util-lib: add new ifname_valid() call that validates interface names

Make use of this in nspawn at a couple of places. A later commit should port
more code over to this, including networkd.

src/basic/socket-util.c

@@ -43,7 +43,9 @@
 #include "socket-util.h"
 #include "string-table.h"
 #include "string-util.h"
+#include "strv.h"
 #include "user-util.h"
+#include "utf8.h"
 #include "util.h"
 
 int socket_address_parse(SocketAddress *a, const char *s) {
@@ -795,6 +797,42 @@ static const char* const ip_tos_table[] = {
 
 DEFINE_STRING_TABLE_LOOKUP_WITH_FALLBACK(ip_tos, int, 0xff);
 
+bool ifname_valid(const char *p) {
+        bool numeric = true;
+
+        /* Checks whether a network interface name is valid. This is inspired by dev_valid_name() in the kernel sources
+         * but slightly stricter, as we only allow non-control, non-space ASCII characters in the interface name. We
+         * also don't permit names that only container numbers, to avoid confusion with numeric interface indexes. */
+
+        if (isempty(p))
+                return false;
+
+        if (strlen(p) >= IFNAMSIZ)
+                return false;
+
+        if (STR_IN_SET(p, ".", ".."))
+                return false;
+
+        while (*p) {
+                if ((unsigned char) *p >= 127U)
+                        return false;
+
+                if ((unsigned char) *p <= 32U)
+                        return false;
+
+                if (*p == ':' || *p == '/')
+                        return false;
+
+                numeric = numeric && (*p >= '0' && *p <= '9');
+                p++;
+        }
+
+        if (numeric)
+                return false;
+
+        return true;
+}
+
 int getpeercred(int fd, struct ucred *ucred) {
         socklen_t n = sizeof(struct ucred);
         struct ucred u;