[pxe] Discard queued PXE UDP packets when under memory pressure

The PXE UDP receive queue may grow without limit if the PXE NBP does
not call PXENV_UDP_READ sufficiently frequently.

Fix by implementing a cache discarder for received PXE UDP packets
(similar to the TCP cache discarder).

Reported-by: Tal Shorer <shorer@amazon.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/interface/pxe/pxe_udp.c

@@ -12,6 +12,7 @@
 #include <ipxe/uaccess.h>
 #include <ipxe/process.h>
 #include <ipxe/netdevice.h>
+#include <ipxe/malloc.h>
 #include <realmode.h>
 #include <pxe.h>
 
@@ -482,3 +483,28 @@ struct pxe_api_call pxe_udp_api[] __pxe_api_call = {
 	PXE_API_CALL ( PXENV_UDP_READ, pxenv_udp_read,
 		       struct s_PXENV_UDP_READ ),
 };
+
+/**
+ * Discard some cached PXE UDP data
+ *
+ * @ret discarded	Number of cached items discarded
+ */
+static unsigned int pxe_udp_discard ( void ) {
+	struct io_buffer *iobuf;
+	unsigned int discarded = 0;
+
+	/* Try to discard the oldest received UDP packet */
+	iobuf = list_first_entry ( &pxe_udp.list, struct io_buffer, list );
+	if ( iobuf ) {
+		list_del ( &iobuf->list );
+		free_iob ( iobuf );
+		discarded++;
+	}
+
+	return discarded;
+}
+
+/** PXE UDP cache discarder */
+struct cache_discarder pxe_udp_discarder __cache_discarder ( CACHE_NORMAL ) = {
+	.discard = pxe_udp_discard,
+};

src/usr/pxemenu.c

@@ -204,7 +204,7 @@ static void pxe_menu_draw_item ( struct pxe_menu *menu,
 	buf[ sizeof ( buf ) - 1 ] = '\0';
 
 	/* Draw row */
-	row = ( LINES - menu->num_items + index );
+	row = ( LINES - menu->num_items + index - 1 );
 	color_set ( ( selected ? CPAIR_PXE : CPAIR_DEFAULT ), NULL );
 	mvprintw ( row, 0, "%s", buf );
 	move ( row, 1 );