llvm dev - May 2020 - ld.lld ignoring --sysroot

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

/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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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!
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On 2020-05-14, Rui Ueyama via llvm-dev wrote:
>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
>>
You misunderstand GNU ld's --sysroot rule, which is applicable in these two
cases:

* when a path begins with "=" or "$SYSROOT"
* https://www.sourceware.org/binutils/docs/ld/File-Commands.html
   "In case a sysroot prefix is configured, and the filename starts with
   the ‘/’ character, and the script being processed was located inside
   the sysroot prefix, the filename will be looked for in the sysroot
   prefix."

For your case, you need a linker script located under /linuxroot/, which
INPUT or GROUP an absolute path.
>--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!
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>_______________________________________________
>LLVM Developers mailing list
>llvm-dev at lists.llvm.org
>https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev