USB: EHCI: Add register array bounds to HCS ports

The original EHCI register struct used a trailing 0-element array for
addressing the N_PORTS-many available registers. However, after commit
a46af4eb ("USB: EHCI: define extension registers like normal ones")
the 0-element array started to overlap the USBMODE extension register.

To avoid future compile-time warnings about accessing indexes within a
0-element array, rearrange the struct to actually describe the expected
layout (max 15 registers) with a union. All offsets remain the same, and
bounds checking becomes possible on accesses to port_status and hostpc.

There are no binary differences, and struct offsets continue to match.
"pahole --hex -C ehci_regs" before:

struct ehci_regs {
	u32                        command;              /*     0   0x4 */
	u32                        status;               /*   0x4   0x4 */
	u32                        intr_enable;          /*   0x8   0x4 */
	u32                        frame_index;          /*   0xc   0x4 */
	u32                        segment;              /*  0x10   0x4 */
	u32                        frame_list;           /*  0x14   0x4 */
	u32                        async_next;           /*  0x18   0x4 */
	u32                        reserved1[2];         /*  0x1c   0x8 */
	u32                        txfill_tuning;        /*  0x24   0x4 */
	u32                        reserved2[6];         /*  0x28  0x18 */
	/* --- cacheline 1 boundary (64 bytes) --- */
	u32                        configured_flag;      /*  0x40   0x4 */
	u32                        port_status[0];       /*  0x44     0 */
	u32                        reserved3[9];         /*  0x44  0x24 */
	u32                        usbmode;              /*  0x68   0x4 */
	u32                        reserved4[6];         /*  0x6c  0x18 */
	/* --- cacheline 2 boundary (128 bytes) was 4 bytes ago --- */
	u32                        hostpc[0];            /*  0x84     0 */
	u32                        reserved5[17];        /*  0x84  0x44 */
	/* --- cacheline 3 boundary (192 bytes) was 8 bytes ago --- */
	u32                        usbmode_ex;           /*  0xc8   0x4 */

	/* size: 204, cachelines: 4, members: 18 */
	/* last cacheline: 12 bytes */
};

after:

struct ehci_regs {
	u32                        command;              /*     0   0x4 */
	u32                        status;               /*   0x4   0x4 */
	u32                        intr_enable;          /*   0x8   0x4 */
	u32                        frame_index;          /*   0xc   0x4 */
	u32                        segment;              /*  0x10   0x4 */
	u32                        frame_list;           /*  0x14   0x4 */
	u32                        async_next;           /*  0x18   0x4 */
	u32                        reserved1[2];         /*  0x1c   0x8 */
	u32                        txfill_tuning;        /*  0x24   0x4 */
	u32                        reserved2[6];         /*  0x28  0x18 */
	/* --- cacheline 1 boundary (64 bytes) --- */
	u32                        configured_flag;      /*  0x40   0x4 */
	union {
		u32                port_status[15];      /*  0x44  0x3c */
		struct {
			u32        reserved3[9];         /*  0x44  0x24 */
			u32        usbmode;              /*  0x68   0x4 */
		};                                       /*  0x44  0x28 */
	};                                               /*  0x44  0x3c */
	/* --- cacheline 2 boundary (128 bytes) --- */
	u32                        reserved4;            /*  0x80   0x4 */
	u32                        hostpc[15];           /*  0x84  0x3c */
	/* --- cacheline 3 boundary (192 bytes) --- */
	u32                        reserved5[2];         /*  0xc0   0x8 */
	u32                        usbmode_ex;           /*  0xc8   0x4 */

	/* size: 204, cachelines: 4, members: 16 */
	/* last cacheline: 12 bytes */
};

With this fixed, adding -Wzero-length-bounds to the build no longer
produces several warnings like this:

In file included from drivers/usb/host/ehci-hcd.c:306:
drivers/usb/host/ehci-hub.c: In function 'ehci_port_handed_over':
drivers/usb/host/ehci-hub.c:1194:8: warning: array subscript '<unknown>' is outside the bounds of an interior zero-length array 'u32[0]' {aka 'unsigned int[]'} [-Wzero-length-bounds]
 1194 |  reg = &ehci->regs->port_status[portnum - 1];
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/usb/host/ehci.h:274,
                 from drivers/usb/host/ehci-hcd.c:97:
./include/linux/usb/ehci_def.h:130:7: note: while referencing 'port_status'
  130 |  u32  port_status[0]; /* up to N_PORTS */
      |       ^~~~~~~~~~~

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Al Cooper <alcooperx@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-usb@vger.kernel.org
Acked-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NKees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210818173018.2259231-2-keescook@chromium.orgSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb/ehci_def.h

@@ -45,6 +45,7 @@ struct ehci_caps {
 #define HCS_PORTROUTED(p)	((p)&(1 << 7))	/* true: port routing */
 #define HCS_PPC(p)		((p)&(1 << 4))	/* true: port power control */
 #define HCS_N_PORTS(p)		(((p)>>0)&0xf)	/* bits 3:0, ports on HC */
+#define HCS_N_PORTS_MAX		15		/* N_PORTS valid 0x1-0xF */
 
 	u32		hcc_params;      /* HCCPARAMS - offset 0x8 */
 /* EHCI 1.1 addendum */
@@ -126,8 +127,9 @@ struct ehci_regs {
 	u32		configured_flag;
 #define FLAG_CF		(1<<0)		/* true: we'll support "high speed" */
 
-	/* PORTSC: offset 0x44 */
-	u32		port_status[0];	/* up to N_PORTS */
+	union {
+		/* PORTSC: offset 0x44 */
+		u32	port_status[HCS_N_PORTS_MAX];	/* up to N_PORTS */
 /* EHCI 1.1 addendum */
 #define PORTSC_SUSPEND_STS_ACK 0
 #define PORTSC_SUSPEND_STS_NYET 1
@@ -164,28 +166,28 @@ struct ehci_regs {
 #define PORT_CSC	(1<<1)		/* connect status change */
 #define PORT_CONNECT	(1<<0)		/* device connected */
 #define PORT_RWC_BITS   (PORT_CSC | PORT_PEC | PORT_OCC)
-
-	u32		reserved3[9];
-
-	/* USBMODE: offset 0x68 */
-	u32		usbmode;	/* USB Device mode */
+		struct {
+			u32	reserved3[9];
+			/* USBMODE: offset 0x68 */
+			u32	usbmode;	/* USB Device mode */
+		};
 #define USBMODE_SDIS	(1<<3)		/* Stream disable */
 #define USBMODE_BE	(1<<2)		/* BE/LE endianness select */
 #define USBMODE_CM_HC	(3<<0)		/* host controller mode */
 #define USBMODE_CM_IDLE	(0<<0)		/* idle state */
-
-	u32		reserved4[6];
+	};
+	u32		reserved4;
 
 /* Moorestown has some non-standard registers, partially due to the fact that
  * its EHCI controller has both TT and LPM support. HOSTPCx are extensions to
  * PORTSCx
  */
 	/* HOSTPC: offset 0x84 */
-	u32		hostpc[0];	/* HOSTPC extension */
+	u32		hostpc[HCS_N_PORTS_MAX];
 #define HOSTPC_PHCD	(1<<22)		/* Phy clock disable */
 #define HOSTPC_PSPD	(3<<25)		/* Port speed detection */
 
-	u32		reserved5[17];
+	u32		reserved5[2];
 
 	/* USBMODE_EX: offset 0xc8 */
 	u32		usbmode_ex;	/* USB Device mode extension */