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    Subject[PATCH 3.12 115/212] n_tty: Fix 4096-byte canonical reads
    Date
    3.12-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Peter Hurley <peter@hurleysoftware.com>

    commit c77569d2f3ef7844ee4ac7005a57da6898b302a8 upstream.

    Although the maximum allowable canonical line is specified to
    be 255 bytes (MAX_CANON), the practical limit has actually been
    the size of the line discipline read buffer (N_TTY_BUF_SIZE == 4096).

    Commit 32f13521ca68bc624ff6effc77f308a52b038bf0,
    n_tty: Line copy to user buffer in canonical mode, limited the
    line copy to 4095 bytes. With a completely full line discipline
    read buffer and a userspace buffer > 4095, _no_ data was copied,
    and the read() syscall returned 0, indicating EOF.

    Fix the interval arithmetic to compute the correct number of bytes
    to copy to userspace in the range [1..4096].

    Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/tty/n_tty.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    --- a/drivers/tty/n_tty.c
    +++ b/drivers/tty/n_tty.c
    @@ -2005,7 +2005,10 @@ static int canon_copy_from_read_buf(stru
    found = 1;

    size = N_TTY_BUF_SIZE - tail;
    - n = (found + eol + size) & (N_TTY_BUF_SIZE - 1);
    + n = eol - tail;
    + if (n > 4096)
    + n += 4096;
    + n += found;
    c = n;

    if (found && read_buf(ldata, eol) == __DISABLED_CHAR) {



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