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

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Date:   Thu, 7 May 2020 14:22:23 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>
Cc:     alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: [PATCH] ALSA: 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/sound/control.h     |    2 +-
 include/sound/intel-nhlt.h  |    6 +++---
 sound/core/oss/pcm_plugin.h |    2 +-
 sound/usb/usx2y/usbusx2y.h  |    2 +-
 4 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/sound/control.h b/include/sound/control.h
index 11feeee31e35..aeaed2a05bae 100644
--- a/include/sound/control.h
+++ b/include/sound/control.h
@@ -75,7 +75,7 @@ struct snd_kcontrol {
 	unsigned long private_value;
 	void *private_data;
 	void (*private_free)(struct snd_kcontrol *kcontrol);
-	struct snd_kcontrol_volatile vd[0];	/* volatile data */
+	struct snd_kcontrol_volatile vd[];	/* volatile data */
 };

 #define snd_kcontrol(n) list_entry(n, struct snd_kcontrol, list)
diff --git a/include/sound/intel-nhlt.h b/include/sound/intel-nhlt.h
index f657fd8fc0ad..743c2f442280 100644
--- a/include/sound/intel-nhlt.h
+++ b/include/sound/intel-nhlt.h
@@ -50,7 +50,7 @@ enum nhlt_device_type {

 struct nhlt_specific_cfg {
 	u32 size;
-	u8 caps[0];
+	u8 caps[];
 } __packed;

 struct nhlt_fmt_cfg {
@@ -60,7 +60,7 @@ struct nhlt_fmt_cfg {

 struct nhlt_fmt {
 	u8 fmt_count;
-	struct nhlt_fmt_cfg fmt_config[0];
+	struct nhlt_fmt_cfg fmt_config[];
 } __packed;

 struct nhlt_endpoint {
@@ -80,7 +80,7 @@ struct nhlt_endpoint {
 struct nhlt_acpi_table {
 	struct acpi_table_header header;
 	u8 endpoint_count;
-	struct nhlt_endpoint desc[0];
+	struct nhlt_endpoint desc[];
 } __packed;

 struct nhlt_resource_desc  {
diff --git a/sound/core/oss/pcm_plugin.h b/sound/core/oss/pcm_plugin.h
index 8d2f7a4e3ab6..46e273bd4a78 100644
--- a/sound/core/oss/pcm_plugin.h
+++ b/sound/core/oss/pcm_plugin.h
@@ -64,7 +64,7 @@ struct snd_pcm_plugin {
 	char *buf;
 	snd_pcm_uframes_t buf_frames;
 	struct snd_pcm_plugin_channel *buf_channels;
-	char extra_data[0];
+	char extra_data[];
 };

 int snd_pcm_plugin_build(struct snd_pcm_substream *handle,
diff --git a/sound/usb/usx2y/usbusx2y.h b/sound/usb/usx2y/usbusx2y.h
index e0f77172ce8f..144b85f57bd2 100644
--- a/sound/usb/usx2y/usbusx2y.h
+++ b/sound/usb/usx2y/usbusx2y.h
@@ -18,7 +18,7 @@ struct snd_usX2Y_AsyncSeq {
 struct snd_usX2Y_urbSeq {
 	int	submitted;
 	int	len;
-	struct urb	*urb[0];
+	struct urb	*urb[];
 };

 #include "usx2yhwdeppcm.h"