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

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Message-ID: <20200221160005.GA13552@embeddedor>
Date:   Fri, 21 Feb 2020 10:00:05 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] video: 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/video/fbdev/mmp/hw/mmp_ctrl.h | 2 +-
 drivers/video/fbdev/ssd1307fb.c       | 2 +-
 include/video/mmp_disp.h              | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbdev/mmp/hw/mmp_ctrl.h b/drivers/video/fbdev/mmp/hw/mmp_ctrl.h
index 335d4983dc52..167585a889d3 100644
--- a/drivers/video/fbdev/mmp/hw/mmp_ctrl.h
+++ b/drivers/video/fbdev/mmp/hw/mmp_ctrl.h
@@ -1406,7 +1406,7 @@ struct mmphw_ctrl {
 
 	/*pathes*/
 	int path_num;
-	struct mmphw_path_plat path_plats[0];
+	struct mmphw_path_plat path_plats[];
 };
 
 static inline int overlay_is_vid(struct mmp_overlay *overlay)
diff --git a/drivers/video/fbdev/ssd1307fb.c b/drivers/video/fbdev/ssd1307fb.c
index 142535267fec..fb2640fe575a 100644
--- a/drivers/video/fbdev/ssd1307fb.c
+++ b/drivers/video/fbdev/ssd1307fb.c
@@ -89,7 +89,7 @@ struct ssd1307fb_par {
 
 struct ssd1307fb_array {
 	u8	type;
-	u8	data[0];
+	u8	data[];
 };
 
 static const struct fb_fix_screeninfo ssd1307fb_fix = {
diff --git a/include/video/mmp_disp.h b/include/video/mmp_disp.h
index 1f9bc133e230..77252cb46361 100644
--- a/include/video/mmp_disp.h
+++ b/include/video/mmp_disp.h
@@ -231,7 +231,7 @@ struct mmp_path {
 
 	/* layers */
 	int overlay_num;
-	struct mmp_overlay overlays[0];
+	struct mmp_overlay overlays[];
 };
 
 extern struct mmp_path *mmp_get_path(const char *name);
-- 
2.25.0