linux-kernel - Re: [PATCH v2 3/4] Makefile: lld: tell clang to use lld

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Date:   Tue, 2 Apr 2019 13:49:44 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Nicholas Piggin <npiggin@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Adrian Reber <adrian@...as.de>,
        Richard Guy Briggs <rgb@...hat.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Smith <Peter.Smith@....com>,
        Stephen Hines <srhines@...gle.com>
Subject: Re: [PATCH v2 3/4] Makefile: lld: tell clang to use lld

Hi Nick,


On Tue, Apr 2, 2019 at 12:54 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Sat, Feb 16, 2019 at 10:09 AM Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
> >
> > On Thu, Feb 14, 2019 at 8:08 AM Nick Desaulniers
> > <ndesaulniers@...gle.com> wrote:
> > >
> > > On Wed, Feb 13, 2019 at 6:59 AM Masahiro Yamada
> > > <yamada.masahiro@...ionext.com> wrote:
> > > >
> > > > On Tue, Feb 12, 2019 at 5:42 AM <ndesaulniers@...gle.com> wrote:
> > > > >
> > > > > This is needed because clang doesn't select which linker to use based on
> > > > > $LD but rather -fuse-ld=lld. This is problematic especially for
> > > > > cc-ldoption, which checks for linker flag support via invoking the
> > > > > compiler, rather than the linker.
> > > >
> > > >
> > > > Sorry, please explain what kind of problem
> > > > this patch solves.
> > > >
> > > >
> > > >
> > > > [1] $(LD) is used to link vmlinux, modules, etc.
> > > >
> > > > [2] $(CC) is used to link vdso, etc.
> > > >     and -fuse-ld= selects which linker is invoked from $(CC)
> > >
> > > It solves case 2.
> > >
> > > >
> > > >
> > > > Is it a problem to use a different
> > > > type of linker for [1] and [2] ?
> > >
> > > Ideally, no, but I can think of at least one case where it might be
> > > problematic to mix linkers like that:
> > > You might be mixing linker flags added via ld-option from one linker
> > > that aren't actually supported by the other linker.
> >
> > You can do this only when you are sure
> > that the _exactly_ same linker is used.
> >
> > In my understanding, -fuse-ld=lld does not guarantee it.
>
> I really don't think we should be mixing and matching linkers during a
> kernel build.  When we compile with clang, we don't have escape
> hatches that allow for some object files to be compiled with GCC
> (mixing clang and GCC compiled object files into one build).
> Following the same logic, I think mixing linkers during kernel build
> should similarly be dissuaded.  This patch AVOIDS clang using a
> different linker than what was specified via $LD, which is CRITICAL
> for cc-ldoption kbuild macro.  Masahiro, I hope this patch can be
> re-evaluated, or if I'm not understanding your point, that you can
> provide additional clarification.
>



You can pass an absolute pass to LD, like

make LD=/path/to/my/llvm/install/dir/bin/ld.lld

This clarifies which linker is being used
even when multiple versions of llvm are installed
on the machine.


However, -fuse-ld=lld is ambiguous.
Will it use the first ld.lld found in PATH?

So, you cannot avoid mixing linkers by this means.


If we could do -fuse=$(LD), I would agree with it.
Clang accepts -fuse=<absolute-path>, GCC does not.

Is there a way to control the linker search path?


-- 
Best Regards
Masahiro Yamada