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

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Date:   Thu, 7 May 2020 14:07:06 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] media: pwc: 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>
---
 drivers/media/usb/pwc/pwc.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/media/usb/pwc/pwc.h b/drivers/media/usb/pwc/pwc.h
index 3362962d0d00..b02a3c7b7742 100644
--- a/drivers/media/usb/pwc/pwc.h
+++ b/drivers/media/usb/pwc/pwc.h
@@ -193,7 +193,7 @@ struct pwc_raw_frame {
 				   decompressor) */
 	__u8   cmd[4];		/* the four byte of the command (in case of
 				   nala, only the first 3 bytes is filled) */
-	__u8   rawframe[0];	/* frame_size = H / 4 * vbandlength */
+	__u8   rawframe[];	/* frame_size = H / 4 * vbandlength */
 } __packed;

 /* intermediate buffers with raw data from the USB cam */