On Tue, Jul 28, 2020 at 6:02 PM Fāng-ruì Sòng <mask...@google.com> wrote: > > On Thu, Jun 25, 2020 at 2:05 PM Fangrui Song <mask...@google.com> wrote: > > > > On 2020-04-21, Fāng-ruì Sòng wrote: > > >On 2020-04-21, Gerd Hoffmann wrote: > > >> Hi, > > > > > >Hi Gerd, thank you for your comments. > > > > > >>>You mean "Makefile: Change ET_EXEC to ET_REL so that lld can link > > >>>bios.bin.elf" > > >>> > > >>>Let me add more descriptions. I am a maintainer of lld ELF (I > > >>>contributed most features and wrote most items of lld 9/10's release > > >>>notes). > > >>> > > >>>A linker accepts ET_REL as regular object files. The input sections > > >>>are taken to make up output sections. The input symbols are taken to > > >>>track dependencies and resolve relocations. > > >>> > > >>>A linker accepts ET_DYN as shared objects. Input sections are ignored. > > >>>Only symbols are used. Many sections are synthesized by a linker and do > > >>>not make sense to be reused as input. > > >>> > > >>>ET_EXEC, position dependent executables. The type is more similar to > > >>>ET_DYN. Many synthesized sections (.hash .gnu.hash .plt .dynstr .dynsym) > > >>>do not make sense to be reused as input. Sometimes there are needs to > > >>>read its symbols so that another component can know which symbols should > > >>>be exported. For this use case, --just-symbols is used. > > >>> > > >>>seabios' use of ET_EXEC is very different. I have built thousands > > >>>third-party pieces of software. seabios and (for a short period because > > >>>I have fixed it) the Linux kernel are the only two cases an ET_EXEC > > >>>input's sections are used. The symbol table appears to be ignored. This > > >>>is really an unusual use case. Another lld maintainer and I think this > > >>>is just a very error-prone case. I hence decide that this is not worth > > >>>supporting. GNU ld accepts it, but note that GNU gold disallows it as > > >>>well > > >>>https://sourceware.org/git/?p=binutils-gdb.git;a=blob;hb=49d1d1f53df552f059… > > >>> > > >>>Since seabios just uses ET_EXEC as something similar to an ET_REL. > > >>>Binary patching the file seems the most portable approach. > > >>>(This Linux kernel patch has been merged into the mainline.) > > >> > > >>Wouldn't it be better to add a switch to ld to request ET_REL output > > >>instead? > > >> > > >>cheers, > > >> Gerd > > >> > > > > > >I added some context in my previous comment. Before I said "binary > > >patching", I analyzed possible linker input types > > >(ET_REL/ET_DYN/ET_EXEC) and discussed why ET_EXEC as ET_REL is really a > > >special case. Let me add a concrete example: > > > > > > % cat a.c > > > void _start() {} > > > % clang -fuse-ld=bfd a.c -nostdlib -o a > > > % clang -fuse-ld=bfd a -nostdlib -o aa > > > /usr/bin/ld.bfd: warning: cannot create .eh_frame_hdr section, > > > --eh-frame-hdr ignored > > > /usr/bin/ld.bfd: error in a(.eh_frame); no .eh_frame_hdr table will be > > > created > > > > > >An ET_EXEC can have many linker synthetized sections (.eh_frame_hdr, > > >.dynamic, .gnu.hash, .hash, .plt, etc) which will really make no sense > > >for a subsequent link. seabios is special in that it has a linker script > > >to explicitly concatenate some sections from the ET_EXEC input and (as I > > >understand it) just ignores all symbols. > > >This usage makes it really special and there is probably no other > > >project doing the same. A lld specific linker option will be neither > > >useful nor good for compatibility because GNU ld does not have the option. > > > > > >For a new linker, we can take time to think what features are > > >error-prone and reject such features in advance to help detect such > > >problems. We are not in GNU ld's situation "our previous versions > > >support this arguably brittle use cases and our future versions may have > > >to be compatible". > > > > > >Hope the explanation makes sense. > > > > Hi Kevin, > > > > Internally we have been using seabios built with LLD and llvm-readelf > > for two months now, without any issue. I hope the stability can give you > > more confidence accepting the patches. > > > > About PATCH 3/5 "Makefile: Change ET_EXEC to ET_REL so that lld can link > > bios.bin.elf" > > The maintainer of binutils agreed with my resolution (ET_EXEC should not be > > accepted as linker input) > > and pushed > > https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=a87e1817a435dab6c6c042f9306497c9f13d4236 > > > > GNU ld from binutils 2.35 onwards will not allow this use case. > > > > Hope you can take another look when you get time. > > Friendly ping. > > ET_EXEC issue was fixed. Hope you can take a look at other patches. Thanks
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