On Fri, Oct 02, 2020 at 08:03:04AM -0500, [email protected] wrote: > From: Corey Minyard <[email protected]> > > If you write to a pty master an immediately close the pty master, the > receiver might get a chunk of data dropped, but then receive some later > data. That's obviously something rather unexpected for a user. It > certainly confused my test program. > > It turns out that tty_vhangup() gets called from pty_close(), and that > causes the data on the slave side to be flushed, but due to races more > data can be copied into the slave side's buffer after that. Consider > the following sequence: > > thread1 thread2 thread3 > ------- ------- ------- > | |-write data into buffer, > | | n_tty buffer is filled > | | along with other buffers > | |-pty_close() > | |--tty_vhangup() > | |---tty_ldisc_hangup() > | |----n_tty_flush_buffer() > | |-----reset_buffer_flags() > |-n_tty_read() | > |--up_read(&tty->termios_rwsem); > | |------down_read(&tty->termios_rwsem) > | |------clear n_tty buffer contents > | |------up_read(&tty->termios_rwsem) > |--tty_buffer_flush_work() | > |--schedules work calling | > | flush_to_ldisc() | > | |-flush_to_ldisc() > | |--receive_buf() > | |---tty_port_default_receive_buf() > | |----tty_ldisc_receive_buf() > | |-----n_tty_receive_buf2() > | |------n_tty_receive_buf_common() > | |-------down_read(&tty->termios_rwsem) > | |-------__receive_buf() > | |-------copies data into n_tty buffer > | |-------up_read(&tty->termios_rwsem) > |--down_read(&tty->termios_rwsem) > |--copy buffer data to user > > >From this sequence, you can see that thread2 writes to the buffer then > only clears the part of the buffer in n_tty. The n_tty receive buffer > code then copies more data into the n_tty buffer. > > This change checks to see if the tty is being hung up before copying > anything in n_tty_receive_buf_common(). It has to be done after the > tty->termios_rwsem semaphore is claimed, for reasons that should be > apparent from the sequence above. > > Signed-off-by: Corey Minyard <[email protected]> > --- > > Changes since v1: Added lines to make the sequence diagram clearer. > > I sent a program to reproduce this, I extended it to prove it wasn't the > test program that caused the issue, and I've uploaded it to: > http://sourceforge.net/projects/ser2net/files/tmp/testpty.c > if you want to run it. It has a lot of comments that explain what is > going on. > > This is not a very satisfying fix, though. It works reliably, but it > doesn't seem right to me. My inclination was to remove the up and down > semaphore around tty_buffer_flush_work() in n_tty_read(), as it just > schedules some work, no need to unlock for that. But that resulted > in a deadlock elsewhere, so that up/down on the semaphore is there for > another reason. > > The locking in the tty code is really hard to follow. I believe this is > actually a locking problem, but fixing it looks daunting to me. > > Another way to fix this that occurred just occurred to me would be to > clear all the buffers in pty_close(). > > -corey > > > drivers/tty/n_tty.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c > index 1794d84e7bf6..1c33c26dc229 100644 > --- a/drivers/tty/n_tty.c > +++ b/drivers/tty/n_tty.c > @@ -1704,6 +1704,9 @@ n_tty_receive_buf_common(struct tty_struct *tty, const > unsigned char *cp, > > down_read(&tty->termios_rwsem); > > + if (test_bit(TTY_HUPPING, &tty->flags)) > + goto out_upsem; > + > do { > /* > * When PARMRK is set, each input char may take up to 3 chars > @@ -1760,6 +1763,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const > unsigned char *cp, > } else > n_tty_check_throttle(tty); > > +out_upsem: > up_read(&tty->termios_rwsem); > > return rcvd; > -- > 2.17.1 >
Jiri, any thoughts about this? At first glance, it looks sane to me, but a second review would be great. thanks, greg k-h
