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

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Message-ID: <20200507192256.GA16375@embeddedor>
Date:   Thu, 7 May 2020 14:22:56 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Kai Vehmanen <kai.vehmanen@...ux.intel.com>,
        Daniel Baluta <daniel.baluta@....com>
Cc:     sound-open-firmware@...a-project.org, linux-kernel@...r.kernel.org
Subject: [PATCH] ASoC: SOF: 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>
---
 sound/soc/sof/probe.h |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/sound/soc/sof/probe.h b/sound/soc/sof/probe.h
index 45daa5552834..250c4817f637 100644
--- a/sound/soc/sof/probe.h
+++ b/sound/soc/sof/probe.h
@@ -36,7 +36,7 @@ struct sof_probe_point_desc {
 struct sof_ipc_probe_dma_add_params {
 	struct sof_ipc_cmd_hdr hdr;
 	unsigned int num_elems;
-	struct sof_probe_dma dma[0];
+	struct sof_probe_dma dma[];
 } __packed;

 struct sof_ipc_probe_info_params {
@@ -51,19 +51,19 @@ struct sof_ipc_probe_info_params {
 struct sof_ipc_probe_dma_remove_params {
 	struct sof_ipc_cmd_hdr hdr;
 	unsigned int num_elems;
-	unsigned int stream_tag[0];
+	unsigned int stream_tag[];
 } __packed;

 struct sof_ipc_probe_point_add_params {
 	struct sof_ipc_cmd_hdr hdr;
 	unsigned int num_elems;
-	struct sof_probe_point_desc desc[0];
+	struct sof_probe_point_desc desc[];
 } __packed;

 struct sof_ipc_probe_point_remove_params {
 	struct sof_ipc_cmd_hdr hdr;
 	unsigned int num_elems;
-	unsigned int buffer_id[0];
+	unsigned int buffer_id[];
 } __packed;

 int sof_ipc_probe_init(struct snd_sof_dev *sdev,