How to overrule default gcc options to the linker? - gcc

Question

On my system when I compile something (with bfin-linux-uclibc-g++ but that is irrelevant), I get hundreds of warnings (not in my own code base) with respect to one of the compiler flags. I want to disable it.
fde encoding in src/SpiMessageUtil.o(.eh_frame) prevents .eh_frame_hdr table being created.
This orginates from a default gcc flag which is handed over to the linker, which is easy to check by adding '-v' to the compilation step:
COLLECT_GCC_OPTIONS=... --eh-frame-hdr ...
I would like to get rid of this option, which is indeed by default defined:
bfin-linux-uclibc-g++ -dumpspecs | grep frame-hdr
%{!static:--eh-frame-hdr}\
%{mfdpic: -m elf32bfinfd -z text} %{shared} %{pie} \
%{static:-dn -Bstatic} %{shared:-G -Bdynamic} \
%{!shared: %{!static: %{rdynamic:-export-dynamic} \
%{!dynamic-linker:-dynamic-linker \
%{mglibc:%{muclibc:%e-mglibc and -muclibc used together;:%e-mglibc not supported for this target};:/lib/ld-uClibc.so.0 \
}}}\
%{static}} -init __init -fini __fini
How can I override this option? I cannot use -Wl,--no-eh-frame-hdr, because there is nothing like that defined.

Answer 1

You can dynamically dump GCC's specs, remove this switch from there and use it when linking, i.e.:
g++ -dumpspecs | sed -e 's,--eh-frame-hdr,,g' > better_specs
g++ -specs=better_specs -o target file1.o file2.o -llib1...
This would replace the specs inline, while keeping original compiler intact.
If you keep your own Makefiles, this could also be handled with something like:
$(TARGET): $(OBJS) | better_specs
$(LINK.o) $(OUTPUT_OPTION) -specs=$| $^
better_specs:
$(CXX) -dumpspecs | sed -e 's,--eh-frame-hdr,,g' > $#
This approach could be also used with configure scripts, provided that you generate better_specs before, you could just use ./configure CXX='g++ -specs=/path/to/better_specs'.

Answer 2

I just got started with back-porting some code to an old system with a bfin controller and ran into the problem with these terribly annoying warnings - 1000s at a time. I didn't find a way to just deactivate the output.
But there are 2 "ways to go" that work:
Fix the source and rebuild the tool-chain:
Remove the code that creates the output in elf-eh-frame.c in the function _bfd_elf_discard_section_eh_frame:
(*info->callbacks->einfo)
(_("%P: fde encoding in %B(%A) prevents .eh_frame_hdr"
" table being created.\n"), abfd, sec);
Patch the ld binary
Take a look at the ld-Binary and patch the binary directly.
I dumped the data segment (.rodata) with objdump to find the address of the string. Then (after creating a disassembly with objdump) I searched where that string was used and replaced the call to the function that creates the output with two NoOps (0xFF 0xD3 -> 0x90 0x90).
Linker still creates the same output, but no more messages.