can: flexcan: Use struct_group() to zero struct flexcan_regs regions

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Add struct_group() to mark both regions of struct flexcan_regs that get
initialized to zero. Avoid the future warnings:

In function 'fortify_memset_chk',
    inlined from 'memset_io' at ./include/asm-generic/io.h:1169:2,
    inlined from 'flexcan_ram_init' at drivers/net/can/flexcan.c:1403:2:
./include/linux/fortify-string.h:199:4: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
  199 |    __write_overflow_field(p_size_field, size);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function 'fortify_memset_chk',
    inlined from 'memset_io' at ./include/asm-generic/io.h:1169:2,
    inlined from 'flexcan_ram_init' at drivers/net/can/flexcan.c:1408:3:
./include/linux/fortify-string.h:199:4: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
  199 |    __write_overflow_field(p_size_field, size);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: linux-can@vger.kernel.org
Cc: netdev@vger.kernel.org
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Kees Cook <keescook@chromium.org>

drivers/net/can/flexcan.c

@@ -290,31 +290,33 @@ struct flexcan_regs {
 	u32 dbg1;		/* 0x58 */
 	u32 dbg2;		/* 0x5c */
 	u32 _reserved3[8];	/* 0x60 */
-	u8 mb[2][512];		/* 0x80 - Not affected by Soft Reset */
-	/* FIFO-mode:
-	 *			MB
-	 * 0x080...0x08f	0	RX message buffer
-	 * 0x090...0x0df	1-5	reserved
-	 * 0x0e0...0x0ff	6-7	8 entry ID table
-	 *				(mx25, mx28, mx35, mx53)
-	 * 0x0e0...0x2df	6-7..37	8..128 entry ID table
-	 *				size conf'ed via ctrl2::RFFN
-	 *				(mx6, vf610)
-	 */
-	u32 _reserved4[256];	/* 0x480 */
-	u32 rximr[64];		/* 0x880 - Not affected by Soft Reset */
-	u32 _reserved5[24];	/* 0x980 */
-	u32 gfwr_mx6;		/* 0x9e0 - MX6 */
-	u32 _reserved6[39];	/* 0x9e4 */
-	u32 _rxfir[6];		/* 0xa80 */
-	u32 _reserved8[2];	/* 0xa98 */
-	u32 _rxmgmask;		/* 0xaa0 */
-	u32 _rxfgmask;		/* 0xaa4 */
-	u32 _rx14mask;		/* 0xaa8 */
-	u32 _rx15mask;		/* 0xaac */
-	u32 tx_smb[4];		/* 0xab0 */
-	u32 rx_smb0[4];		/* 0xac0 */
-	u32 rx_smb1[4];		/* 0xad0 */
+	struct_group(init,
+		u8 mb[2][512];		/* 0x80 - Not affected by Soft Reset */
+		/* FIFO-mode:
+		 *			MB
+		 * 0x080...0x08f	0	RX message buffer
+		 * 0x090...0x0df	1-5	reserved
+		 * 0x0e0...0x0ff	6-7	8 entry ID table
+		 *				(mx25, mx28, mx35, mx53)
+		 * 0x0e0...0x2df	6-7..37	8..128 entry ID table
+		 *				size conf'ed via ctrl2::RFFN
+		 *				(mx6, vf610)
+		 */
+		u32 _reserved4[256];	/* 0x480 */
+		u32 rximr[64];		/* 0x880 - Not affected by Soft Reset */
+		u32 _reserved5[24];	/* 0x980 */
+		u32 gfwr_mx6;		/* 0x9e0 - MX6 */
+		u32 _reserved6[39];	/* 0x9e4 */
+		u32 _rxfir[6];		/* 0xa80 */
+		u32 _reserved8[2];	/* 0xa98 */
+		u32 _rxmgmask;		/* 0xaa0 */
+		u32 _rxfgmask;		/* 0xaa4 */
+		u32 _rx14mask;		/* 0xaa8 */
+		u32 _rx15mask;		/* 0xaac */
+		u32 tx_smb[4];		/* 0xab0 */
+		u32 rx_smb0[4];		/* 0xac0 */
+		u32 rx_smb1[4];		/* 0xad0 */
+	);
 	u32 mecr;		/* 0xae0 */
 	u32 erriar;		/* 0xae4 */
 	u32 erridpr;		/* 0xae8 */
@@ -328,9 +330,11 @@ struct flexcan_regs {
 	u32 fdcbt;		/* 0xc04 - Not affected by Soft Reset */
 	u32 fdcrc;		/* 0xc08 */
 	u32 _reserved9[199];	/* 0xc0c */
-	u32 tx_smb_fd[18];	/* 0xf28 */
-	u32 rx_smb0_fd[18];	/* 0xf70 */
-	u32 rx_smb1_fd[18];	/* 0xfb8 */
+	struct_group(init_fd,
+		u32 tx_smb_fd[18];	/* 0xf28 */
+		u32 rx_smb0_fd[18];	/* 0xf70 */
+		u32 rx_smb1_fd[18];	/* 0xfb8 */
+	);
 };
 
 static_assert(sizeof(struct flexcan_regs) ==  0x4 * 18 + 0xfb8);
@@ -1400,14 +1404,10 @@ static void flexcan_ram_init(struct net_device *dev)
 	reg_ctrl2 |= FLEXCAN_CTRL2_WRMFRZ;
 	priv->write(reg_ctrl2, &regs->ctrl2);
 
-	memset_io(&regs->mb[0][0], 0,
-		  offsetof(struct flexcan_regs, rx_smb1[3]) -
-		  offsetof(struct flexcan_regs, mb[0][0]) + 0x4);
+	memset_io(&regs->init, 0, sizeof(regs->init));
 
 	if (priv->can.ctrlmode & CAN_CTRLMODE_FD)
-		memset_io(&regs->tx_smb_fd[0], 0,
-			  offsetof(struct flexcan_regs, rx_smb1_fd[17]) -
-			  offsetof(struct flexcan_regs, tx_smb_fd[0]) + 0x4);
+		memset_io(&regs->init_fd, 0, sizeof(regs->init_fd));
 
 	reg_ctrl2 &= ~FLEXCAN_CTRL2_WRMFRZ;
 	priv->write(reg_ctrl2, &regs->ctrl2);