[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/arch/i386/include/bits/compiler.h

@@ -3,6 +3,9 @@
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
+/** Dummy relocation type */
+#define RELOC_TYPE_NONE R_386_NONE
+
 #ifndef ASSEMBLY
 
 /** Declare a function with standard calling conventions */

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

@@ -346,6 +346,7 @@ int pxe_start_nbp ( void ) {
 	return 0;
 }
 
+REQUIRING_SYMBOL ( pxe_api_call );
 REQUIRE_OBJECT ( pxe_preboot );
 REQUIRE_OBJECT ( pxe_undi );
 REQUIRE_OBJECT ( pxe_udp );

src/arch/i386/prefix/kkkpxeprefix.S

@@ -8,6 +8,7 @@
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
 
 /* Provide the PXENV_FILE_EXIT_HOOK API call */
+REQUIRING_SYMBOL ( _kkkpxe_start )
 REQUIRE_OBJECT ( pxe_exit_hook )
 
 #define PXELOADER_KEEP_UNDI

src/arch/i386/prefix/romprefix.S

@@ -875,5 +875,8 @@ wait_for_tick:
 	ret
 	.size wait_for_tick, . - wait_for_tick
 
+/* Drag in objects via _rom_start */
+REQUIRING_SYMBOL ( _rom_start )
+
 /* Drag in ROM configuration */
 REQUIRE_OBJECT ( config_romprefix )

src/arch/i386/transitions/librm_test.c

@@ -118,4 +118,5 @@ struct self_test librm_test __self_test = {
 	.exec = librm_test_exec,
 };
 
+REQUIRING_SYMBOL ( librm_test );
 REQUIRE_OBJECT ( test );

src/arch/x86/drivers/hyperv/hyperv.c

@@ -590,5 +590,8 @@ struct root_device hv_root_device __root_device = {
 	.driver = &hv_root_driver,
 };
 
+/* Drag in objects via hv_root_device */
+REQUIRING_SYMBOL ( hv_root_device );
+
 /* Drag in netvsc driver */
 REQUIRE_OBJECT ( netvsc );

src/arch/x86/drivers/xen/hvm.c

@@ -496,5 +496,8 @@ struct pci_driver hvm_driver __pci_driver = {
 	.remove = hvm_remove,
 };
 
+/* Drag in objects via hvm_driver */
+REQUIRING_SYMBOL ( hvm_driver );
+
 /* Drag in netfront driver */
 REQUIRE_OBJECT ( netfront );

src/arch/x86_64/include/bits/compiler.h

@@ -1,6 +1,9 @@
 #ifndef _BITS_COMPILER_H
 #define _BITS_COMPILER_H
 
+/** Dummy relocation type */
+#define RELOC_TYPE_NONE R_X86_64_NONE
+
 #ifndef ASSEMBLY
 
 /** Declare a function with standard calling conventions */

src/config/config.c

@@ -44,6 +44,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  * in the final iPXE executable built.
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in all requested console types
  *

src/config/config_ethernet.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in Ethernet-specific protocols
  */

src/config/config_fc.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in Fibre Channel-specific commands
  *

src/config/config_infiniband.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in Infiniband-specific protocols
  */

src/config/config_net80211.c

@@ -25,6 +25,8 @@ FILE_LICENCE ( GPL2_OR_LATER );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in 802.11-specific commands
  *

src/config/config_romprefix.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Provide UNDI loader if PXE stack is requested
  *

src/config/config_route.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in routing management for relevant protocols
  *

src/config/config_usb.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in USB controllers
  */

src/crypto/x509.c

@@ -1765,5 +1765,8 @@ int x509_validate_chain ( struct x509_chain *chain, time_t time,
 	return -EACCES_USELESS;
 }
 
+/* Drag in objects via x509_validate() */
+REQUIRING_SYMBOL ( x509_validate );
+
 /* Drag in certificate store */
 REQUIRE_OBJECT ( certstore );

src/drivers/bus/usb.c

@@ -1903,6 +1903,9 @@ struct usb_port * usb_root_hub_port ( struct usb_device *usb ) {
 	return usb->port;
 }
 
+/* Drag in objects via register_usb_bus() */
+REQUIRING_SYMBOL ( register_usb_bus );
+
 /* Drag in USB configuration */
 REQUIRE_OBJECT ( config_usb );
 

src/drivers/net/rtl818x/rtl8180.c

@@ -4,6 +4,7 @@ FILE_LICENCE(GPL2_OR_LATER);
 
 #include <ipxe/pci.h>
 
+PROVIDE_REQUIRING_SYMBOL();
 REQUIRE_OBJECT(rtl818x);
 REQUIRE_OBJECT(rtl8180_grf5101);
 REQUIRE_OBJECT(rtl8180_max2820);

src/drivers/net/rtl818x/rtl8185.c

@@ -4,6 +4,7 @@ FILE_LICENCE(GPL2_OR_LATER);
 
 #include <ipxe/pci.h>
 
+PROVIDE_REQUIRING_SYMBOL();
 REQUIRE_OBJECT(rtl818x);
 REQUIRE_OBJECT(rtl8185_rtl8225);
 

src/drivers/net/vxge/vxge.c

@@ -9,6 +9,7 @@ FILE_LICENCE(GPL2_OR_LATER);
 
 #include <ipxe/pci.h>
 
+PROVIDE_REQUIRING_SYMBOL();
 REQUIRE_OBJECT(vxge_main);
 
 /** vxge PCI IDs for util/parserom.pl which are put into bin/NIC */

src/hci/commands/image_trust_cmd.c

@@ -173,6 +173,9 @@ struct command image_trust_commands[] __command = {
 	},
 };
 
+/* Drag in objects via command list */
+REQUIRING_SYMBOL ( image_trust_commands );
+
 /* Drag in objects typically required for signature verification */
 REQUIRE_OBJECT ( rsa );
 REQUIRE_OBJECT ( md5 );

src/include/compiler.h

@@ -57,54 +57,101 @@
  * @{
  */
 
-/** Provide a symbol within this object file */
+/**
+ * Provide a symbol within this object file
+ *
+ * @v symbol		Symbol name
+ */
 #ifdef ASSEMBLY
-#define PROVIDE_SYMBOL( _sym )				\
-	.section ".provided", "a", @nobits ;		\
-	.hidden _sym ;					\
-	.globl	_sym ;					\
-	_sym: ;						\
+#define PROVIDE_SYMBOL( symbol )				\
+	.section ".provided", "a", @nobits ;			\
+	.hidden symbol ;					\
+	.globl	symbol ;					\
+	symbol: ;						\
 	.previous
-#else /* ASSEMBLY */
-#define PROVIDE_SYMBOL( _sym )				\
-	char _sym[0]					\
+#else
+#define PROVIDE_SYMBOL( symbol )				\
+	char symbol[0]						\
 	  __attribute__ (( section ( ".provided" ) ))
-#endif /* ASSEMBLY */
+#endif
 
-/** Require a symbol within this object file
+/**
+ * Request a symbol
+ *
+ * @v symbol		Symbol name
  *
- * The symbol is referenced by a relocation in a discarded section, so
- * if it is not available at link time the link will fail.
+ * Request a symbol to be included within the link.  If the symbol
+ * cannot be found, the link will succeed anyway.
  */
 #ifdef ASSEMBLY
-#define REQUIRE_SYMBOL( _sym )				\
-	.section ".discard", "a", @progbits ;		\
-	.extern	_sym ;					\
-	.long	_sym ;					\
-	.previous
-#else /* ASSEMBLY */
-#define REQUIRE_SYMBOL( _sym )				\
-	extern char _sym;				\
-	static char * _C2 ( _C2 ( __require_, _sym ), _C2 ( _, __LINE__ ) ) \
-		__attribute__ (( section ( ".discard" ), used )) \
-		= &_sym
+#define REQUEST_SYMBOL( symbol )				\
+	.equ __request_ ## symbol, symbol
+#else
+#define REQUEST_SYMBOL( symbol )				\
+	__asm__ ( ".equ __request_" #symbol ", " #symbol )
 #endif
 
-/** Request that a symbol be available at runtime
+/**
+ * Require a symbol
  *
- * The requested symbol is entered as undefined into the symbol table
- * for this object, so the linker will pull in other object files as
- * necessary to satisfy the reference. However, the undefined symbol
- * is not referenced in any relocations, so the link can still succeed
- * if no file contains it.
+ * @v symbol		Symbol name
+ *
+ * Require a symbol to be included within the link.  If the symbol
+ * cannot be found, the link will fail.
+ *
+ * To use this macro within a file, you must also specify the file's
+ * "requiring symbol" using the REQUIRING_SYMBOL() or
+ * PROVIDE_REQUIRING_SYMBOL() macros.
  */
 #ifdef ASSEMBLY
-#define REQUEST_SYMBOL( _sym )				\
-	.equ	__need_ ## _sym, _sym
-#else /* ASSEMBLY */
-#define REQUEST_SYMBOL( _sym )				\
-	__asm__ ( ".equ\t__need_" #_sym ", " #_sym )
-#endif /* ASSEMBLY */
+#define REQUIRE_SYMBOL( symbol )				\
+	.reloc __requiring_symbol__, RELOC_TYPE_NONE, symbol
+#else
+#define REQUIRE_SYMBOL( symbol )				\
+	__asm__ ( ".reloc __requiring_symbol__, "		\
+		  _S2 ( RELOC_TYPE_NONE ) ", " #symbol )
+#endif
+
+/**
+ * Specify the file's requiring symbol
+ *
+ * @v symbol		Symbol name
+ *
+ * REQUIRE_SYMBOL() works by defining a dummy relocation record
+ * against a nominated "requiring symbol".  The presence of the
+ * nominated requiring symbol will drag in all of the symbols
+ * specified using REQUIRE_SYMBOL().
+ */
+#ifdef ASSEMBLY
+#define REQUIRING_SYMBOL( symbol )				\
+	.equ __requiring_symbol__, symbol
+#else
+#define REQUIRING_SYMBOL( symbol )				\
+	__asm__ ( ".equ __requiring_symbol__, " #symbol )
+#endif
+
+/**
+ * Provide a file's requiring symbol
+ *
+ * If the file contains no symbols that can be used as the requiring
+ * symbol, you can provide a dummy one-byte-long symbol using
+ * PROVIDE_REQUIRING_SYMBOL().
+ */
+#ifdef ASSEMBLY
+#define PROVIDE_REQUIRING_SYMBOL()				\
+	.section ".tbl.requiring_symbols", "a", @progbits ;	\
+	__requiring_symbol__:	.byte 0 ;			\
+	.size __requiring_symbol__, . - __requiring_symbol__ ;	\
+	.previous
+#else
+#define PROVIDE_REQUIRING_SYMBOL()				\
+	__asm__ ( ".section \".tbl.requiring_symbols\", "	\
+		  "         \"a\", @progbits\n"			\
+		  "__requiring_symbol__:\t.byte 0\n"		\
+		  ".size __requiring_symbol__, "		\
+		  "      . - __requiring_symbol__\n"		\
+		  ".previous" )
+#endif
 
 /** @} */
 
@@ -119,11 +166,29 @@
 /** Always provide the symbol for the current object (defined by -DOBJECT) */
 PROVIDE_SYMBOL ( OBJECT_SYMBOL );
 
-/** Explicitly require another object */
-#define REQUIRE_OBJECT( _obj ) REQUIRE_SYMBOL ( obj_ ## _obj )
+/**
+ * Request an object
+ *
+ * @v object		Object name
+ *
+ * Request an object to be included within the link.  If the object
+ * cannot be found, the link will succeed anyway.
+ */
+#define REQUEST_OBJECT( object ) REQUEST_SYMBOL ( obj_ ## object )
 
-/** Pull in another object if it exists */
-#define REQUEST_OBJECT( _obj ) REQUEST_SYMBOL ( obj_ ## _obj )
+/**
+ * Require an object
+ *
+ * @v object		Object name
+ *
+ * Require an object to be included within the link.  If the object
+ * cannot be found, the link will fail.
+ *
+ * To use this macro within a file, you must also specify the file's
+ * "requiring symbol" using the REQUIRING_SYMBOL() or
+ * PROVIDE_REQUIRING_SYMBOL() macros.
+ */
+#define REQUIRE_OBJECT( object ) REQUIRE_SYMBOL ( obj_ ## object )
 
 /** @} */
 
@@ -685,8 +750,8 @@ int __debug_disable;
 
 /** @} */
 
-/* This file itself is under GPLv2-or-later */
-FILE_LICENCE ( GPL2_OR_LATER );
+/* This file itself is under GPLv2+/UBDL */
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <bits/compiler.h>
 

src/include/nic.h

@@ -276,6 +276,7 @@ static inline void * legacy_isa_get_drvdata ( void *hwdev ) {
 	_name ## _isa_legacy_remove ( struct isa_device *isa ) {	  \
 		return legacy_remove ( isa, legacy_isa_get_drvdata,	  \
 				       _name ## _disable );		  \
-	}
+	}								  \
+	PROVIDE_REQUIRING_SYMBOL()
 
 #endif	/* NIC_H */

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 );