greybus: 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 <[e-mail]>
Link: https://lore.kernel.org/r/20200507185318.GA14393@embeddedor
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/greybus/arpc.h

···
 	__le16	id;		/* RPC unique id */
 	__le16	size;		/* Size in bytes of header + payload */
 	__u8	type;		/* RPC type */
-	__u8	data[0];	/* ARPC data */
+	__u8	data[];	/* ARPC data */
 } __packed;
 
 struct arpc_response_message {

include/linux/greybus/greybus_protocols.h

···
 
 struct gb_cap_get_ims_certificate_response {
 	__u8			result_code;
-	__u8			certificate[0];
+	__u8			certificate[];
 } __packed;
 
 /* CAP authenticate request/response */
···
 struct gb_cap_authenticate_response {
 	__u8			result_code;
 	__u8			response[64];
-	__u8			signature[0];
+	__u8			signature[];
 } __packed;
 
 
···
 struct gb_hid_set_report_request {
 	__u8				report_type;
 	__u8				report_id;
-	__u8				report[0];
+	__u8				report[];
 } __packed;
 
 /* HID input report request, via interrupt pipe */
···
 
 struct gb_i2c_transfer_request {
 	__le16				op_count;
-	struct gb_i2c_transfer_op	ops[0];		/* op_count of these */
+	struct gb_i2c_transfer_op	ops[];		/* op_count of these */
 } __packed;
 struct gb_i2c_transfer_response {
 	__u8				data[0];	/* inbound data */
···
 	__u8			chip_select;	/* of the spi device */
 	__u8			mode;		/* of the spi device */
 	__le16			count;
-	struct gb_spi_transfer	transfers[0];	/* count of these */
+	struct gb_spi_transfer	transfers[];	/* count of these */
 } __packed;
 
 struct gb_spi_transfer_response {
···
 
 struct gb_svc_pwrmon_rail_names_get_response {
 	__u8	status;
-	__u8	name[0][GB_SVC_PWRMON_RAIL_NAME_BUFSIZE];
+	__u8	name[][GB_SVC_PWRMON_RAIL_NAME_BUFSIZE];
 } __packed;
 
 #define GB_SVC_PWRMON_TYPE_CURR			0x01
···
 
 struct gb_raw_send_request {
 	__le32	len;
-	__u8	data[0];
+	__u8	data[];
 } __packed;
 
 
···
 /* Represents data from AP -> Module */
 struct gb_uart_send_data_request {
 	__le16	size;
-	__u8	data[0];
+	__u8	data[];
 } __packed;
 
 /* recv-data-request flags */
···
 struct gb_uart_recv_data_request {
 	__le16	size;
 	__u8	flags;
-	__u8	data[0];
+	__u8	data[];
 } __packed;
 
 struct gb_uart_receive_credits_request {
···
 	__le32	len;
 	__le32  reserved0;
 	__le32  reserved1;
-	__u8	data[0];
+	__u8	data[];
 } __packed;
 
 struct gb_loopback_transfer_response {
 	__le32	len;
 	__le32	reserved0;
 	__le32	reserved1;
-	__u8	data[0];
+	__u8	data[];
 } __packed;
 
 /* SDIO */
···
 
 	__le16	data_blocks;
 	__le16	data_blksz;
-	__u8	data[0];
+	__u8	data[];
 } __packed;
 
 struct gb_sdio_transfer_response {
 	__le16	data_blocks;
 	__le16	data_blksz;
-	__u8	data[0];
+	__u8	data[];
 } __packed;
 
 /* event request: generated by module and is defined as unidirectional */
···
 	__u8 flags;
 #define GB_CAMERA_CONFIGURE_STREAMS_TEST_ONLY	0x01
 	__le16 padding;
-	struct gb_camera_stream_config_request config[0];
+	struct gb_camera_stream_config_request config[];
 } __packed;
 
 /* Greybus Camera Configure Streams response payload */
···
 	__u8 flags;
 	__u8 padding[2];
 	__le32 data_rate;
-	struct gb_camera_stream_config_response config[0];
+	struct gb_camera_stream_config_response config[];
 };
 
 /* Greybus Camera Capture request payload - response has no payload */
···
 	__u8 streams;
 	__u8 padding;
 	__le16 num_frames;
-	__u8 settings[0];
+	__u8 settings[];
 } __packed;
 
 /* Greybus Camera Flush response payload - request has no payload */
···
 	__le16 frame_number;
 	__u8 stream;
 	__u8 padding;
-	__u8 metadata[0];
+	__u8 metadata[];
 } __packed;
 
 /* Lights */
···
 struct gb_audio_enumerated {
 	__le32	items;
 	__le16	names_length;
-	__u8	names[0];
+	__u8	names[];
 } __packed;
 
 struct gb_audio_ctl_elem_info { /* See snd_ctl_elem_info in Linux source */
···
 	__u8	type;		/* GB_AUDIO_WIDGET_TYPE_* */
 	__u8	state;		/* GB_AUDIO_WIDGET_STATE_* */
 	__u8	ncontrols;
-	struct gb_audio_control	ctl[0];	/* 'ncontrols' entries */
+	struct gb_audio_control	ctl[];	/* 'ncontrols' entries */
 } __packed;
 
 struct gb_audio_route {
···
 	 * struct gb_audio_widget	widgets[num_widgets];
 	 * struct gb_audio_route	routes[num_routes];
 	 */
-	__u8	data[0];
+	__u8	data[];
 } __packed;
 
 struct gb_audio_get_topology_size_response {
···
 
 struct gb_audio_send_data_request {
 	__le64	timestamp;
-	__u8	data[0];
+	__u8	data[];
 } __packed;
 
 
···
 
 struct gb_log_send_log_request {
 	__le16	len;
-	__u8    msg[0];
+	__u8    msg[];
 } __packed;
 
 #endif /* __GREYBUS_PROTOCOLS_H */