Re: [PATCH] riscv: fix vdso build with lld

[Fangrui Song]

The comment of `With ld -R we can then ...` should be fixed as well.

On 2020-04-02, Nick Desaulniers wrote:
> + Jordan, Fangrui
>
> On Thu, Apr 2, 2020 at 1:56 AM Ilie Halip <ilie.halip@xxxxxxxxx> wrote:
> > When building with the LLVM linker this error occurrs:
> >     LD      arch/riscv/kernel/vdso/vdso-syms.o
> >   ld.lld: error: no input files
> >
> > This happens because the lld treats -R as an alias to -rpath, as opposed
> > to ld where -R means --just-symbols.
> >
> > Use the long option name for compatibility between the two.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/805
> > Reported-by: Dmitry Golovin <dima@xxxxxxxxxx>
> > Signed-off-by: Ilie Halip <ilie.halip@xxxxxxxxx>
> > ---
> >  arch/riscv/kernel/vdso/Makefile | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> > index 33b16f4212f7..19f7b9ea10ab 100644
> > --- a/arch/riscv/kernel/vdso/Makefile
> > +++ b/arch/riscv/kernel/vdso/Makefile
> > @@ -41,7 +41,8 @@ SYSCFLAGS_vdso.so.dbg = -shared -s -Wl,-soname=linux-vdso.so.1 \
> >  $(obj)/vdso-dummy.o: $(src)/vdso.lds $(obj)/rt_sigreturn.o FORCE
> >         $(call if_changed,vdsold)
> >
> > -LDFLAGS_vdso-syms.o := -r -R
> > +# lld aliases -R to -rpath; use the longer option name
>
> Thanks for the patch.  Maybe the comment can be dropped? It doesn't
> make sense if there's no -R in the source file you're touching.  If
> someone cares about why `--just-symbols` is spelled out, that's what
> `git log` or vim fugitive is for.  Maybe the maintainer would be kind
> enough to just drop that line for you when merging?
>
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
>
> Jordan, Fangrui, thoughts on this? Sounds like something other users
> of LLD might run into porting their codebase to LLVM's linker.

Independently, I noticed this ~2 days ago.
https://reviews.llvm.org/D76885#1952860

GNU ld parses options with getopt_long and -j is recognized as
--just-symbols ('R') because there is no other long options prefixed with -j.

Now, the following comment applies. Basically, --just-symbols/-R is an
overloaded option. It can be used as an -rpath if the argument is a directory.
The best practice is to use either -rpath or --just-symbols, never -R.

// binutils-gdb/ld/lexsup.c
	case 'R':
	  /* The GNU linker traditionally uses -R to mean to include
	     only the symbols from a file.  The Solaris linker uses -R
	     to set the path used by the runtime linker to find
	     libraries.  This is the GNU linker -rpath argument.  We
	     try to support both simultaneously by checking the file
	     named.  If it is a directory, rather than a regular file,
	     we assume -rpath was meant.  */
	  {
	    struct stat s;

	    if (stat (optarg, &s) >= 0
		&& ! S_ISDIR (s.st_mode))
	      {
		lang_add_input_file (optarg,
				     lang_input_file_is_symbols_only_enum,
				     NULL);
		break;
	      }
	  }




> $ ld.lld --help | grep \\-R
>  -R <value>              Alias for --rpath
> $ ld.bfd --help | grep \\-R
>  -R FILE, --just-symbols FILE
>
> > +LDFLAGS_vdso-syms.o := -r --just-symbols
> >  $(obj)/vdso-syms.o: $(obj)/vdso-dummy.o FORCE
> >         $(call if_changed,ld)
> >
> > --
> > 2.17.1
>
>
> --
> Thanks,
> ~Nick Desaulniers