Re: [PATCH 5/5] SLIP: Fix transmission segmentation mechanism

Greg Kroah-Hartman
2013-07-24T22:18:29Z

On Tue, Jul 02, 2013 at 04:31:34PM +0100, Dean Jenkins wrote:
> Binding SLIP with a PTY/TTY can cause truncated SLIP frames to be
> transmitted when the first write to the TTY fails to consume all
> the characters of the SLIP frame.
>
> Asynchronous to the write function is a wakeup event from the TTY
> that indicates the TTY can accept more data. The wakeup event
> calls tty_wakeup() which calls slip_write_wakeup() when
> TTY_DO_WRITE_WAKEUP is set.
>
> To complete the transmission of a SLIP frame to the TTY,
> slip_write_wakeup() must run at least once. Unfortunately, pty_write()
> also calls tty_wakeup() and when TTY_DO_WRITE_WAKEUP is set,
> slip_write_wakeup() is called. xleft is always zero and
> causes slip_write_wakeup() to complete the transmission and
> clears the TTY_DO_WRITE_WAKEUP flag. This can cause a truncated
> SLIP frame because any remaining characters will not get sent.
>
> The wakeup event is unable to process the remaining characters
> because the TTY_DO_WRITE_WAKEUP flag has been cleared.
>
> The code modification fixes the transmission segmentation
> mechanism by preventing pty_write() from calling
> slip_write_wakeup() by clearing TTY_DO_WRITE_WAKEUP before calling
> pty_write(). After pty_write() returns, TTY_DO_WRITE_WAKEUP is set
> to allow the TTY wakeup event to call slip_write_wakeup() to
> attempt to complete the transmission of the SLIP frame.
>
> This may not be foolproof because a timeout is needed to
> break out of the cycle of transmission attempts. Note error
> codes from the TTY layer will break out of the cycle of
> transmission attempts.
>
> Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>