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SubjectRe: [PATCH] serial: mvebu-uart: Fix reporting of effective CSIZE to userspace
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On 2018-08-26 19:49, Jan Kiszka wrote:
> From: Jan Kiszka <jan.kiszka@siemens.com>
>
> Apparently, this driver (or the hardware) does not support character
> length settings. It's apparently running in 8-bit mode, but it makes
> userspace believe it's in 5-bit mode. That makes tcsetattr with CS8
> incorrectly fail, breaking e.g. getty from busybox, thus the login shell
> on ttyMVx.
>
> Fix by hard-wiring CS8 into c_cflag.
>
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>
> It's a bit of a shame that still maturing drivers can break userspace
> that easily and subtly. I had to debug my way from old but working
> buildroot to busybox, libc (tcsetattr) and then finally this driver.
> This wasn't the first bug of this kind, and maybe it's not the last (I
> didn't check all that termio flags). Could the kernel help in some way
> with sanity checks or sane defaults driver have to make insane
> willingly?
>
> drivers/tty/serial/mvebu-uart.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-uart.c
> index d04b5eeea3c6..170e446a2f62 100644
> --- a/drivers/tty/serial/mvebu-uart.c
> +++ b/drivers/tty/serial/mvebu-uart.c
> @@ -511,6 +511,7 @@ static void mvebu_uart_set_termios(struct uart_port *port,
> termios->c_iflag |= old->c_iflag & ~(INPCK | IGNPAR);
> termios->c_cflag &= CREAD | CBAUD;
> termios->c_cflag |= old->c_cflag & ~(CREAD | CBAUD);
> + termios->c_cflag |= CS8;
> }
>
> spin_unlock_irqrestore(&port->lock, flags);
>

FWIW, below is the analogous fix for stable trees <= 4.14.

Jan

---8<---

From: Jan Kiszka <jan.kiszka@siemens.com>
Subject: [PATCH] serial: mvebu-uart: Fix reporting of effective CSIZE to userspace

Apparently, this driver (or the hardware) does not support character
length settings. It's apparently running in 8-bit mode, but it makes
userspace believe it's in 5-bit mode. That makes tcsetattr with CS8
incorrectly fail, breaking e.g. getty from busybox, thus the login shell
on ttyMVx.

Fix by hard-wiring CS8 into c_cflag.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
drivers/tty/serial/mvebu-uart.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-uart.c
index 45b57c294d13..401c983ec5f3 100644
--- a/drivers/tty/serial/mvebu-uart.c
+++ b/drivers/tty/serial/mvebu-uart.c
@@ -327,8 +327,10 @@ static void mvebu_uart_set_termios(struct uart_port *port,
if ((termios->c_cflag & CREAD) == 0)
port->ignore_status_mask |= STAT_RX_RDY | STAT_BRK_ERR;

- if (old)
+ if (old) {
tty_termios_copy_hw(termios, old);
+ termios->c_cflag |= CS8;
+ }

baud = uart_get_baud_rate(port, termios, old, 0, 460800);
uart_update_timeout(port, termios->c_cflag, baud);
--
2.16.4
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