net: Enhancements for dhcp option 209

- Enable option 209 by default
- Set pxelinux_configfile to NULL to avoid potential double free
- change hardcoded 209 to a define

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>

cmd/Kconfig

@@ -1898,6 +1898,7 @@ config BOOTP_PXE_CLIENTARCH
 
 config BOOTP_PXE_DHCP_OPTION
 	bool "Request & store 'pxe_configfile' from BOOTP/DHCP server"
+	default y
 	depends on BOOTP_PXE
 
 config BOOTP_VCI_STRING

cmd/pxe.c

@@ -65,6 +65,8 @@ static int pxe_dhcp_option_path(struct pxe_context *ctx, unsigned long pxefile_a
 	int ret = get_pxe_file(ctx, pxelinux_configfile, pxefile_addr_r);
 
 	free(pxelinux_configfile);
+	/* set to NULL to avoid double-free if DHCP is tried again */
+	pxelinux_configfile = NULL;
 
 	return ret;
 }

net/bootp.c

@@ -603,7 +603,7 @@ static int dhcp_extended(u8 *e, int message_type, struct in_addr server_ip,
 	*cnt += 1;
 #endif
 	if (IS_ENABLED(CONFIG_BOOTP_PXE_DHCP_OPTION)) {
-		*e++ = 209;	/* PXELINUX Config File */
+		*e++ = DHCP_OPTION_PXE_CONFIG_FILE;	/* PXELINUX Config File */
 		*cnt += 1;
 	}
 	/* no options, so back up to avoid sending an empty request list */
@@ -922,7 +922,7 @@ static void dhcp_process_options(uchar *popt, uchar *end)
 				net_boot_file_name[size] = 0;
 			}
 			break;
-		case 209:	/* PXELINUX Config File */
+		case DHCP_OPTION_PXE_CONFIG_FILE:	/* PXELINUX Config File */
 			if (IS_ENABLED(CONFIG_BOOTP_PXE_DHCP_OPTION)) {
 				/* In case it has already been allocated when get DHCP Offer packet,
 				 * free first to avoid memory leak.

net/bootp.h

@@ -91,6 +91,8 @@ typedef enum { INIT,
 #define DHCP_NAK      6
 #define DHCP_RELEASE  7
 
+#define DHCP_OPTION_PXE_CONFIG_FILE	209	/* "ConfigFile" option according to rfc5071 */
+
 /**********************************************************************/
 
 #endif /* __BOOTP_H__ */