sc_gnrc_ipv6_nib: check interface existence

Currently an interface's existence is not checked when it is supplied
by the user with the `nib` command. This can lead to assertion errors
as soon as the generated entry tries to resolve an address or route
generated with that command and the network interface not being found.

sys/shell/commands/sc_gnrc_ipv6_nib.c

@@ -78,6 +78,15 @@ static void _usage_nib_route(char **argv)
     printf("       %s %s show [iface]\n", argv[0], argv[1]);
 }
 
+static inline gnrc_netif_t *_get_iface(unsigned iface)
+{
+     /* To prevent integer overflow we can't use pid_is_valid() since it
+      * itself would cause an overflow due to the cast to `kernel_pid_t` */
+    return (iface <= ((unsigned)KERNEL_PID_LAST))
+           ? gnrc_netif_get_by_pid(iface)
+           : NULL;
+}
+
 static int _nib_neigh(int argc, char **argv)
 {
     if ((argc == 2) || (strcmp(argv[2], "show") == 0)) {
@@ -101,6 +110,10 @@ static int _nib_neigh(int argc, char **argv)
         size_t l2addr_len = 0;
         unsigned iface = atoi(argv[3]);
 
+        if (_get_iface(iface) == NULL) {
+            printf("Interface %u does not exist\n", iface);
+            return 1;
+        }
         if (ipv6_addr_from_str(&ipv6_addr, argv[4]) == NULL) {
             _usage_nib_neigh(argv);
             return 1;
@@ -116,6 +129,10 @@ static int _nib_neigh(int argc, char **argv)
         ipv6_addr_t ipv6_addr;
         unsigned iface = atoi(argv[3]);
 
+        if (_get_iface(iface) == NULL) {
+            printf("Interface %u does not exist\n", iface);
+            return 1;
+        }
         if (ipv6_addr_from_str(&ipv6_addr, argv[4]) == NULL) {
             _usage_nib_neigh(argv);
             return 1;
@@ -138,6 +155,10 @@ static int _nib_prefix(int argc, char **argv)
 
         if (argc > 3) {
             iface = atoi(argv[3]);
+            if (_get_iface(iface) == NULL) {
+                printf("Interface %u does not exist\n", iface);
+                return 1;
+            }
         }
         while (gnrc_ipv6_nib_pl_iter(iface, &state, &entry)) {
             gnrc_ipv6_nib_pl_print(&entry);
@@ -152,6 +173,10 @@ static int _nib_prefix(int argc, char **argv)
         unsigned pfx_len = ipv6_addr_split_prefix(argv[4]);
         uint32_t valid_ltime = UINT32_MAX, pref_ltime = UINT32_MAX;
 
+        if (_get_iface(iface) == NULL) {
+            printf("Interface %u does not exist\n", iface);
+            return 1;
+        }
         if (ipv6_addr_from_str(&pfx, argv[4]) == NULL) {
             _usage_nib_prefix(argv);
             return 1;
@@ -175,6 +200,10 @@ static int _nib_prefix(int argc, char **argv)
         unsigned iface = atoi(argv[3]);
         unsigned pfx_len = ipv6_addr_split_prefix(argv[4]);
 
+        if (_get_iface(iface) == NULL) {
+            printf("Interface %u does not exist\n", iface);
+            return 1;
+        }
         if (ipv6_addr_from_str(&pfx, argv[4]) == NULL) {
             _usage_nib_prefix(argv);
             return 1;
@@ -197,6 +226,10 @@ static int _nib_route(int argc, char **argv)
 
         if (argc > 3) {
             iface = atoi(argv[3]);
+            if (_get_iface(iface) == NULL) {
+                printf("Interface %u does not exist\n", iface);
+                return 1;
+            }
         }
         while (gnrc_ipv6_nib_ft_iter(NULL, iface, &state, &entry)) {
             gnrc_ipv6_nib_ft_print(&entry);
@@ -211,6 +244,10 @@ static int _nib_route(int argc, char **argv)
         unsigned pfx_len = ipv6_addr_split_prefix(argv[4]);
         uint16_t ltime = 0;
 
+        if (_get_iface(iface) == NULL) {
+            printf("Interface %u does not exist\n", iface);
+            return 1;
+        }
         if (ipv6_addr_from_str(&pfx, argv[4]) == NULL) {
             /* check if string equals "default"
              * => keep pfx as unspecified address == default route */