From: Lukas Wunner lukas@wunner.de
commit f85e04503f369b3f2be28c83fc48b74e19936ebc upstream.
Commit f45709df7731 ("serial: 8250: Don't touch RTS modem control while in rs485 mode") sought to prevent user space from interfering with rs485 communication by ignoring a TIOCMSET ioctl() which changes RTS polarity.
It did so in serial8250_do_set_mctrl(), which turns out to be too deep in the call stack: When a uart_port is opened, RTS polarity is set by the rs485-aware function uart_port_dtr_rts(). It calls down to serial8250_do_set_mctrl() and that particular RTS polarity change should *not* be ignored.
The user-visible result is that on 8250_omap ports which use rs485 with inverse polarity (RTS bit in MCR register is 1 to receive, 0 to send), a newly opened port initially sets up RTS for sending instead of receiving. That's because omap_8250_startup() sets the cached value up->mcr to 0 and omap_8250_restore_regs() subsequently writes it to the MCR register. Due to the commit, serial8250_do_set_mctrl() preserves that incorrect register value:
do_sys_openat2 do_filp_open path_openat vfs_open do_dentry_open chrdev_open tty_open uart_open tty_port_open uart_port_activate uart_startup uart_port_startup serial8250_startup omap_8250_startup # up->mcr = 0 uart_change_speed serial8250_set_termios omap_8250_set_termios omap_8250_restore_regs serial8250_out_MCR # up->mcr written tty_port_block_til_ready uart_dtr_rts uart_port_dtr_rts serial8250_set_mctrl omap8250_set_mctrl serial8250_do_set_mctrl # mcr[1] = 1 ignored
Fix by intercepting RTS changes from user space in uart_tiocmset() instead.
Link: https://lore.kernel.org/linux-serial/20211027111644.1996921-1-baocheng.su@si... Fixes: f45709df7731 ("serial: 8250: Don't touch RTS modem control while in rs485 mode") Cc: Chao Zeng chao.zeng@siemens.com Cc: stable@vger.kernel.org # v5.7+ Reported-by: Su Bao Cheng baocheng.su@siemens.com Reported-by: Jan Kiszka jan.kiszka@siemens.com Tested-by: Su Bao Cheng baocheng.su@siemens.com Signed-off-by: Lukas Wunner lukas@wunner.de Link: https://lore.kernel.org/r/21170e622a1aaf842a50b32146008b5374b3dd1d.163759643... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/serial/8250/8250_port.c | 7 ------- drivers/tty/serial/serial_core.c | 5 +++++ 2 files changed, 5 insertions(+), 7 deletions(-)
--- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2024,13 +2024,6 @@ void serial8250_do_set_mctrl(struct uart struct uart_8250_port *up = up_to_u8250p(port); unsigned char mcr;
- if (port->rs485.flags & SER_RS485_ENABLED) { - if (serial8250_in_MCR(up) & UART_MCR_RTS) - mctrl |= TIOCM_RTS; - else - mctrl &= ~TIOCM_RTS; - } - mcr = serial8250_TIOCM_to_MCR(mctrl);
mcr = (mcr & up->mcr_mask) | up->mcr_force | up->mcr; --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -1075,6 +1075,11 @@ uart_tiocmset(struct tty_struct *tty, un goto out;
if (!tty_io_error(tty)) { + if (uport->rs485.flags & SER_RS485_ENABLED) { + set &= ~TIOCM_RTS; + clear &= ~TIOCM_RTS; + } + uart_update_mctrl(uport, set, clear); ret = 0; }