USB: 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 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200220132017.GA29262@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/atm/usbatm.h

@@ -164,7 +164,7 @@ struct usbatm_data {
 	unsigned char *cell_buf;	/* holds partial rx cell */
 	unsigned int buf_usage;
 
-	struct urb *urbs[0];
+	struct urb *urbs[];
 };
 
 static inline void *to_usbatm_driver_data(struct usb_interface *intf)

drivers/usb/dwc2/hcd.h

@@ -199,7 +199,7 @@ struct dwc2_hcd_urb {
 	u32 flags;
 	u16 interval;
 	struct dwc2_hcd_pipe_info pipe_info;
-	struct dwc2_hcd_iso_packet_desc iso_descs[0];
+	struct dwc2_hcd_iso_packet_desc iso_descs[];
 };
 
 /* Phases for control transfers */

drivers/usb/host/ehci-tegra.c

@@ -282,7 +282,7 @@ static int tegra_ehci_hub_control(
 struct dma_aligned_buffer {
 	void *kmalloc_ptr;
 	void *old_xfer_buffer;
-	u8 data[0];
+	u8 data[];
 };
 
 static void free_dma_aligned_buffer(struct urb *urb)

drivers/usb/host/ehci.h

@@ -255,7 +255,7 @@ struct ehci_hcd {			/* one per controller */
 	struct list_head	tt_list;
 
 	/* platform-specific data -- must come last */
-	unsigned long		priv[0] __aligned(sizeof(s64));
+	unsigned long		priv[] __aligned(sizeof(s64));
 };
 
 /* convert between an HCD pointer and the corresponding EHCI_HCD */
@@ -460,7 +460,7 @@ struct ehci_iso_sched {
 	struct list_head	td_list;
 	unsigned		span;
 	unsigned		first_packet;
-	struct ehci_iso_packet	packet[0];
+	struct ehci_iso_packet	packet[];
 };
 
 /*

drivers/usb/host/fotg210.h

@@ -490,7 +490,7 @@ struct fotg210_iso_packet {
 struct fotg210_iso_sched {
 	struct list_head	td_list;
 	unsigned		span;
-	struct fotg210_iso_packet	packet[0];
+	struct fotg210_iso_packet	packet[];
 };
 
 /*

drivers/usb/host/ohci.h

@@ -337,7 +337,7 @@ typedef struct urb_priv {
 	u16			length;		// # tds in this request
 	u16			td_cnt;		// tds already serviced
 	struct list_head	pending;
-	struct td		*td [0];	// all TDs in this request
+	struct td		*td[];		// all TDs in this request
 
 } urb_priv_t;
 
@@ -435,7 +435,7 @@ struct ohci_hcd {
 	struct dentry		*debug_dir;
 
 	/* platform-specific data -- must come last */
-	unsigned long           priv[0] __aligned(sizeof(s64));
+	unsigned long           priv[] __aligned(sizeof(s64));
 
 };
 

drivers/usb/host/xhci-mtk.h

@@ -95,7 +95,7 @@ struct mu3h_sch_ep_info {
 	u32 pkts;
 	u32 cs_count;
 	u32 burst_mode;
-	u32 bw_budget_table[0];
+	u32 bw_budget_table[];
 };
 
 #define MU3C_U3_PORT_MAX 4

drivers/usb/host/xhci.h

@@ -1642,7 +1642,7 @@ struct xhci_scratchpad {
 struct urb_priv {
 	int	num_tds;
 	int	num_tds_done;
-	struct	xhci_td	td[0];
+	struct	xhci_td	td[];
 };
 
 /*
@@ -1893,7 +1893,7 @@ struct xhci_hcd {
 
 	void			*dbc;
 	/* platform-specific data -- must come last */
-	unsigned long		priv[0] __aligned(sizeof(s64));
+	unsigned long		priv[] __aligned(sizeof(s64));
 };
 
 /* Platform specific overrides to generic XHCI hc_driver ops */

drivers/usb/serial/io_usbvend.h

@@ -593,7 +593,7 @@ struct ti_i2c_desc {
 	__u8	Type;			// Type of descriptor
 	__le16	Size;			// Size of data only not including header
 	__u8	CheckSum;		// Checksum (8 bit sum of data only)
-	__u8	Data[0];		// Data starts here
+	__u8	Data[];		// Data starts here
 } __attribute__((packed));
 
 // for 5152 devices only (type 2 record)
@@ -601,7 +601,7 @@ struct ti_i2c_desc {
 struct ti_i2c_firmware_rec {
 	__u8	Ver_Major;		// Firmware Major version number
 	__u8	Ver_Minor;		// Firmware Minor version number
-	__u8	Data[0];		// Download starts here
+	__u8	Data[];		// Download starts here
 } __attribute__((packed));
 
 

drivers/usb/serial/ti_usb_3410_5052.c

@@ -219,7 +219,7 @@ struct ti_write_data_bytes {
 	u8	bDataCounter;
 	__be16	wBaseAddrHi;
 	__be16	wBaseAddrLo;
-	u8	bData[0];
+	u8	bData[];
 } __packed;
 
 struct ti_read_data_request {
@@ -234,7 +234,7 @@ struct ti_read_data_bytes {
 	__u8	bCmdCode;
 	__u8	bModuleId;
 	__u8	bErrorCode;
-	__u8	bData[0];
+	__u8	bData[];
 } __packed;
 
 /* Interrupt struct */

include/linux/usb.h

@@ -325,7 +325,7 @@ struct usb_interface_cache {
 
 	/* variable-length array of alternate settings for this interface,
 	 * stored in no particular order */
-	struct usb_host_interface altsetting[0];
+	struct usb_host_interface altsetting[];
 };
 #define	ref_to_usb_interface_cache(r) \
 		container_of(r, struct usb_interface_cache, ref)
@@ -1589,7 +1589,7 @@ struct urb {
 	int error_count;		/* (return) number of ISO errors */
 	void *context;			/* (in) context for completion */
 	usb_complete_t complete;	/* (in) completion routine */
-	struct usb_iso_packet_descriptor iso_frame_desc[0];
+	struct usb_iso_packet_descriptor iso_frame_desc[];
 					/* (in) ISO ONLY */
 };
 

include/linux/usb/audio-v2.h

@@ -153,7 +153,7 @@ struct uac2_feature_unit_descriptor {
 	__u8 bSourceID;
 	/* bmaControls is actually u32,
 	 * but u8 is needed for the hybrid parser */
-	__u8 bmaControls[0]; /* variable length */
+	__u8 bmaControls[]; /* variable length */
 } __attribute__((packed));
 
 /* 4.9.2 Class-Specific AS Interface Descriptor */

include/linux/usb/audio-v3.h

@@ -109,7 +109,7 @@ struct uac3_feature_unit_descriptor {
 	__u8 bSourceID;
 	/* bmaControls is actually u32,
 	 * but u8 is needed for the hybrid parser */
-	__u8 bmaControls[0]; /* variable length */
+	__u8 bmaControls[]; /* variable length */
 	/* wFeatureDescrStr omitted */
 } __attribute__((packed));
 

include/linux/usb/gadget.h

@@ -767,7 +767,7 @@ struct usb_gadget_strings {
 
 struct usb_gadget_string_container {
 	struct list_head        list;
-	u8                      *stash[0];
+	u8                      *stash[];
 };
 
 /* put descriptor for string with that id into buf (buflen >= 256) */

include/linux/usb/hcd.h

@@ -228,7 +228,7 @@ struct usb_hcd {
 	/* The HC driver's private data is stored at the end of
 	 * this structure.
 	 */
-	unsigned long hcd_priv[0]
+	unsigned long hcd_priv[]
 			__attribute__ ((aligned(sizeof(s64))));
 };
 

include/linux/usbdevice_fs.h

@@ -69,7 +69,7 @@ struct usbdevfs_urb32 {
 	compat_int_t error_count;
 	compat_uint_t signr;
 	compat_caddr_t usercontext; /* unused */
-	struct usbdevfs_iso_packet_desc iso_frame_desc[0];
+	struct usbdevfs_iso_packet_desc iso_frame_desc[];
 };
 
 struct usbdevfs_ioctl32 {