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SubjectRe: [PATCH][v4] tty: fix race between flush_to_ldisc and tty_open
On Fri, Jan 18, 2019 at 05:27:17PM +0800, Li RongQing wrote:
> There still is a race window after the commit b027e2298bd588
> ("tty: fix data race between tty_init_dev and flush of buf"),
> and we encountered this crash issue if receive_buf call comes
> before tty initialization completes in n_tty_open and
> tty->driver_data may be NULL.
>
> CPU0 CPU1
> ---- ----
> n_tty_open
> tty_init_dev
> tty_ldisc_unlock
> schedule
> flush_to_ldisc
> receive_buf
> tty_port_default_receive_buf
> tty_ldisc_receive_buf
> n_tty_receive_buf_common
> __receive_buf
> uart_flush_chars
> uart_start
> /*tty->driver_data is NULL*/
> tty->ops->open
> /*init tty->driver_data*/
>
> it can be fixed by extending ldisc semaphore lock in tty_init_dev
> to driver_data initialized completely after tty->ops->open(), but
> this will lead to put lock on one function and unlock in some other
> function, and hard to maintain, so fix this race only by checking
> tty->driver_data when receiving, and return if tty->driver_data
> is NULL
>
> Signed-off-by: Wang Li <wangli39@baidu.com>
> Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
> V4: add version information
> V3: not used ldisc semaphore lock, only checking tty->driver_data with NULL
> V2: fix building error by EXPORT_SYMBOL tty_ldisc_unlock
> V1: extend ldisc lock to protect that tty->driver_data is inited
>
> drivers/tty/tty_port.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
> index 044c3cbdcfa4..86d0bec38322 100644
> --- a/drivers/tty/tty_port.c
> +++ b/drivers/tty/tty_port.c
> @@ -31,6 +31,9 @@ static int tty_port_default_receive_buf(struct tty_port *port,
> if (!tty)
> return 0;
>
> + if (!tty->driver_data)
> + return 0;
> +

How is this working? What is setting driver_data to NULL to "stop" this
race?

There's no requirement that a tty driver set this field to NULL when it
is "done" with the tty device, so I think you are just getting lucky in
that your specific driver happens to be doing this.

What driver are you testing this against?

thanks,

greg k-h

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