Re: [PATCH] n_tty: Fix unordered accesses to lockless read buffer

[Peter Hurley]

Hi Christian,

On 01/01/2015 08:55 AM, Christian Riesch wrote:
> On Thu, Jan 1, 2015 at 12:00 PM, Christian Riesch >> @@ -164,15
> +160,17 @@ static inline int tty_put_user(struct tty_struct *tty,
> unsigned char x,
>>>  static int receive_room(struct tty_struct *tty)
>>>  {
>>>         struct n_tty_data *ldata = tty->disc_data;
>>> +       size_t head = ACCESS_ONCE(ldata->commit_head);
>>> +       size_t tail = ACCESS_ONCE(ldata->read_tail);
>>>         int left;
>>>
>>>         if (I_PARMRK(tty)) {
>>> -               /* Multiply read_cnt by 3, since each byte might take up to
>>> +               /* Multiply count by 3, since each byte might take up to
>>>                  * three times as many spaces when PARMRK is set (depending on
>>>                  * its flags, e.g. parity error). */
>>> -               left = N_TTY_BUF_SIZE - read_cnt(ldata) * 3 - 1;
>>> +               left = N_TTY_BUF_SIZE - (head - tail) * 3 - 1;
>>>         } else
>>> -               left = N_TTY_BUF_SIZE - read_cnt(ldata) - 1;
>>> +               left = N_TTY_BUF_SIZE - (head - tail) - 1;
>>
>> Actually, less room may be available, if read_head != commit_head.
>> Could this cause problems? I guess yes, at least in
>> n_tty_receive_buf_common, where this could lead to a buffer overflow,
>> right?
> 
> Sorry, should not be a problem, at least not for
> n_tty_receive_buf_common, since this is producer path, right?

Yeah, that's what I was in the process of writing just now.
BTW, I did see your note about the I_PARMRK computation being
overly conservative; I'll address that in a separate patch
on top of this.

> But how about the other calls of receive_room()?

Those are all either consumer-side or exclusive, ie., when both
producer and consumer are prevented from running by the termios_rwsem
write lock (eg., n_tty_set_termios()).

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