mmc: 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 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20200226223125.GA20630@embeddedor
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/sdhci-acpi.c

@@ -75,7 +75,7 @@ struct sdhci_acpi_host {
 	bool				use_runtime_pm;
 	bool				is_intel;
 	bool				reset_signal_volt_on_suspend;
-	unsigned long			private[0] ____cacheline_aligned;
+	unsigned long			private[] ____cacheline_aligned;
 };
 
 enum {

drivers/mmc/host/sdhci-cadence.c

@@ -68,7 +68,7 @@ struct sdhci_cdns_priv {
 	void __iomem *hrs_addr;
 	bool enhanced_strobe;
 	unsigned int nr_phy_params;
-	struct sdhci_cdns_phy_param phy_params[0];
+	struct sdhci_cdns_phy_param phy_params[];
 };
 
 struct sdhci_cdns_phy_cfg {

drivers/mmc/host/sdhci-pci.h

@@ -163,7 +163,7 @@ struct sdhci_pci_slot {
 	bool			cd_override_level;
 
 	void (*hw_reset)(struct sdhci_host *host);
-	unsigned long		private[0] ____cacheline_aligned;
+	unsigned long		private[] ____cacheline_aligned;
 };
 
 struct sdhci_pci_chip {

drivers/mmc/host/sdhci-pltfm.h

@@ -25,7 +25,7 @@ struct sdhci_pltfm_host {
 	unsigned int clock;
 	u16 xfer_mode_shadow;
 
-	unsigned long private[0] ____cacheline_aligned;
+	unsigned long private[] ____cacheline_aligned;
 };
 
 #ifdef CONFIG_MMC_SDHCI_BIG_ENDIAN_32BIT_BYTE_SWAPPER

drivers/mmc/host/sdhci.h

@@ -614,7 +614,7 @@ struct sdhci_host {
 
 	u64			data_timeout;
 
-	unsigned long private[0] ____cacheline_aligned;
+	unsigned long private[] ____cacheline_aligned;
 };
 
 struct sdhci_ops {

drivers/mmc/host/vub300.c

@@ -95,7 +95,7 @@ struct sd_response_header {
 	u8 port_number;
 	u8 command_type;
 	u8 command_index;
-	u8 command_response[0];
+	u8 command_response[];
 } __packed;
 
 struct sd_status_header {

include/linux/mmc/host.h

@@ -468,7 +468,7 @@ struct mmc_host {
 	/* Host Software Queue support */
 	bool			hsq_enabled;
 
-	unsigned long		private[0] ____cacheline_aligned;
+	unsigned long		private[] ____cacheline_aligned;
 };
 
 struct device_node;

include/linux/mmc/sdio_func.h

@@ -25,7 +25,7 @@ struct sdio_func_tuple {
 	struct sdio_func_tuple *next;
 	unsigned char code;
 	unsigned char size;
-	unsigned char data[0];
+	unsigned char data[];
 };
 
 /*

include/uapi/linux/mmc/ioctl.h

@@ -57,7 +57,7 @@ struct mmc_ioc_cmd {
  */
 struct mmc_ioc_multi_cmd {
 	__u64 num_of_cmds;
-	struct mmc_ioc_cmd cmds[0];
+	struct mmc_ioc_cmd cmds[];
 };
 
 #define MMC_IOC_CMD _IOWR(MMC_BLOCK_MAJOR, 0, struct mmc_ioc_cmd)