[build] Fix the REQUIRE_SYMBOL mechanism

At some point in the past few years, binutils became more aggressive
at removing unused symbols.  To function as a symbol requirement, a
relocation record must now be in a section marked with @progbits and
must not be in a section which gets discarded during the link (either
via --gc-sections or via /DISCARD/).

Update REQUIRE_SYMBOL() to generate relocation records meeting these
criteria.  To minimise the impact upon the final binary size, we use
existing symbols (specified via the REQUIRING_SYMBOL() macro) as the
relocation targets where possible.  We use R_386_NONE or R_X86_64_NONE
relocation types to prevent any actual unwanted relocation taking
place.  Where no suitable symbol exists for REQUIRING_SYMBOL() (such
as in config.c), the macro PROVIDE_REQUIRING_SYMBOL() can be used to
generate a one-byte-long symbol to act as the relocation target.

If there are versions of binutils for which this approach fails, then
the fallback will probably involve killing off REQUEST_SYMBOL(),
redefining REQUIRE_SYMBOL() to use the current definition of
REQUEST_SYMBOL(), and postprocessing the linked ELF file with
something along the lines of "nm -u | wc -l" to check that there are
no undefined symbols remaining.

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

src/net/80211/net80211.c

@@ -2827,5 +2827,8 @@ struct errortab common_wireless_errors[] __errortab = {
 	__einfo_errortab ( EINFO_ECONNREFUSED_AUTH_ALGO_UNSUPP ),
 };
 
+/* Drag in objects via net80211_ll_protocol */
+REQUIRING_SYMBOL ( net80211_ll_protocol );
+
 /* Drag in 802.11 configuration */
 REQUIRE_OBJECT ( config_net80211 );

src/net/80211/wpa.c

@@ -912,4 +912,5 @@ struct eapol_handler eapol_key_handler __eapol_handler = {
 };
 
 /* WPA always needs EAPOL in order to be useful */
+REQUIRING_SYMBOL ( eapol_key_handler );
 REQUIRE_OBJECT ( eapol );

src/net/ethernet.c

@@ -239,6 +239,9 @@ struct net_device * alloc_etherdev ( size_t priv_size ) {
 	return netdev;
 }
 
+/* Drag in objects via ethernet_protocol */
+REQUIRING_SYMBOL ( ethernet_protocol );
+
 /* Drag in Ethernet configuration */
 REQUIRE_OBJECT ( config_ethernet );
 

src/net/fc.c

@@ -1940,5 +1940,8 @@ struct fc_ulp * fc_ulp_get_port_id_type ( struct fc_port *port,
 	return NULL;
 }
 
+/* Drag in objects via fc_ports */
+REQUIRING_SYMBOL ( fc_ports );
+
 /* Drag in Fibre Channel configuration */
 REQUIRE_OBJECT ( config_fc );

src/net/infiniband.c

@@ -999,6 +999,9 @@ struct ib_device * last_opened_ibdev ( void ) {
 	return ibdev;
 }
 
+/* Drag in objects via register_ibdev() */
+REQUIRING_SYMBOL ( register_ibdev );
+
 /* Drag in Infiniband configuration */
 REQUIRE_OBJECT ( config_infiniband );
 

src/net/ipv4.c

@@ -841,5 +841,8 @@ struct settings_applicator ipv4_settings_applicator __settings_applicator = {
 	.apply = ipv4_create_routes,
 };
 
+/* Drag in objects via ipv4_protocol */
+REQUIRING_SYMBOL ( ipv4_protocol );
+
 /* Drag in ICMPv4 */
 REQUIRE_OBJECT ( icmpv4 );

src/net/ipv6.c

@@ -1104,6 +1104,9 @@ struct net_driver ipv6_driver __net_driver = {
 	.remove = ipv6_remove,
 };
 
+/* Drag in objects via ipv6_protocol */
+REQUIRING_SYMBOL ( ipv6_protocol );
+
 /* Drag in ICMPv6 */
 REQUIRE_OBJECT ( icmpv6 );