net: tap: replace snprintf with g_strdup_printf calls

When invoking qemu-bridge-helper in 'net_bridge_run_helper',
instead of using fixed sized buffers, use dynamically allocated
ones initialised and returned by g_strdup_printf().

Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/tap.c

@@ -498,9 +498,9 @@ static int net_bridge_run_helper(const char *helper, const char *bridge,
     }
     if (pid == 0) {
         int open_max = sysconf(_SC_OPEN_MAX), i;
-        char fd_buf[6+10];
-        char br_buf[6+IFNAMSIZ] = {0};
-        char helper_cmd[PATH_MAX + sizeof(fd_buf) + sizeof(br_buf) + 15];
+        char *fd_buf = NULL;
+        char *br_buf = NULL;
+        char *helper_cmd = NULL;
 
         for (i = 3; i < open_max; i++) {
             if (i != sv[1]) {
@@ -508,17 +508,17 @@ static int net_bridge_run_helper(const char *helper, const char *bridge,
             }
         }
 
-        snprintf(fd_buf, sizeof(fd_buf), "%s%d", "--fd=", sv[1]);
+        fd_buf = g_strdup_printf("%s%d", "--fd=", sv[1]);
 
         if (strrchr(helper, ' ') || strrchr(helper, '\t')) {
             /* assume helper is a command */
 
             if (strstr(helper, "--br=") == NULL) {
-                snprintf(br_buf, sizeof(br_buf), "%s%s", "--br=", bridge);
+                br_buf = g_strdup_printf("%s%s", "--br=", bridge);
             }
 
-            snprintf(helper_cmd, sizeof(helper_cmd), "%s %s %s %s",
-                     helper, "--use-vnet", fd_buf, br_buf);
+            helper_cmd = g_strdup_printf("%s %s %s %s", helper,
+                            "--use-vnet", fd_buf, br_buf ? br_buf : "");
 
             parg = args;
             *parg++ = (char *)"sh";
@@ -527,10 +527,11 @@ static int net_bridge_run_helper(const char *helper, const char *bridge,
             *parg++ = NULL;
 
             execv("/bin/sh", args);
+            g_free(helper_cmd);
         } else {
             /* assume helper is just the executable path name */
 
-            snprintf(br_buf, sizeof(br_buf), "%s%s", "--br=", bridge);
+            br_buf = g_strdup_printf("%s%s", "--br=", bridge);
 
             parg = args;
             *parg++ = (char *)helper;
@@ -541,6 +542,8 @@ static int net_bridge_run_helper(const char *helper, const char *bridge,
 
             execv(helper, args);
         }
+        g_free(fd_buf);
+        g_free(br_buf);
         _exit(1);
 
     } else {