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    Subject[PATCH 3.12 038/176] n_tty: Fix read_buf race condition, increment read_head after pushing data
    Date
    From: Christian Riesch <christian.riesch@omicron.at>

    3.12-stable review patch. If anyone has any objections, please let me know.

    ===============

    commit 8bfbe2de769afda051c56aba5450391670e769fc upstream.

    Commit 19e2ad6a09f0c06dbca19c98e5f4584269d913dd ("n_tty: Remove overflow
    tests from receive_buf() path") moved the increment of read_head into
    the arguments list of read_buf_addr(). Function calls represent a
    sequence point in C. Therefore read_head is incremented before the
    character c is placed in the buffer. Since the circular read buffer is
    a lock-less design since commit 6d76bd2618535c581f1673047b8341fd291abc67
    ("n_tty: Make N_TTY ldisc receive path lockless"), this creates a race
    condition that leads to communication errors.

    This patch modifies the code to increment read_head _after_ the data
    is placed in the buffer and thus fixes the race for non-SMP machines.
    To fix the problem for SMP machines, memory barriers must be added in
    a separate patch.

    Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    ---
    drivers/tty/n_tty.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
    index 1197767b3019..d711dbb6d9fb 100644
    --- a/drivers/tty/n_tty.c
    +++ b/drivers/tty/n_tty.c
    @@ -319,7 +319,8 @@ static void n_tty_check_unthrottle(struct tty_struct *tty)

    static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
    {
    - *read_buf_addr(ldata, ldata->read_head++) = c;
    + *read_buf_addr(ldata, ldata->read_head) = c;
    + ldata->read_head++;
    }

    /**
    --
    2.2.2


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