util.c: check first/last char and allow extra chars

In `invalid_name`.

src/firejail/util.c

@@ -1476,21 +1476,43 @@ int ascii_isxdigit(unsigned char c) {
  return ret;
 }
 
-// allow strict ASCII letters and numbers; names with only numbers are rejected; spaces are rejected
+// Allow only ASCII letters, digits and a few special characters; names with
+// only numbers are rejected; spaces and control characters are rejected.
 int invalid_name(const char *name) {
  const char *c = name;
  int only_numbers = 1;
 
  if (strlen(name) > 253)
  return 1;
 
+ // must start with alnum
+ if (!ascii_isalnum(*c))
+ return 1;
+ if (!ascii_isdigit(*c))
+ only_numbers = 0;
+ ++c;
+
  while (*c) {
- if (!ascii_isalnum(*c))
- return 1;
- if (!ascii_isdigit(*c))
+ switch (*c) {
+ case '-':
+ case '.':
+ case '_':
  only_numbers = 0;
+ break;
+ default:
+ if (!ascii_isalnum(*c))
+ return 1;
+ if (!ascii_isdigit(*c))
+ only_numbers = 0;
+ }
  ++c;
  }
+
+ // must end with alnum
+ --c;
+ if (!ascii_isalnum(*c))
+ return 1;
+
  if (only_numbers)
  return 1;