Re: [PATCH] tty: discard flip-buffer input for TCSAFLUSH

[Greg KH]

On Tue, Oct 27, 2026 at 05:43:04AM +0000, Hungyu Lin wrote:
> set_termios() handles TERMIOS_FLUSH by clearing only the line discipline's
> input buffer. Data already committed to the tty flip buffers can be
> delivered after that flush, leaving old input readable after a successful
> tcsetattr(TCSAFLUSH).
> 
> This can be reproduced with a raw PTY: complete an 8192-byte master write,
> wait until the slave reports 4095 unread bytes, and apply TCSAFLUSH.
> The remaining 4097 bytes are still readable afterwards. Small amounts of
> pending input can also survive if they have not reached the line
> discipline yet.
> 
> Use tty_buffer_flush() with the referenced line discipline to discard both
> receive queues under the existing buffer lock. The helper checks for the
> line discipline's flush_buffer callback itself.
> 
> Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>
> ---
>  drivers/tty/tty_ioctl.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
> index 90c70d8d14e3..45cb77185dbb 100644
> --- a/drivers/tty/tty_ioctl.c
> +++ b/drivers/tty/tty_ioctl.c
> @@ -496,8 +496,8 @@ static int set_termios(struct tty_struct *tty, void __user *arg, int opt)
>  
>  		ld = tty_ldisc_ref(tty);
>  		if (ld != NULL) {
> -			if ((opt & TERMIOS_FLUSH) && ld->ops->flush_buffer)
> -				ld->ops->flush_buffer(tty);
> +			if (opt & TERMIOS_FLUSH)
> +				tty_buffer_flush(tty, ld);
>  			tty_ldisc_deref(ld);
>  		}
>  
> -- 
> 2.43.0
> 
> 

What commit id does this fix?  How was this tested?

thanks,

greg k-h