tty: Avoid the use of one-element arrays

One-element arrays are being deprecated[1]. Replace the one-element arrays
with simple value types 'char reserved_char' and 'compat_int_t reserved'[2],
once it seems these are just placeholders for alignment.

Also, while there, use the preferred form for passing a size of a struct.
The alternative form where struct name is spelled out hurts readability
and introduces an opportunity for a bug when the variable type is changed
but the corresponding sizeof that is passed as argument is not.

Lastly, fix the checkpatch.pl warnings below:

ERROR: code indent should use tabs where possible
+        char    reserved_char;$

WARNING: please, no spaces at the start of a line
+        char    reserved_char;$

ERROR: code indent should use tabs where possible
+        compat_int_t    reserved;$

WARNING: please, no spaces at the start of a line
+        compat_int_t    reserved;$

[1] KSPP#79
[2] KSPP#86

Tested-by: kernel test robot <[email protected]>
Link: https://github.com/GustavoARSilva/linux-hardening/blob/master/cii/0-day/tty-20200716.md
Acked-by: Jiri Slaby <[email protected]>
Signed-off-by: Gustavo A. R. Silva <[email protected]>

drivers/tty/tty_io.c

@@ -2673,25 +2673,25 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 #ifdef CONFIG_COMPAT
 
 struct serial_struct32 {
-        compat_int_t    type;
-        compat_int_t    line;
-        compat_uint_t   port;
-        compat_int_t    irq;
-        compat_int_t    flags;
-        compat_int_t    xmit_fifo_size;
-        compat_int_t    custom_divisor;
-        compat_int_t    baud_base;
-        unsigned short  close_delay;
-        char    io_type;
-        char    reserved_char[1];
-        compat_int_t    hub6;
-        unsigned short  closing_wait; /* time to wait before closing */
-        unsigned short  closing_wait2; /* no longer used... */
-        compat_uint_t   iomem_base;
-        unsigned short  iomem_reg_shift;
-        unsigned int    port_high;
-     /* compat_ulong_t  iomap_base FIXME */
-        compat_int_t    reserved[1];
+	compat_int_t    type;
+	compat_int_t    line;
+	compat_uint_t   port;
+	compat_int_t    irq;
+	compat_int_t    flags;
+	compat_int_t    xmit_fifo_size;
+	compat_int_t    custom_divisor;
+	compat_int_t    baud_base;
+	unsigned short  close_delay;
+	char    io_type;
+	char    reserved_char;
+	compat_int_t    hub6;
+	unsigned short  closing_wait; /* time to wait before closing */
+	unsigned short  closing_wait2; /* no longer used... */
+	compat_uint_t   iomem_base;
+	unsigned short  iomem_reg_shift;
+	unsigned int    port_high;
+	/* compat_ulong_t  iomap_base FIXME */
+	compat_int_t    reserved;
 };
 
 static int compat_tty_tiocsserial(struct tty_struct *tty,
@@ -2705,7 +2705,7 @@ static int compat_tty_tiocsserial(struct tty_struct *tty,
 	struct serial_struct v;
 	int flags;
 
-	if (copy_from_user(&v32, ss, sizeof(struct serial_struct32)))
+	if (copy_from_user(&v32, ss, sizeof(*ss)))
 		return -EFAULT;
 
 	memcpy(&v, &v32, offsetof(struct serial_struct32, iomem_base));
@@ -2743,7 +2743,7 @@ static int compat_tty_tiocgserial(struct tty_struct *tty,
 			0xfffffff : ptr_to_compat(v.iomem_base);
 		v32.iomem_reg_shift = v.iomem_reg_shift;
 		v32.port_high = v.port_high;
-		if (copy_to_user(ss, &v32, sizeof(struct serial_struct32)))
+		if (copy_to_user(ss, &v32, sizeof(v32)))
 			err = -EFAULT;
 	}
 	return err;