Use philipc's object::write to emit object files

[pchickey]

No description provided.

[jedisct1]

This is producing an invalid object on MacOS:

ld: illegal text-relocation to 'guest_table_0' in 
    /var/folders/gq/pbvv220x79156lw2f_f22r5w0000gn/T/lucetcfv14BM/tmp.o 
    from 'guest_table_0'

How to reproduce

int main(void)
{
    return 0;
}

clang --target=wasm32-wasi --sysroot /opt/wasi-libc -O2 -s -o a.wasm a.c

lucetc-wasi a.wasm

WASM file and resulting object files:

a.wasm.gz
a.obj.gz

[philipc]

Looks like I moved too much into __TEXT/__const (it used to all be in __DATA/__data). I've added gimli-rs/object@1f8a490 which should let us use __DATA/__const instead for const data that has relocations. This will require a small change in cranelift-object too.

[jedisct1]

Pretty ugly, but the following diff over object rev a2758023ea001c5e9fba79ebc476bd81b0bda8fa seems to help lucetc build valid Mach-O files.

https://gist.github.com/4eccc9f79d8db408e1c1717a4d446d49

BTW, that file can come in handy: https://opensource.apple.com/source/ld64/ld64-128.2/src/ld/parsers/macho_relocatable_file.cpp.auto.html

[philipc]

@jedisct1 Thanks for figuring out those fixes!

Can you explain more about the underscore prefix, or point to some documentation about it? I see that faerie writes the underscore for every symbol, but you've only added for globals.

I might have to come up with a way of making the underscore an option in add_symbol, since always adding it during writing is going to break the objcopy round trip (unless I remove it again when reading?).

[jedisct1]

Hi @philipc ,

The Mach-O documentation says:

The name of a symbol representing a function that conforms to standard C calling conventions is the name of the function with an underscore prefix. Thus, the name of the symbol representing the function main would be _main.

So I guess it indeed should also be applied to non-exported functions.

COFF symbols also need leading underscores.

[jedisct1]

I don't think any other calling convention has even been used on MacOS.

LLVM skips leading underscores unconditionally on Mach-O files.

So maybe it would make sense to heave the read interface remove these underscores, so we have symmetry between the read and write interfaces.

[philipc]

I've opened gimli-rs/object#127 to fix those issues, if you could review that.

I chosen to make the mangling optional, and disable this for objcopy. That LLVM link is only for the symbolizer, which is a bit higher level than the API that the object crate wants to provide. I've based this on this LLVM code.

[pchickey]

Closing in favor of #295