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    SubjectRe: [Bug #14388] keyboard under X with 2.6.31
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    On Tue, 2009-10-13 at 15:42 -0700, Linus Torvalds wrote:

    > They are added to the tail only if the tail is non-NULL.

    True

    tail is null only after:
    * initialization - no flush_to_ldisc in progress
    * tty_flush_buffer - protected from flush_to_ldisc by TTY_FLUSHING

    flush_to_ldisc does not currently set tail to null after
    flushing data from the last buffer. Each buffer is marked
    individually as consumed so no more data will be added to it.
    The buffer is marked empty again as it is recycled.

    However, looking at this does reveal a problem.

    If an individual buffer of 512 bytes or larger gets into the
    free and full lists, that buffer is kfreed in tty_buffer_free()
    instead of being recycled. That means that tty->buf.tail can
    point to a freed block of memory, at least until another buffer
    is allocated.

    If that buffer is written to while in this state, the fields
    will become incoherent and may result in more data being added to it.

    I think the patch below fixes this problem. It sets tail to null
    when all buffers are flushed. This is only executed after the buffer
    has been passed to the ldisc and the spinlock is held so there is
    no place for more data to be added incorrectly. I will test it myself
    tomorrow when I get back to the office.

    > I do object to the whole crazy subtle TTY locking. I'm convinced it's
    > wrong, and I'm convinced it's wrong exactly _because_ it tries to be so
    > subtle and does non-obvious things.

    I understand. The patch below should fix the hole above, and I'm not
    aware of any other hole. But I you prefer reworking the locking
    to make things more obvious, I have no objection.

    --- a/drivers/char/tty_buffer.c 2009-09-09 17:13:59.000000000 -0500
    +++ b/drivers/char/tty_buffer.c 2009-10-13 18:34:34.000000000 -0500
    @@ -423,6 +423,8 @@ static void flush_to_ldisc(struct work_s
    break;
    tbuf = head;
    head = head->next;
    + if (!head)
    + tty->buf.tail = NULL;
    tty_buffer_free(tty, tbuf);
    continue;
    }



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