linux-kernel - Re: [PATCH] riscv: fix vdso build with lld

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Message-ID: <20200402175354.pzhzhumlqsjk66nu@google.com>
Date:   Thu, 2 Apr 2020 10:53:54 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Ilie Halip <ilie.halip@...il.com>
Cc:     linux-riscv@...ts.infradead.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mao Han <han_mao@...ky.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Jordan Rupprecht <rupprecht@...gle.com>
Subject: Re: [PATCH] riscv: fix vdso build with lld

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@...il.com> 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@...ovin.in>
>> Signed-off-by: Ilie Halip <ilie.halip@...il.com>
>> ---
>>  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@...gle.com>
>
>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