USB: cdc-acm: fix TIOCGSERIAL implementation

TIOCSSERIAL is a horrid, underspecified, legacy interface which for most
serial devices is only useful for setting the close_delay and
closing_wait parameters.

The xmit_fifo_size parameter could be used to set the hardware transmit
fifo size of a legacy UART when it could not be detected, but the
interface is limited to eight bits and should be left unset when it is
not used.

Similarly, baud_base could be used to set the UART base clock when it
could not be detected, but might as well be left unset when it is not
known (which is the case for CDC).

Fix the cdc-acm TIOCGSERIAL implementation by dropping its custom
interpretation of the unused xmit_fifo_size and baud_base fields, which
overflowed the former with the URB buffer size and set the latter to the
current line speed. Also return the port line number, which is the only
other value used besides the close parameters.

Note that the current line speed can still be retrieved through the
standard termios interfaces.

Fixes: 18c75720 ("USB: allow users to run setserial with cdc-acm")
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210408131602.27956-4-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -929,8 +929,7 @@ static int get_serial_info(struct tty_struct *tty, struct serial_struct *ss)
 {
 	struct acm *acm = tty->driver_data;
 
-	ss->xmit_fifo_size = acm->writesize;
-	ss->baud_base = le32_to_cpu(acm->line.dwDTERate);
+	ss->line = acm->minor;
 	ss->close_delay	= jiffies_to_msecs(acm->port.close_delay) / 10;
 	ss->closing_wait = acm->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
 				ASYNC_CLOSING_WAIT_NONE :