recv_packet: calculate valid package sizes from struct size

src/main.c

@@ -30,6 +30,10 @@ typedef struct {
     uint8_t _padding;
 } rcon_packet_t;
 
+// Minimum and maximum values for the `size` field of a valid packet.
+#define MIN_SIZE (sizeof(int32_t) * 2 + 2)
+#define MAX_SIZE (sizeof(rcon_packet_t) - sizeof(int32_t))
+
 int read_env_int(const char *var) {
     const char* port_str = getenv(var);
     if (port_str == NULL) {
@@ -125,7 +129,7 @@ int recv_packet(int sockfd, rcon_packet_t *packet) {
     }
     packet->size = le32toh(packet->size);
 
-    if (packet->size < 10 || packet->size > 4110) {
+    if (packet->size < (int32_t)MIN_SIZE || packet->size > (int32_t)MAX_SIZE) {
         fprintf(stderr, "invalid packet size: %d\n", packet->size);
         return -1;
     }