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

[Fangrui Song via llvm-dev]

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
>>

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