lib/, src/: Fix error handling after strto[u]l[l](3)

-  Set errno = 0 before the call.  Otherwise, it may contain anything.
-  ERANGE is not the only possible errno value of these functions.  They
   can also set it to EINVAL.
-  Any errno value after these calls is bad; just compare against 0.
-  Don't check for the return value; just errno.  This function is
   guaranteed to not modify errno on success (POSIX).
-  Check endptr == str, which may or may not set EINVAL.

Suggested-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

src/check_subid_range.c

@@ -34,11 +34,12 @@ int main(int argc, char **argv)
 
 	owner = argv[1];
 	check_uids = argv[2][0] == 'u';
+	errno = 0;
 	start = strtoul(argv[3], NULL, 10);
-	if (start == ULONG_MAX && errno == ERANGE)
+	if (errno != 0)
 		exit(1);
 	count = strtoul(argv[4], NULL, 10);
-	if (count == ULONG_MAX && errno == ERANGE)
+	if (errno != 0)
 		exit(1);
 	if (check_uids) {
 		if (have_sub_uids(owner, start, count))

src/usermod.c

@@ -318,13 +318,13 @@ static struct ulong_range getulong_range(const char *str)
 
 	errno = 0;
 	first = strtoll(str, &pos, 10);
-	if (('\0' == *str) || ('-' != *pos ) || (ERANGE == errno) ||
+	if (('\0' == *str) || ('-' != *pos ) || (0 != errno) ||
 	    (first != (unsigned long)first))
 		goto out;
 
 	errno = 0;
 	last = strtoll(pos + 1, &pos, 10);
-	if (('\0' != *pos ) || (ERANGE == errno) ||
+	if (('\0' != *pos ) || (0 != errno) ||
 	    (last != (unsigned long)last))
 		goto out;