tty: fix termios input-speed encoding when using BOTHER

[ Upstream commit 1cee38f0 ]

When the termios CIBAUD bits are left unset (i.e. B0), we use the same
output and input speed and should leave CIBAUD unchanged.

When the user requests a rate using BOTHER and c_ospeed which the driver
cannot set exactly, the driver can report back the actual baud rate
using tty_termios_encode_baud_rate(). If this rate is close enough to a
standard rate however, we could end up setting CIBAUD to a Bfoo value
despite the user having left it unset.

This in turn could lead to an unexpected input rate being set on
subsequent termios updates.

Fix this by using a zero tolerance value also for the input rate when
CIBAUD is clear so that the matching logic works as expected.

Fixes: 78137e3b ("[PATCH] tty: improve encode_baud_rate logic")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_baudrate.c

@@ -156,16 +156,20 @@ void tty_termios_encode_baud_rate(struct ktermios *termios,
 	termios->c_ospeed = obaud;
 
 #ifdef BOTHER
+	if ((termios->c_cflag >> IBSHIFT) & CBAUD)
+		ibinput = 1;	/* An input speed was specified */
+
 	/* If the user asked for a precise weird speed give a precise weird
 	   answer. If they asked for a Bfoo speed they may have problems
 	   digesting non-exact replies so fuzz a bit */
 
-	if ((termios->c_cflag & CBAUD) == BOTHER)
+	if ((termios->c_cflag & CBAUD) == BOTHER) {
 		oclose = 0;
+		if (!ibinput)
+			iclose = 0;
+	}
 	if (((termios->c_cflag >> IBSHIFT) & CBAUD) == BOTHER)
 		iclose = 0;
-	if ((termios->c_cflag >> IBSHIFT) & CBAUD)
-		ibinput = 1;	/* An input speed was specified */
 #endif
 	termios->c_cflag &= ~CBAUD;