Messages in this thread Patch in this message | | | From | minyard@acm ... | Subject | [PATCH v2] drivers:tty:pty: Fix a race causing data loss on close | Date | Fri, 2 Oct 2020 08:03:04 -0500 |
| |
From: Corey Minyard <cminyard@mvista.com>
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 <cminyard@mvista.com> ---
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
| |