linux-kernel - Re: [PATCH] kbuild: merge module sections under CONFIG_LD_DEAD_CODE_DATA

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Message-ID: <CAK7LNAQBYxZ4s4Eg2TPMoLA+NDWXsNrnq2ch1ZQsBgcCXPk7Sw@mail.gmail.com>
Date:   Wed, 7 Apr 2021 23:24:30 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Alexander Lobakin <alobakin@...me>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Jessica Yu <jeyu@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Sean Christopherson <seanjc@...gle.com>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kbuild: merge module sections under CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
 too

On Tue, Apr 6, 2021 at 11:42 PM Alexander Lobakin <alobakin@...me> wrote:
>
> On Friday, 2 April 2021, 18:09, Sami
> Tolvanen <samitolvanen@...gle.com> wrote:
>
> > On Fri, Apr 2, 2021 at 5:40 AM Alexander Lobakin alobakin@...me wrote:
> >
> > > When building with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
> > > -fdata-sections and -ffunction-sections are being enabled by the
> > > top-level Makefile, and module section merging is also needed.
> > > Expand the ifdef (and the comment block) to cover that case too.
> > > Fixes: 6a3193cdd5e5 ("kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled")


Did you test this patch before submission?


See the top Makefile closely:

ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
LDFLAGS_vmlinux += --gc-sections
endif


-ffunction-sections -fdata-sections are passed to only
built-in objects, but not to module objects in the
first place.

KBUILD_CFLAGS_KERNEL is only passed to built-in objects.


The situation you claimed never happens.







> > Wouldn't this trigger the ld.bfd bug described in the commit message
> > when LD_DEAD_CODE_DATA_ELIMINATION is enabled? LTO_CLANG always uses
> > LLD, so it won't have this issue.
>
> LD_DEAD_CODE_DATA_ELIMINATION is marked
> “EXPERIMENTAL“ in the config prompt, and
> arches have to opt-in
> HAS_LD_DEAD_CODE_DATA_ELIMINATION to give
> an access to it (only a few does). This
> should be relatively safe.
>
> > Sami
>
> Thanks,
> Al



-- 
Best Regards
Masahiro Yamada