tty: n_gsm: fix sometimes uninitialized warning in gsm_dlci_modem_output()

'size' may be used uninitialized in gsm_dlci_modem_output() if called with
an adaption that is neither 1 nor 2. The function is currently only called
by gsm_modem_upd_via_data() and only for adaption 2.
Properly handle every invalid case by returning -EINVAL to silence the
compiler warning and avoid future regressions.

Fixes: c19ffe00fed6 ("tty: n_gsm: fix invalid use of MSC in advanced option")
Cc: stable@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220425104726.7986-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 8652308..a38b922 100644 (file)
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -932,18 +932,21 @@ static int gsm_dlci_modem_output(struct gsm_mux *gsm, struct gsm_dlci *dlci,
 {
        u8 *dp = NULL;
        struct gsm_msg *msg;
-       int size;
+       int size = 0;
 
        /* for modem bits without break data */
-       if (dlci->adaption == 1) {
-               size = 0;
-       } else if (dlci->adaption == 2) {
-               size = 1;
+       switch (dlci->adaption) {
+       case 1: /* Unstructured */
+               break;
+       case 2: /* Unstructured with modem bits. */
+               size++;
                if (brk > 0)
                        size++;
-       } else {
+               break;
+       default:
                pr_err("%s: unsupported adaption %d\n", __func__,
                       dlci->adaption);
+               return -EINVAL;
        }
 
        msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);