[pxe] Clarify comments regarding shrinking of cached DHCP packet

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

diff --git a/src/arch/i386/core/cachedhcp.c b/src/arch/i386/core/cachedhcp.c
index a5c624035483df363ccb14d98b01cd89d1bb3e11..ff35b9256959bf1139696eae8f1521d9b287a3ac 100644 (file)
--- a/src/arch/i386/core/cachedhcp.c
+++ b/src/arch/i386/core/cachedhcp.c
@@ -58,6 +58,7 @@ static void cachedhcp_init ( void ) {
        struct dhcp_packet *dhcppkt;
        struct dhcp_packet *tmp;
        struct dhcphdr *dhcphdr;
+       size_t max_len;
        size_t len;
 
        /* Do nothing if no cached DHCPACK is present */
@@ -69,23 +70,25 @@ static void cachedhcp_init ( void ) {
        /* No reliable way to determine length before parsing packet;
         * start by assuming maximum length permitted by PXE.
         */
-       len = sizeof ( BOOTPLAYER_t );
+       max_len = sizeof ( BOOTPLAYER_t );
 
        /* Allocate and populate DHCP packet */
-       dhcppkt = zalloc ( sizeof ( *dhcppkt ) + len );
+       dhcppkt = zalloc ( sizeof ( *dhcppkt ) + max_len );
        if ( ! dhcppkt ) {
                DBGC ( colour, "CACHEDHCP could not allocate copy\n" );
                return;
        }
        dhcphdr = ( ( ( void * ) dhcppkt ) + sizeof ( *dhcppkt ) );
        copy_from_user ( dhcphdr, phys_to_user ( cached_dhcpack_phys ), 0,
-                        len );
-       dhcppkt_init ( dhcppkt, dhcphdr, len );
+                        max_len );
+       dhcppkt_init ( dhcppkt, dhcphdr, max_len );
 
-       /* Resize packet to required length.  If reallocation fails,
-        * just continue to use the original packet.
+       /* Shrink packet to required length.  If reallocation fails,
+        * just continue to use the original packet and waste the
+        * unused space.
         */
        len = dhcppkt_len ( dhcppkt );
+       assert ( len <= max_len );
        tmp = realloc ( dhcppkt, ( sizeof ( *dhcppkt ) + len ) );
        if ( tmp )
                dhcppkt = tmp;