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

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Date:   Tue, 06 Apr 2021 14:42:27 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     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>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "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 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")
>
> 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