[llvm-dev] ld.lld ignoring --sysroot

[Rui Ueyama via llvm-dev]

On Thu, May 14, 2020 at 11:04 AM Ivan Medoedov via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
> I'm trying to compile a Linux hello world executable on macOS.
>
> The first step is simple:
>
> clang -c -target x86_64-linux-gnu -c -o hello.o hello.c
>
> But linking results in an error:
>
> ld.lld --sysroot=/linuxroot/ -o hello -m elf_x86_64 \
>   -dynamic-linker /lib64/ld-linux-x86-64.so.2 \
>   /lib/crt1.o \
>   /usr/lib/x86_64-linux-gnu/crti.o ../hello.o \
>   /usr/lib/x86_64-linux-gnu/libc.so \
>   /usr/lib/x86_64-linux-gnu/crtn.o
>
> ld.lld: error: cannot open /lib/crt1.o: No such file or directory
> ld.lld: error: cannot open /usr/lib/x86_64-linux-gnu/crti.o: No such file
> or directory
>

--sysroot affects some library search paths, but it doesn't affect paths
directly given to the linker. In the above command line, you are directly
specifying object file and so file paths without -l, so the linker looks at
the only given locations. This is an expected behavior.

I think, unlike some other linkers, --sysroot option isn't very useful for
lld, because lld doesn't have a notion of built-in default search paths.
You always have to specify library search paths by -L option to lld. So why
don't you specify library directories with -L, like this?

$ ld.lld -o hello -m elf_x86-64 -dynamic-linker /lib64/ld-linux-x86-64.so.2
-L /linuxroot/usr/lib -lc /linuxroot/usr/lib/x86_64-linux-gnu/crtn.o


> /linuxroot/ contains all the necessary files copied from a Linux machine:
> /linuxroot/lib/crt1.o, /linuxroot/usr/lib/x86_64-linux-gnu/crti.o, etc
>
> It works if I use a full path for each file (-dynamic-linker
> /linuxroot/lib64/ld-linux-x86-64.so.2 ...), but the resulting binary
> doesn't work on Linux, because it's dynamically linked to /linuxroot/...
> which is missing on the Linux box.
>
> file hi
> hi: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically
> linked, interpreter /linuxroot/lib64/ld-linux-x86-64.so.2, for GNU/Linux
> 3.2.0, not stripped
>
> The only way I can make it work is to have actual
> /usr/lib/x86_64-linux-gnu/crti.o etc on my macOS box, which --sysroot is
> supposed to help avoid.
>
> I'm sure I'm missing something simple here. I've been following the docs,
> but couldn't figure it out on my own.
>
> Thanks!
>
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