[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>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/drivers/bus/usb.c b/src/drivers/bus/usb.c
index 91d52b0c..22a57e6e 100644
--- a/src/drivers/bus/usb.c
+++ b/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 );