ino; 		/* == zero for unlink */
 	uint8_t nsize;		/* dirent name size */
 	uint8_t type;		/* dirent type */
-	uint8_t name[0];	/* dirent name */
+	uint8_t name[];	/* dirent name */
 } __attribute__((packed));
 
 struct jffs2_sum_xattr_flash
@@ -117,7 +117,7 @@ struct jffs2_sum_dirent_mem
 	jint32_t ino; 		/* == zero for unlink */
 	uint8_t nsize;		/* dirent name size */
 	uint8_t type;		/* dirent type */
-	uint8_t name[0];	/* dirent name */
+	uint8_t name[];	/* dirent name */
 } __attribute__((packed));
 
 struct jffs2_sum_xattr_mem
-- 
2.25.0