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    Subject[PATCH 5.4 75/84] tty: n_gsm: fix wrong signal octet encoding in convergence layer type 2
    Date
    From: Daniel Starke <daniel.starke@siemens.com>

    commit 06d5afd4d640eea67f5623e76cd5fc03359b7f3c upstream.

    n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
    See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
    The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
    the newer 27.010 here. Chapter 5.5.2 describes that the signal octet in
    convergence layer type 2 can be either one or two bytes. The length is
    encoded in the EA bit. This is set 1 for the last byte in the sequence.
    gsmtty_modem_update() handles this correctly but gsm_dlci_data_output()
    fails to set EA to 1. There is no case in which we encode two signal octets
    as there is no case in which we send out a break signal.
    Therefore, always set the EA bit to 1 for the signal octet to fix this.

    Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
    Cc: stable@vger.kernel.org
    Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
    Link: https://lore.kernel.org/r/20220414094225.4527-5-daniel.starke@siemens.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/tty/n_gsm.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/drivers/tty/n_gsm.c
    +++ b/drivers/tty/n_gsm.c
    @@ -823,7 +823,7 @@ static int gsm_dlci_data_output(struct g
    break;
    case 2: /* Unstructed with modem bits.
    Always one byte as we never send inline break data */
    - *dp++ = gsm_encode_modem(dlci);
    + *dp++ = (gsm_encode_modem(dlci) << 1) | EA;
    break;
    }
    WARN_ON(kfifo_out_locked(dlci->fifo, dp , len, &dlci->lock) != len);

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