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[build] Fix the REQUIRE_SYMBOL mechanism

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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>
This commit is contained in:
Michael Brown
2015-03-04 18:48:19 +00:00
parent 86ae6e6c18
commit fbc4ba4b4e
37 changed files with 185 additions and 46 deletions
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3
src/util/elf2efi.c
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@@ -478,6 +478,9 @@ static void process_reloc ( bfd *bfd __attribute__ (( unused )),
/* Skip absolute symbols; the symbol value won't
* change when the object is loaded.
*/
} else if ( ( strcmp ( howto->name, "R_386_NONE" ) == 0 ) ||
( strcmp ( howto->name, "R_X86_64_NONE" ) == 0 ) ) {
/* Ignore dummy relocations used by REQUIRE_SYMBOL() */
} else if ( strcmp ( howto->name, "R_X86_64_64" ) == 0 ) {
/* Generate an 8-byte PE relocation */
generate_pe_reloc ( pe_reltab, offset, 8 );