From: Daniel Starke <daniel.starke@xxxxxxxxxxx> commit 317f86af7f5d19f286ed2d181cbaef4a188c7f19 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. The value of the modem status command (MSC) frame contains an address field, control signal and optional break signal octet. The address field is encoded as described in chapter 5.2.1.2 with only one octet (may be extended to more in future versions of the standard). Whereas the control signal and break signal octet are always one byte each. This is strange at first glance as it makes the EA bit redundant. However, the same two octets are also encoded as header in convergence layer type 2 as described in chapter 5.5.2. No header length field is given and the only way to test if there is an optional break signal octet is via the EA flag which extends the control signal octet with a break signal octet. Now it becomes obvious how the EA bit for those two octets shall be encoded in the MSC frame. The current implementation treats the signal octet different for MSC frame and convergence layer type 2 header even though the standard describes it for both in the same way. Use the EA bit to encode the signal octets not only in the convergence layer type 2 header but also in the MSC frame in the same way with either 1 or 2 bytes in case of an optional break signal. Adjust the receiving path accordingly in gsm_control_modem(). Fixes: 3ac06b905655 ("tty: n_gsm: Fix for modems with brk in modem status control") Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Daniel Starke <daniel.starke@xxxxxxxxxxx> Link: https://lore.kernel.org/r/20220414094225.4527-13-daniel.starke@xxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/tty/n_gsm.c | 18 +++++------------- 1 file changed, 5 insertions(+), 13 deletions(-) --- a/drivers/tty/n_gsm.c +++ b/drivers/tty/n_gsm.c @@ -1094,7 +1094,6 @@ static void gsm_control_modem(struct gsm { unsigned int addr = 0; unsigned int modem = 0; - unsigned int brk = 0; struct gsm_dlci *dlci; int len = clen; int slen; @@ -1124,17 +1123,8 @@ static void gsm_control_modem(struct gsm return; } len--; - if (len > 0) { - while (gsm_read_ea(&brk, *dp++) == 0) { - len--; - if (len == 0) - return; - } - modem <<= 7; - modem |= (brk & 0x7f); - } tty = tty_port_tty_get(&dlci->port); - gsm_process_modem(tty, dlci, modem, slen); + gsm_process_modem(tty, dlci, modem, slen - len); if (tty) { tty_wakeup(tty); tty_kref_put(tty); @@ -2963,8 +2953,10 @@ static int gsmtty_modem_update(struct gs int len = 2; modembits[0] = (dlci->addr << 2) | 2 | EA; /* DLCI, Valid, EA */ - modembits[1] = (gsm_encode_modem(dlci) << 1) | EA; - if (brk) { + if (!brk) { + modembits[1] = (gsm_encode_modem(dlci) << 1) | EA; + } else { + modembits[1] = gsm_encode_modem(dlci) << 1; modembits[2] = (brk << 4) | 2 | EA; /* Length, Break, EA */ len++; }
