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

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Message-ID: <CAK7LNAQMoYNevPns0zSaSx0iU5LA+2146snNDmy9fF8v5ej5tg@mail.gmail.com>
Date:   Sat, 16 Feb 2019 12:07:59 +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>
Subject: Re: [PATCH v2 3/4] Makefile: lld: tell clang to use lld

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.



-- 
Best Regards
Masahiro Yamada