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.
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