[PATCH] ASoC: Intel: Skylake: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

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@xxxxxxxxxx>
---
 sound/soc/intel/atom/sst-atom-controls.h |    2 +-
 sound/soc/intel/skylake/skl-i2s.h        |    2 +-
 sound/soc/intel/skylake/skl-topology.h   |    4 ++--
 sound/soc/intel/skylake/skl.h            |    2 +-
 4 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/sound/soc/intel/atom/sst-atom-controls.h b/sound/soc/intel/atom/sst-atom-controls.h
index 5356e954a732..620b48d2a064 100644
--- a/sound/soc/intel/atom/sst-atom-controls.h
+++ b/sound/soc/intel/atom/sst-atom-controls.h
@@ -410,7 +410,7 @@ struct sst_cmd_set_gain_dual {
 struct sst_cmd_set_params {
 	struct sst_destination_id dst;
 	u16 command_id;
-	char params[0];
+	char params[];
 } __packed;
 
 
diff --git a/sound/soc/intel/skylake/skl-i2s.h b/sound/soc/intel/skylake/skl-i2s.h
index d7c15873c0d4..dfce91e11be1 100644
--- a/sound/soc/intel/skylake/skl-i2s.h
+++ b/sound/soc/intel/skylake/skl-i2s.h
@@ -46,7 +46,7 @@ struct skl_i2s_config_mclk {
 struct skl_i2s_config_mclk_ext {
 	u32 mdivctrl;
 	u32 mdivr_count;
-	u32 mdivr[0];
+	u32 mdivr[];
 } __packed;
 
 struct skl_i2s_config_blob_signature {
diff --git a/sound/soc/intel/skylake/skl-topology.h b/sound/soc/intel/skylake/skl-topology.h
index e967800dbb62..d2cd8ef8e97f 100644
--- a/sound/soc/intel/skylake/skl-topology.h
+++ b/sound/soc/intel/skylake/skl-topology.h
@@ -119,7 +119,7 @@ struct skl_cpr_gtw_cfg {
 struct skl_dma_control {
 	u32 node_id;
 	u32 config_length;
-	u32 config_data[0];
+	u32 config_data[];
 } __packed;
 
 struct skl_cpr_cfg {
@@ -152,7 +152,7 @@ struct skl_up_down_mixer_cfg {
 
 struct skl_algo_cfg {
 	struct skl_base_cfg  base_cfg;
-	char params[0];
+	char params[];
 } __packed;
 
 struct skl_base_outfmt_cfg {
diff --git a/sound/soc/intel/skylake/skl.h b/sound/soc/intel/skylake/skl.h
index 2bfbf59277c4..26057f38a014 100644
--- a/sound/soc/intel/skylake/skl.h
+++ b/sound/soc/intel/skylake/skl.h
@@ -49,7 +49,7 @@ struct skl_astate_param {
 
 struct skl_astate_config {
 	u32 count;
-	struct skl_astate_param astate_table[0];
+	struct skl_astate_param astate_table[];
 };
 
 struct skl_fw_config {