[pxe] Construct all fake DHCP packets before starting PXE NBP

Commit edf74df ("[pxe] Always reconstruct packet for
PXENV_GET_CACHED_INFO") fixed the problems caused by returning stale
DHCP packets (e.g. from an earlier boot attempt using a different
network device), but broke interoperability with NBPs such as WDS
which may overwrite our cached (fake) DHCP packets and expect the
modified packets to be returned by a subsequent call to
PXENV_GET_CACHED_INFO.

Fix by constructing the fake DHCP packets immediately before
transferring control to a PXE NBP.  Calls to PXENV_GET_CACHED_INFO
will now never modify the cached packets.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/image/pxe_image.c

@@ -78,6 +78,9 @@ static int pxe_exec ( struct image *image ) {
 	/* Activate PXE */
 	pxe_activate ( netdev );
 
+	/* Construct fake DHCP packets */
+	pxe_fake_cached_info();
+
 	/* Set PXE command line */
 	pxe_cmdline = image->cmdline;
 

src/arch/i386/include/pxe.h

@@ -192,6 +192,7 @@ extern struct net_device *pxe_netdev;
 extern const char *pxe_cmdline;
 
 extern void pxe_set_netdev ( struct net_device *netdev );
+extern void pxe_fake_cached_info ( void );
 extern PXENV_EXIT_t pxenv_tftp_read_file ( struct s_PXENV_TFTP_READ_FILE
 					   *tftp_read_file );
 extern PXENV_EXIT_t undi_loader ( struct s_UNDI_LOADER *undi_loader );

src/arch/i386/interface/pxe/pxe_preboot.c

@@ -128,6 +128,38 @@ pxenv_get_cached_info_name ( int packet_type ) {
 static union pxe_cached_info __bss16_array ( cached_info, [NUM_CACHED_INFOS] );
 #define cached_info __use_data16 ( cached_info )
 
+/**
+ * Construct cached DHCP packets
+ *
+ */
+void pxe_fake_cached_info ( void ) {
+	struct pxe_dhcp_packet_creator *creator;
+	union pxe_cached_info *info;
+	unsigned int i;
+	int rc;
+
+	/* Sanity check */
+	assert ( pxe_netdev != NULL );
+
+	/* Erase any stale packets */
+	memset ( cached_info, 0, sizeof ( cached_info ) );
+
+	/* Construct all DHCP packets */
+	for ( i = 0 ; i < ( sizeof ( pxe_dhcp_packet_creators ) /
+			    sizeof ( pxe_dhcp_packet_creators[0] ) ) ; i++ ) {
+
+		/* Construct DHCP packet */
+		creator = &pxe_dhcp_packet_creators[i];
+		info = &cached_info[i];
+		if ( ( rc = creator->create ( pxe_netdev, info,
+					      sizeof ( *info ) ) ) != 0 ) {
+			DBGC ( &pxe_netdev, " failed to build packet: %s\n",
+			       strerror ( rc ) );
+			/* Continue constructing remaining packets */
+		}
+	}
+}
+
 /**
  * UNLOAD BASE CODE STACK
  *
@@ -149,44 +181,26 @@ pxenv_unload_stack ( struct s_PXENV_UNLOAD_STACK *unload_stack ) {
  */
 static PXENV_EXIT_t
 pxenv_get_cached_info ( struct s_PXENV_GET_CACHED_INFO *get_cached_info ) {
-	struct pxe_dhcp_packet_creator *creator;
 	union pxe_cached_info *info;
 	unsigned int idx;
 	size_t len;
 	userptr_t buffer;
-	int rc;
 
 	DBGC ( &pxe_netdev, "PXENV_GET_CACHED_INFO %s to %04x:%04x+%x",
 	       pxenv_get_cached_info_name ( get_cached_info->PacketType ),
 	       get_cached_info->Buffer.segment,
 	       get_cached_info->Buffer.offset, get_cached_info->BufferSize );
 
-	/* Sanity check */
-	if ( ! pxe_netdev ) {
-		DBGC ( &pxe_netdev, "PXENV_GET_CACHED_INFO called with no "
-		       "network device\n" );
-		get_cached_info->Status = PXENV_STATUS_UNDI_INVALID_STATE;
-		return PXENV_EXIT_FAILURE;
-	}
-
 	/* Sanity check */
         idx = ( get_cached_info->PacketType - 1 );
 	if ( idx >= NUM_CACHED_INFOS ) {
 		DBGC ( &pxe_netdev, " bad PacketType %d\n",
 		       get_cached_info->PacketType );
-		goto err;
+		get_cached_info->Status = PXENV_STATUS_UNSUPPORTED;
+		return PXENV_EXIT_FAILURE;
 	}
 	info = &cached_info[idx];
 
-	/* Construct DHCP packet */
-	creator = &pxe_dhcp_packet_creators[idx];
-	if ( ( rc = creator->create ( pxe_netdev, info,
-				      sizeof ( *info ) ) ) != 0 ) {
-		DBGC ( &pxe_netdev, " failed to build packet: %s\n",
-		       strerror ( rc ) );
-		goto err;
-	}
-
 	/* Copy packet (if applicable) */
 	len = get_cached_info->BufferSize;
 	if ( len == 0 ) {
@@ -238,10 +252,6 @@ pxenv_get_cached_info ( struct s_PXENV_GET_CACHED_INFO *get_cached_info ) {
 	DBGC ( &pxe_netdev, "\n" );
 	get_cached_info->Status = PXENV_STATUS_SUCCESS;
 	return PXENV_EXIT_SUCCESS;
-
- err:
-	get_cached_info->Status = PXENV_STATUS_OUT_OF_RESOURCES;
-	return PXENV_EXIT_FAILURE;
 }
 
 /* PXENV_RESTART_TFTP