cxgb3: Avoid potential string truncation in desc

Builds with W=1 were warning about potential string truncations:

drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c: In function 'cxgb_up':
drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c:394:38: warning: '%d' directive output may be truncated writing between 1 and 11 bytes into a region of size between 5 and 20 [-Wformat-truncation=]
  394 |                                  "%s-%d", d->name, pi->first_qset + i);
      |                                      ^~
In function 'name_msix_vecs',
    inlined from 'cxgb_up' at drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c:1264:3: drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c:394:34: note: directive argument in the range [-2147483641, 509]
  394 |                                  "%s-%d", d->name, pi->first_qset + i);
      |                                  ^~~~~~~
drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c:393:25: note: 'snprintf' output between 3 and 28 bytes into a destination of size 21
  393 |                         snprintf(adap->msix_info[msi_idx].desc, n,
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  394 |                                  "%s-%d", d->name, pi->first_qset + i);
      |                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Avoid open-coded %NUL-termination (this code was assuming snprintf
wasn't %NUL terminating when it does -- likely thinking of strncpy),
and grow the size of the string to handle a maximal value.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312100937.ZPZCARhB-lkp@intel.com/
Cc: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231212220954.work.219-kees@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/chelsio/cxgb3/adapter.h

@@ -237,7 +237,7 @@ struct adapter {
 	int msix_nvectors;
 	struct {
 		unsigned short vec;
-		char desc[22];
+		char desc[IFNAMSIZ + 1 + 12];	/* Needs space for "%s-%d" */
 	} msix_info[SGE_QSETS + 1];
 
 	/* T3 modules */

drivers/net/ethernet/chelsio/cxgb3/cxgb3_main.c

@@ -380,19 +380,18 @@ static irqreturn_t t3_async_intr_handler(int irq, void *cookie)
  */
 static void name_msix_vecs(struct adapter *adap)
 {
-	int i, j, msi_idx = 1, n = sizeof(adap->msix_info[0].desc) - 1;
+	int i, j, msi_idx = 1;
 
-	snprintf(adap->msix_info[0].desc, n, "%s", adap->name);
-	adap->msix_info[0].desc[n] = 0;
+	strscpy(adap->msix_info[0].desc, adap->name, sizeof(adap->msix_info[0].desc));
 
 	for_each_port(adap, j) {
 		struct net_device *d = adap->port[j];
 		const struct port_info *pi = netdev_priv(d);
 
 		for (i = 0; i < pi->nqsets; i++, msi_idx++) {
-			snprintf(adap->msix_info[msi_idx].desc, n,
+			snprintf(adap->msix_info[msi_idx].desc,
+				 sizeof(adap->msix_info[0].desc),
 				 "%s-%d", d->name, pi->first_qset + i);
-			adap->msix_info[msi_idx].desc[n] = 0;
 		}
 	}
 }